ACTIVITY RHYTHMS EXPRESSED BY JUVENILE ... - nsgl.gso.uri.edu
C.J. Madden and R. D. DeLaune - nsgl.gso.uri.edu
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CHAPTER 3 CHEMISTRY AND NUTRIENT DYNAMICS by C.J. Madden and R. D. DeLaune 3.1 INTRODUCTION Nutrient dynamics in Barataria Basin are controlled by the amount of allochthonous materials entering the basin, hydrology, and biological and chemical processes actingwithinthe basin. The hydrologic flow from the upperbasinto the Gulf of Mexicocarries nutrientsenteringfrom the heavily populated uplands surrounding the upper basin to the upper, middle, and lower basins. 'The nutrients undergo a series of transformations as they move through the basin. Upper Barataria Basin is composed of fresh swamp forest, marsh, and lake, Around the upland perimeter, increasing urban, agricultural, and industrial pollution has introduced nutrient-rich runoff to the basin. The rniddle basin grades into brackish marsh surrounding a complex of lakes which act as receiving basinsfor uplandand upper basin runoff. In lower Barataria Basin, two large saline bays, Barataria Bay, and Carninada Bay, which open into the Gulf of Mexico, are bordered by tidal salt rnarshes. The distinct upper, middle, and lower basin habitats of Barataria Basin are distinguished by their salinity and nutrient regimes andbythe amount of production they sustain. Historically, regular inundation of the basin by floodwaters topping the natural levees introduced riverine water and nutrients to the swamp and marsh. As cultural development occurred along the river and basin, artificial levees were constructed,eliminating all riverine input to the basin, Today, the sole hydrologic input is precipitation, which averages 160 cm/yr Sklar 1983!. Nevertheless, the hydrologic transport of sediments and nutrients remains the major mechanism of nutrient supply in the Barataria system. Fertilizer and dead organic material which accumulate in the uplands during the summer months are flushed through the basin during the fall in freshwater pulses. Biological processes allow nutrient exchange among atmosphere, biosphere, and sediments and also make nutrients available for hydrologic transport. A number of biological processes important to nutrient dynamics have been studied in the basin and will be discussed. These include nitrogen fixation, nitrification, denitrification,ammonification, photo- synthesis, respiration, and mineralization. Salinity fluctuations are important in determining the chemical properties of the water as well as the distribution of biota in the wetlands. By monitoring the conservative propertyof salinity, investigators have traced long-term changes in hydrologic inputs and flows through the basin. The seasonal salinity regime of Barataria Basin is a complex one controlled by precipitation, winds, and the proportion of Mississippi River and gulf water importedthrough the bay mouths. Strong northwest frontal passages drive water from the upper basin to the gulf, freshening the entire basin during winter. At this time of year, the gulf water level is at its lowest, assisting the southward flow. In summer, predominantly southeasterly winds and a high gulf water level push salt water into Barataria Basin from the gulf. l-ligh evapotranspira- tion:precipitation ET:P! ratios coupled with the gulf backpressure generallycause freshwater flow downbasin to cease in July and August, raising basin salinity. Precipitation in autumn exceeds evapotranspiration and freshwater again feeds the lower basin. The upper basin swamp forest never experiences salinity. Middle basin salinities range 18
Transcript of C.J. Madden and R. D. DeLaune - nsgl.gso.uri.edu
CHAPTER 3
CHEMISTRY AND NUTRIENT DYNAMICS
by
C.J. Madden and R. D. DeLaune
3.1 INTRODUCTION
Nutrient dynamics in Barataria Basin arecontrolled by the amount of allochthonousmaterials entering the basin, hydrology, andbiological and chemical processes acting within thebasin. The hydrologic flow from the upper basin tothe Gulf of Mexico carries nutrients entering fromthe heavily populated uplands surrounding theupper basin to the upper, middle, and lowerbasins. 'The nutrients undergo a series oftransformations as they move through the basin.
Upper Barataria Basin is composed offresh swamp forest, marsh, and lake, Around theupland perimeter, increasing urban, agricultural,and industrial pollution has introduced nutrient-richrunoff to the basin. The rniddle basin grades intobrackish marsh surrounding a complex of lakeswhich act as receiving basins for upland and upperbasin runoff. In lower Barataria Basin, two largesaline bays, Barataria Bay, and Carninada Bay,which open into the Gulf of Mexico, are borderedby tidal salt rnarshes. The distinct upper, middle,and lower basin habitats of Barataria Basin aredistinguished by their salinity and nutrient regimesand by the amount of production they sustain.
Historically, regular inundation of the basinby floodwaters topping the natural leveesintroduced riverine water and nutrients to the
swamp and marsh. As cultural developmentoccurred along the river and basin, artificial leveeswere constructed, eliminating all riverine input tothe basin, Today, the sole hydrologic input isprecipitation, which averages 160 cm/yr Sklar1983!. Nevertheless, the hydrologic transport ofsediments and nutrients remains the majormechanism of nutrient supply in the Barataria
system. Fertilizer and dead organic material whichaccumulate in the uplands during the summermonths are flushed through the basin during thefall in freshwater pulses. Biological processesallow nutrient exchange among atmosphere,biosphere, and sediments and also make nutrientsavailable for hydrologic transport. A number ofbiological processes important to nutrientdynamics have been studied in the basin and willbe discussed. These include nitrogen fixation,nitrification, denitrification, ammonification, photo-synthesis, respiration, and mineralization.
Salinity fluctuations are important indetermining the chemical properties of the wateras well as the distribution of biota in the wetlands.By monitoring the conservative property of salinity,investigators have traced long-term changes inhydrologic inputs and flows through the basin.The seasonal salinity regime of Barataria Basin is acomplex one controlled by precipitation, winds,and the proportion of Mississippi River and gulfwater imported through the bay mouths. Strongnorthwest frontal passages drive water from theupper basin to the gulf, freshening the entire basinduring winter. At this time of year, the gulf waterlevel is at its lowest, assisting the southward flow.In summer, predominantly southeasterly winds anda high gulf water level push salt water into BaratariaBasin from the gulf. l-ligh evapotranspira-tion:precipitation ET:P! ratios coupled with thegulf backpressure generally cause freshwater flowdownbasin to cease in July and August, raisingbasin salinity. Precipitation in autumn exceedsevapotranspiration and freshwater again feeds thelower basin.
The upper basin swamp forest neverexperiences salinity. Middle basin salinities range
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between 0 and 5 ppt depending on precipitationand the amount of gulf water driven inland bysoutheasterly winds. The lower basin-bay areaexperiences salinities of 18-30 ppt throughout theyear, Studies have shown salinity in the middlebasin to be increasing by almost 0.11 ppUyr VanSickle et al, 1976!. At the turn of the century, themiddle basin was fresh. Encroachment by salinewater has increased middle basin salinity to 5 ppt.Toward the gulf, salinity in upper Barataria Bay hasincreased to 11 ppt where 75 years ago it was only6 ppt. Predictive models indicate that overallsalinity will increase about 30% by the year 2000 Van Sickle et al. 1976!.
Increases in salinity are directly andindirectly related to human activity, The con-struction of artificial levees has stopped the inflowof fresh river water to the basin. The dredging oflinear canals has given saline gulf water directaccess far upbasin Craig and Day 1977!. Marshsubsidence has converted much of the marsh areato open water, allowing saltwater intrusion. Naturalsubsidence is a consequence of sediment con-solidation and eustatic sea level rise. Theindiscriminate dredging of canals and constructionof artificial levees that block the influx of importantmarsh-building sediments from the river hasunnaturally increased land loss rates to more than100 km2t'yr Gagliano et al. 1981!,
Salinity increases are also affecting theproduction of marsh macrophytes within the basin.Recent studies Pezeshki et al. in press! show thatsalinity increases currently occurring in brackishmarshes are altering normal physiological functionsof fga5rg gaia@, the dominant species in theBarataria brackish marshes. Reduction in primaryproduction of marsh macrophytes will affectestuarine carbon cycling and organic carbon poolswhich are important to vertical marsh accretion.Organic carbon accumulation is important inmaintaining marsh elevations in rapidly subsidingenvironments such as the Barataria Basin.
3.2 UPPER BASIN NUTRIENT PROFILEAND TROPHIC STATE
The upper Barataria Basin is dominated byswamp forests of baldcypress and water tupeloand, in better drained areas, by bottomlandhardwoods. Lac des Allemands, the major waterbody in this section, is always fresh and serves as
the catchment for all upper basin drainage. It is fedby upland runoff through Bayou Chevreuil andGrand Bayou. Fresh marsh borders the lake to thesouth and east.
The natural chemistry of the upper basinhas been disturbed by urban and agriculturalactivity on the surrounding natural levees, Thedevelopment of population centers has intro-duced high levels of nutrients and sediments intoupland runoff. Impervious urban land areas andhighly channelled drainage networks in farm fieldshave increased upland runoff volumes andchanged the natural hydrology of the upper basin.These impacts are compounded by canalization ofthe swamp and marsh by oil and gas interests. Inshort, the burgeoning development of the area isincreasing nutrient input and reducing the capacityof the wetlands to process these inputs. The resultis eutrophication of the basin's waters, a disturbedswamp system chemistry, and reduced swampproductivity Day et al. 1982!.
Annual loadings of 1.2 million kg nitrogen{N! and 130,000 kg phosphorus P! are estimatedto enter the upper basin from all upland sources Kernp 1978!, Runoff from agricultural land addsmore N and P per area than any other land use,comprising an estimated 75% of all N and 95% of Pentering the basin. Seventy-one percent of theupland Barataria watershed is now devoted toagriculture Mopkinson 1978! and cropland ero-sion accounts for over 80% of the 63 million kg or63,000 metric tons of sediment lost annually fromthe watershed. Based on predictions of land use,N loads are expected to increase by 28% and Ploads by 16% before 1995 Hopkinson and Day1980b!.
As a means of quantitatively assessing thewater quality of Barataria Basin, Seaton and Day�979! modified a statistical technique developedby Brezonik and Shannon �971! which yields anindex of the degree of eutrophication in a waterbody. The Trophic State Index TSI! ranks bodiesof water based on a multivariate statisticalformulation including total N, total P, secchi depth,and chlorophyll g data Table 3!. By ranking allwater bodies in Barataria Basin from most negativeTSI oligotrophic! to most positive hypereutrophic!it was shown that all waterways in the upper basinclustered together in the eutrophic tohypereutrophic range, Poor water quality char-acterized by high nutrient levels, high algal
Table 3. Average yearly nutrient and chlorophyll a concentrations and secchi depths at several sitesin Barataria Basin. GIVAV Gulf Intracoastal Waterway. Nutrient data are in mg/I; chlorophyll data lnmg/rrt and secchi in cm,a
NH4+ NO3- Inorg N PO4 3 TON TP Secchi CHLOEArea/Location
0.13 0.08 0.210.10 1.88 1,98
1.17 0.22 50201 019 8
2
53Upper/swampLlpper/St. James Canal
0.09 0.11
0.06 0.95
0.06 0.270.08 0.15
0.73 0.11 46
531.59 0.15 32
53
0.20
1 01 0.060.33
0.23 0.09
38
2747
Middle/Lake SalvadorhNddle/Lake SalvadokMiddle/Lake Cataoua~Middle/Lake Cataouaf~
0.10 0,48 0.58
0.15 0.12 0.27
0.10 0.21 0.31 0.060.06 0.04
0.95 0.89 110.71 0.07 65
54
7
510
+wer/GIWWLower/Little LakeLower/Little Lak@Lower/Airplane Laic
~ Data from Seaten and Day �979! unless otherwise noted,b From Hopkinson and Day �979!c From Ho and Lane �973!
reduce the runoff nutrient loads, resulting in thehigh loading to Lac des Allemands Butler 1975!.
3.2 1
20
ing crop, and low water clarity persistedughout the upper bagn. Downbasin, waters
gradually improved in quality. Caminada andI4rataria Bays had negative TSI, indicatingmesotrophy to mesowliootrcphy Table 4!.
Oil access cycle and agricultural drainagegtohee cutting through the basin Figure 17! have
n cited as having exacerbated the problem ofgggrient enrichment. In the swamp's naturaldcyditlon, before the dredging of linear canals,runcjff was distributed as sheet flow over the«camp forest. Now, hydraulically efficient canalsguide water and nutrients directly to receivinge@erbodies, bypassing the swamp forest.Stfjdies have shown that the Barataria swamps hadbeen acting as effective nutrient filters, removinginorganic nutrients from percolating floodwaters Meo et al, 1975; Kemp 1978!. But in the currentsituation, nutrient removal and uptake processesoccurring in the canals alone are insufficient to
By constraining canal flow, spoil banksrestrict the natural lateral transport of waternecessary for the removal of wastes, thereplenishment of marsh sediments, and the importof new nutrients to the swamp forest. Obstructionof overbank flooding by levees can result innutrient starvation and the blocking of swampsubstrate accretion. As the density of canalsincreases, there is a concomitant reduction inoverland flow and basin water storage capacity.
Horton �945! developed a means ofquantifying the impact of canals on wetlands. TheDrainage Density Index DDI! reflects the degree ofcanal development in a wetland and is defined asthe total length of the drainage network per squareunit of drainage basin. A total of 37,2 linear mi ofcanals criss-cross the upper Barataria Basinrepresenting a DDI of 40.0 rni/mi 2 in uplands Gaeland Hopkinson 1979!. When upland DDI is
Table 4. Trophic state index TSI! classification of Barataria Basin, LouISiarta, LB = lower basin, MB =middle basin, UB = upper basin, 0 = oligotrophic, M = mesotrophic, E = eut~ic, and H = hypereutrophic modified from Witzig and Day1983a, b!.
TSIscore
TrophlcgroupStation Location
recalculated to include aII split and quarter ditchesin sugarcane fields, the index rises to 800 mi ofcanal/mi2.
large number of canals contributed to the high totalN loads of 2.0-2.2 mg/I. Catchments consisting ofunderdeveloped natural swamp exhibited goodwater quality and had lower total N valuesaveraging 0.67 to 1.1 mg/I,A strong correlation has been noted
between drainage canal density and the conditionof receiving water bodies. In the Gael andHopkinson study, Barataria Basin was divided into24 subcatchment areas, including water qualitystations that had been previously sampled bySeaton and Day �979!. Within subcatchments, adirect relationship between DDI and thedeterioration of water quality was found indicatingthat eutrophication in the upper basin is a functionof the density of drainage canals. In regions of lowcanal density, the upper basin is generallyoligotrophic to mesotrophic Craig and Day 1977;Seaton and Day 1979!. In regions of extensivechannelization and agricultural development, theupper basin has become eutrophic tohypereutrophic. Large percentages of agriculturalland included in two of the subcatchments with a
Most of the canal development hasoccurred in the last two decades. The nutrient-concentrating effect of canals can be illustrated byrecalculating loading rates to the swamp as theywould have been without the presence of canalsand levees. If the 1.2 million kg of N entering thebasin from upland sources were allowed to moveas sheetf low over the entire upper basin, theannual N load woukf total about 1.27 g/m2, atwenty-fold reduction from current levels of 30.1g/m2 to Lac des Allemands. Likewise, the132,283 kg of P entering from uplands, distributedover the basin floor, would add 0.14 g/m2 annually,instead of the 4.3 g/m2 loaded to I ac desAllemands by canal-directed flow Hopkinson andDay 1980a, b!.
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Caminada PassBayou RigolettesBarataria BayLake Salvador
Bayou PerotLittle Lake
Bayou BaratariaBarataria WatenvayNatural swamp streamJohn-the-Fool BayouLittle Lake oil and gas fieldBayou Chevreuil 1Lake Cataouatche
Bayou des AllemandsBurtchell Canal
Bayou CitamonLac des Allemands
Bayou Chevreuil 2St. James Canal
LBLBLB
MBLB
LB
LBLB
UBLBLB
UB
UB
MB
MB
UBUBUB
UB
-4.8
-4,3
-3,8
-3.3-2.8-2.7
-1.8
-1.6
-1,4
-0.6-0.4
6.00.7
2.62.7
3.7
3.84.0
6.4
M-0M M M M M M M M M M E E E EE-H
E-HE-H
H
Figure 17. Drainage network of the upper Barataria Basin, Stippled areas areintensively drained naturallevees modified and used with permission, fromGael and Hopkinson 1979, copyright LSU Division of Continuing Education!.
3.2.2 N ri
22
The disruption of basin hydrology bycanals and the increasing inputs of pollution haveresulted in high nutrient concentrations in waterbodies of the upper basin Stow et al. 1985!.However, a number of studies Seaton and Day1979; Hopkinson and Day 1979; Kemp 1978;Butler 1975! have shown striking differencesbetween the nutrient levels in impacted lakes andnatural swamps in the upper basin. While bayousand lakes receiving large inputs of upland runoffhave N:P ratios of 6:1 Kemp 1978! to 9.2:1 Gaeland Hopkinson 1979!, natural swamp watersexhibit ratios of 2:1 Kemp 1978!, Inorganic N isremoved from swamp floodwaters throughde nitrification and the sedimentation ofammonium-bearing clays Kemp 1978; Engler andPatrick 1974!. Although the sedimented claysremain in the system, the end product ofdenitrification is N2 gas which is released to theatmosphere.
Ten percent of ammonium inputs toswamps is removed through plant uptake. Another
large percentage is adsorbed to sediments.Swamp sediment pore water ammoniumconcentrations are 15 times the concentrations inoverlying waters Kemp 1978!, in passing throughthe swamps, water column nitrate levels in bayouInlets are reduced by one-half and N is exportedfrom the swamps largely in organic form, as detritusand ieachates. Thus, nitrate concentrations inswamp streams are kept low throughout the year,averaging 0,05 to 0.14 mg/I Kernp 1978; Butler1975!, while bayou and lake sites always exhibithigh concentrations of nitrate �.28 to 0,32 mg/I!,The high levels of nitrate and ammonium found inBayou Chevreuil and other receiving watersdraining the uplands show a seasonality thatcoincides in spring with peak fertilizer applicationand peak hydrologic flushing, and in late summerwith the decomposition of large blooms of N-fixingalgae Figure 18!.
Nitrogen-limited freshwater marshes in theupper basin can improve water quality by removingnutrients from inflowing waters. In one uptakestudy DeLaune et al. 1986!, eight percent of thelabelled N applied to the freshwater marshes
0.6~ -PGAMIC P
0.4
IL
e 02
0.0
Q ORGANIC N2.5
-N
2.0
'f.5
R e1.0
0.5
0,0
MONTH
23
Figure 18. Phosphorus P! and nitrogen N!levels in bayous of the upper Barataria Basin Kemp 1978!.
remained in the plant-soil system, indicatingefficient N use,
Kemp �978! speculates that originally theentire upper basin aquatic system was N-limited.Although this condition persists in the lowerBarataria Basin Sklar 1976!, the upper basin hasrecently been described as P-limited by Day et al.�977! and Seaton and Day �979!, Lantz �970!reported that increases in eutrophication haveparalleled accelerated P loading while N loadinghas remained constant indicating limitation by P.Nitrogen fixation rather than upland drainage isreported to be the primary source of N to Lac desAllemands Stow et al. 1985!. This is attributed tothe fixation of atmospheric N by massive algaeblooms in the P-enriched lake. Annual P loadingof 4.3 g/m2/yr to I ac des Allemands has risen toten times the critical specific hading leveldiscussed by Vollenweider �968! and 30 timesthat calculated by Brezonik and Shannon �971!
as being the threshold for eutrophic conditions inlakes. Fresh upper reaches of Barataria Basin areespecially susceptible to eutrophication becauseof the lack of tidal flushing.
Studies Butler 1975; Craig and Day 1977!concur that the P retention capacity of Lac desAllemands is approximately 55'/o and while the lakeacts as a strong sink for P, almost half of the P loadis exported to water bodies downstream. Aconsiderable portion of the P in Lac des Allemandsis tied up in plant material, and on average, there isthree times more P in particulate than in dissolvedform. The high water column productivity netdaytime productivity = 611 g C/m2/yr; Day et al.1977! incorporates 10'/-15'/ of the P importedinto organic matter, but by far the iargest fraction ofthe des Allemands P load is bound in lake
sediments Stow et al. 1985!. Equilibrium studiesshow bottom sediments in Lac des Allemands are
a major sink for incoming dissolved ortho-phosphate Stow et al. 1985!. Well-oxygenatedwaters and the presence of an oxidized sedimentmicrozone encourage the trapping of precipitatedferric-phosphates. These compounds, plusphosphate adsorbed to clay particles are trappedin reduced sediments below the oxidized zone.
Day et al. �977!, using a mass balance model,estimated that 45/. of the P in the lake flows to the
sediments, adding 2,0 g P/m2/yr to the sedimentpool. The lake bottom, in reducing dissolved Pconcentrations, significantly reduces the potentialfor eutrophication and buffers the impact of upperbasin P loads on middle basin waters.
Sedimentation in the lake, ranging from 0.44 cm/yrto 0.81 crn/yr, also removes large amounts of C, N,and P. Carbon, nitrogen, and phosphorus werefound to accumulate in the sediment at the rate of
60, 7,1, and 1.1 g/m2/yr, respectively Stow et al.1985!.
3.3 MIDDLE BASIN NUTRIENTS
Lake Salvador lies at what is consideredthe interface of the upper and rniddle basins andserves to filter out much of the high nutrient loaddelivered from the upper basin, Much dissolvedorganic and inorganic material is removed byflocculation in Salvador's brackish waters. Theprocess of flocculation is common in estuaries,occurring where salinity is 2-3 ppt. The ionicenvironment at this salinity encourages theclumping of clays, silts, and organic particulatesinto larger particles which readily settle out.
The rniddle basin, in contrast to the upperbasin, experiences gulf tidal effects and alternateseasonal inundations of fresh and salt water,flushing the marshes more efficiently than those inthe non-tidal upper basin. As in the upper basin,canals have altered the chemistry and sheet flowhydrology in the middle basin by causing flow tobypass swamp and marsh., 'FurtheAnore, canalshave altered the circulation and flow patterns withinthe lake corn'plex itself. Whereas LakeCataouatche once emptied into Lake Salvador, thecreation of the Barataria Waterway, a linearnavigation canal, has proved an efficient alternatedrainage for Lake Cataouatche, which nowempties directly into lower Barataria Bay, effectivelyisolating Lake Salvador from the rniddle basin lakesystem. As a result, nutrient and chlorophyll glevels in Lake Salvador are much lower than in theother middle basin lakes. In Lake Cataouatche, theBarataria Waterway, and Barataria Bay, nutrientsand chlorophyll p concentrations are similarly high Seaton and Day 1979!, indicating that theseenriched water bodies form a tightly integratedcomplex with free circulation among them Day etal. 1982!.
The uncoupling of Lake Salvador from thecirculation of the middle basin has resulted in anuneven distribution of nutrients and sediments inthe water bodies and a less stable hydrologicregime. Because of its isolation, Lake Salvador,the largest body of water in both the upper andmiddle basins, can no longer serve as a runoffnutrient processor, and the other lakes areoverloaded. Slow moving bayous that oncetended to damp the movement of fresh and saltwater are now replaced by channels which movewater rapidly through the basin. Water level andsalinity variations are rapid and extreme, tiedclosely to precipitation or tidal fluctuations, Saltwater now intrudes farther upbasin, and hasaltered the species composition of the wetlandbiota,
I and use in the middle basin differs fromthat in the agriculturally-dominated upper basin.Hopkinson �978! reported that 40/o of the middlebasin upland is woodland or open, while only 6/o isagricultural as compared to 71 /o agricultural in theupper basin!. However, much of the input to therniddle basin is nutrient-rich urban runoff drainingthe populated eastern levees near New Orleans.Nitrate concentrations �.91 mg/I! in the runoff are
quadruple the average runoff concentration inother parts of the basin. Through Bayous Verretand Segnette, 935,000 kg N and 134,000 kg Pare imported to the rniddle basin annually in runofffram the uplands surrounding the middle basin.Additionally, a substantial loading of nutrients isimported to the middle basin through Bayou desAllemands from the upper basin. Butler �975!calculated this annual export from the upperBarataria Basin to be 8,016,000 kg arganic carbon C!, 1,047,000 kg N, and 154,000 kg P. Added tothis huge unprocessed export from the upperbasin, middle basin upland runoff raises the totalloading on the middle basin system to almost 2million kg N and 228,000 kg P annually, or nearlytwice the loading on the upper basin.
Import of C, N and P from the upper basinto Lake Salvador peaks in Febnjary with the surgeof wet season runoff Figure 19!. Phosphorusloading to Lake Salvador frown the upper basintotals 176,000 kg or 0.97 g/W/yr, nearly 100'/o ofwhich is retained in the lake Craig and Day 1977!.Despite such high nutrient loads, the lake is noteutrophic because the sediments and denitri-fication act as effective sinks for P and N.
The net daytime productivity �02 g02/m2/yr GPP! and chlorophyll a concentration �-12 mg/m3! of clear Lake Salvador are low relative toother middle basin waters, and with a communityrespiration uptake of 602 g 02/m2/yr this lakedisplays a moderate degree of heterotrophy -198g 02/m /yr net community productivity!. Heter-otrophy is a condition whereby a greater amount oforganic material and oxygen! is consumed than isproduced by a system. This indicates that thesystem is not autosufficient; that is, it requires theimport of organic material from outside the system.Lake Cataouatche, although contiguous with LakeSalvador, differs greatly from Salvador in nutrientcharacter. Net daytime productivity 876 g02/m /yr! is double that of Salvador, and theresulting organic material when decomposed bybactena fuels a strong heterotrophic demand foroxygen. Net respiration in Lake Cataouatche is-1,205 g 02/m2/yr, resulting in a net communityoxygen consumption of -350 g 02/m2/yr,approaching the heterotrophy of Lac desAllemands -450 g 02/m2/yr! and far exceedingthat of Lake Salvador and of Little Lake -117 gO~/m2/yr NCP; Wopkinson and Day 1979!.
24
285
200180160140120100806040
1600140012001000800
600400200
0
TOO 13664 mt/yr!
12001000800
600400200
020
0
MoNTH
Figure 19. Monthly material export in metric tons! from the upperBarataria Basin to the lower basin as measured in Bayou des Alle-rnands A!, Bayou Chevreuil B!, and Bayou Boeuf C! Butler 1975!.
25
C 0 E EX 0 Cl
1400O 1200
1000800
630 600
40020O
o o
0 I K2III
The P loading rates for Lake Cataouatcheand Lake Salvador are similar �.6 and 0.97g/mar!, but surprisingly Salvador displays none ofthe eutrophy plaguing Lake Cataouatche. Severalfactors in addition to the 40'/o lower P loading rateaccount for the high water quality in Lake Salvador:
1, Lake Salvador, with more than four times thevolume of Cataouatche, effectively dilutes thenutrient load.
2. Circulation patterns in Lake Salvador tend tohold nutrient inputs along the western shore,
20 0 m X 0 IlI
01601401201008060 Z4020 00
3160140 o120100806040
leaving the majority of the lake unimpacted.3. Bayou des Allemands began carrying large
nutrient surpluses into Lake Salvador onlyrecently and the sediments in Lake Salvadorstill possess a great capacity to store nutrients.In contrast, Lake Cataouatche has drained anurban center for over three hundred years.Continuous inputs during that period haveexhausted the sediment buffer.
4. Whereas Lake Cataouatche's reducedsediments release phosphorus to the overlyingwater column, Lake Salvador is oxygenated
throughout the water column and into thesediments trapping phosphorus in the form offerric-phosphates and adsorbed phosphatesbelow the oxidized zone Craig and Day 1977!.
25:1
20:1
Like Lake Salvador, I ake Cataouatcheretains 95% of its P input, but in Lake Cataouatchesediment sink processes are not sufficient toprevent continuous algal blooms. The dissolvednitrate level �.58 mg/I! in Cataouatche is threetimes Lake Salvador's and particulate organicnitrate PON! is six times that of Lake Salvador.The mean chlorophyll g value in Cataouatche of 55mg/m3 is five times the concentration in LakeSalvador. The average basin-wide chlorophyll gconcentration is 11 mg/m~.
Bayou Perot and Bayou Rigolettes areparallel, elongated lakes draining Lakes Salvacforand Cataouatche, respectively. Although thewater bodies they drain are of very different quality,these bayous are similarly high in water quality andlow in nutrient and chlorophyll concentrations.This is of interest considering the very eutrophiccharacter of the Lake Cataouatche source water forBayou Rigollettes. The 95% P retention capacity ofCataouatche effectively reduces the nutrientlevels in its outflow, so at this point in the basin,nutrients are reduced to near-natural levelsthrough biological and chemical processes andfrom this point on, the basin system shifts from aheterotrophic to an autotrophic state.
Nutrient levels in Bayou Chevreuil in theupper basin exhibit rapid increases in inorganicnutrients during the days following storms Figure20; Kemp 1978!. This suggests that accumulatednutrients are mobilized quickly by storms andconcentrated in the bayous. These precipitationevents play an important part in the chemistry ofthe middle basin; seasonal variations in thenutrient regime are linked to precipitation patterns.In summer, gulf backpressure and highevapotranspiration relative to precipitationcombine to stall hydrologic flow, also shuttingdown the export of nutrients from the upper to therniddle basin Butler 1975!. In autumn, whenprecipitation increases, accumulated nutrients areflushed into the estuary. High concentrations oftotal P and dissolved organic N are pulsed throughthe system from the flushing of leachates anddetritus.
15:1
3.88
- 1.922
WEIGHTED 5-DAY PRECIPITATIONAVERAGE cm!
Figure 20, Relationship of precipitation to N:Pratios in Bayou Chevreuil A! and swamp flood-water B! reprinted, with permission, from Kempand Day 'f984, copyright University Presses ofFlorida!.
Hopkinson et al. �978! described yetanother mechanism of nutrient enrichmentimportant in the middle basin. High rates ofbenthic bacterial respiration have been measuredin lake sediments, largely driven by the largeorganic input from upstream. Bacteria decomposeorganic material and release N and P in theirmineral ammonium and phosphate forms to theenvironment. Theoretically, this source ofliberated "rernineralized" P could add another 27
g/m2/yr inorganic P to the water column annually.
3.4 LOWER BASINNUTRIENT DYNAMICS
Most of the water reaching the lower basinat Barataria and Caminada Bays is imported from
0
10:1OX0
5:1
20
U10:1
2
0 0.5 1.0 1.5 2.0
4.0
3.5 NO +NO !-N3 2
~ Sellnlty1.2 3.0
B2.5
0I
2.0CO0
1.53
1.0
1.00.8
0.8EZ
0,7
Zz o.6
CI 0.5Z
0.4Z
0.3CV
Z0 0.2
+ 0.1
Z0 00
O.a
�
Z2 08
0.8
0.1 P
0,6X
0.5 0Z
0,43
0.3
0.06
0.0 5
B O.O4~ 0.03
P 0.02 0,2 0.50.01 0.1
0 00 5 I 10 15 20 255 . 10 15 20 25
CAMINADA CAMINADA NEARSNPRE ZPNEBAY PASS
CAMINADA CAMINADA NEARSHPRE ZDNEBAY PASS
DISTANCE FROM BAY TO RtVER MOUTH ml!
Figure 21. Nutrient profile for Caminada Bay and nearshore Gulf of Mexico during spring flood, 1973 reprinted, with permission, from Ho and Barrett 1975!.
27
Little Lake and is of good quality with Iowchlorophyll p of about 9 rng/m3, total P of 0,18rrg/I, nitrate of 0,39 mg/I, and ammonium of 0.12mg/I. The artificially dredged Barataria Waterwayserves as a conduit, discharging enriched upperbasin water directly into Barataria Bay. However, inthe down-basin direction, freshwater runoffbecomes less influential to basin chemistry. TheGulf of Mexico, laden with Mississippi River water,becomes increasingly important toward the lowerbasin and the proportion of Mississippi and Gulf ofMexico water mixing and entering the mouth of theBarataria estuary is controlled by wind, current,tide, sea level, discharge, and precipitation. Whilethe river plume can flow directly into the basin forsustained intervals during spring flood, duringperiods of low flow, the river does not impact thelower estuary at all. The seasonal importance ofthe Mississippi River to lower basin nutrientchemistry is evident in the high correlationbetween river discharge and nutrient levels in thelower Barataria estuary,
In 1973, the Mississippi River floodpeaked at its greatest discharge in several years.Ho and Barrett �977! took this opportunity tomake detailed measurements of nutrients along
the Mississippi River plume westward past theBarataria estuary throughout the year. Theirobjective was to determine the influence of theriver on nearshore gulf and estuarine waters undervarious discharge conditions.
Salinity transects in January 1973 from theupper estuary in Caminada Bay to the gulf crosseda number of of distinct water masses. The upperregion of Caminada Bay draining freshwatermarshes was characterized by low salinities. AtCaminada Pass, salinities increased in a gulf watermass trapped by the river plume, but againdropped farther offshore where stations lay in theturbid river plume itself Figure 21!. Silicate,phosphate, ammonium, and nitrate levels had astrong negative correlation with salinity, risingsharply in the riverine water mass, indicating theimportance of the river as a source of inorganicnutrients to the lower basin during high-flowperiods. In contrast, marsh-drained upper estuarywaters were an order of magnitude higher inorganic N than gulf or riverine levels.
When river flow is adequate and currentsare favorable for directing the plurne into the bay,the increased input of nutrients stimulate
productivity in lower Barataria Basin. During themonths of spring flood, Barataria and CaminadaBays often experience an inverted spatial nutrientpattern opposite that of the upper and middlebasin pattern of decreasing concentrationsdownbay. Instead of decreasing at these times,downstream concentrations of inorganic nutrientsin the lower basin increase toward the gulfbecause of the effect of the river. During theseperiods of high river flow, organic N levels alsoincrease toward the gulf due to increasedincorporation into organic rnatter by phytoplanktonproduction. During periods of low river flow,concentrations of inorganic nutrients andchlorophyll g are low and further decrease into thegulf.
The marshes in the lower saline portion ofthe Barataria estuary are highly productive. Annualmarsh net production of 1,176 g organic matter/m2measured by Kirby and Gosselink �976! isequivalent to 590 g C/m2/yr. The salt marshes playan active role in the nutrient cycling of the basin. Ingeneral, salt marshes are perceived to be a majorsource of carbon to estuaries and the nearshorezone, and many investigators Day et al. 1973;Happ et al, 1977; Craig et al. 1979!, in estimatinghigh rates of export of C from Barataria Basin, havelinked the productivity of the vast Louisiana fisheryto the high primary productivity of Louisiana saltmarshes. Carbon, in its particulate organic form POC! and its dissolved form DOC! is produced insalt marshes as a result of the breakdown of deadorganic material and leaching. Day et al. �973!created a budget to estimate the total communityproductivity and carbon flow through the estuary.Total marsh aboveground net primary~roductionwas estimated to equal 759 g C/m~ annually.Consumption of the vascular plant and algalproduction by primary consumers was calculated toapproximate 50% of the NPP, leaving 382 gC/m2/yr available as detritus for sedimentation inthe lower basin, as food, or for export from theestuary. Combined with phytoplankton andbenthic algal production, the input of marshdetritus to the waters of the lower basin wascalculated to be 750 g C/m2/yr for water surface i.e. not including marsh! of which 432 g C/rn2/yrwas consumed in the water. The fate of the
remaining 318 g C/re/yr input to the estuary is thesubject of some debate, specifically concerningwhether the carbon is exported or sedimentedwithin the basin, Converted to mass per unit of theentire estuary surface in the lower Barataria Basin
including marsh surface!, the surplus available is178 g C/m2/yr,
Day et al. �973! expected that the entireamount of the surplus was exported from theestuary to the nearshore zone and very little wassedimented. Although no flux measurementswere available on which to base this estimate, theyfind support in the fact that there is no net gain ofsediment level in the basin and not more than 0.81cm of sediment can accumulate each year basedon the rate of subsidence. Mineral sediments fromresuspension in the basin and Mississippi Riverinputs contribute to sedimentation, and theorganic content of the basin sediment is onlyabout 13%. Day et al. �973! calculated that 1,053g/m2/yr organic rnatter or 527 g C/m2/yr wouldaccount for this input, most of which would beavailable from root and belowground production.Although there are no root productionmeasurements for these rnarshes, their argumentthat most of the organic material withheld fromexport would come from biomass alreadyemplaced in the ground is reasonable. One mayexpect that belowground production is greaterthan aboveground production, Furthermore,since much of the lower basin is not keeping upwith subsidence, somewhat less than the 527 gC/rn0yr is required to account for annual peataccumulation,
Happ et al. �977! measured concen-trations and fluxes of DOC, TOC, and chlorophyll gin the lower basin as a means of determining thefate of C and found evidence for the export ofcarbon from the Barataria estuary. A gradient ofTOC from a high of 8.5 rng/I in waters of fringingmarshes to 5.9 mg/I in the lower bay to 2.8 mg/I inthe nearshore gulf indicated that marshes are asource of carbon and that there is movement intothe gulf. Based on calculations of mixing rates inthe estuary by Kjerfve �972! and a flushing timeof 23 days Austin 1955!, Happ et al. concludedthat outweiting exports 140-190 g C/m2/yr from theestuary, which s near the value calculated by Dayet al. �973!.
DeLaune et al. �978!, Hatton �981!, andDeLaune and Smith �984! measured verticalaccretion rates in Barataria Basin rnarshes using
Cs profiles and determined that streamside andinland salt marshes are accreting at the rate of 1.35cm/yr and 0,75 crn/yr, respectively, from theaccumulation of organic detritus and tidally
imported clays and silts. The organic contentmeasured in these soils is about 20/o,representing an accumulation of about 240-390 gof organic C/m2/yl' of marsh surface in the form ofpeat Smith et al. 1983!, indicatirg that 50/~75'/oof annual marsh production must remain on themarsh, Another large percentage is lost as CO2and CH4 gas, leaving much less net carbon to beexported than other studies would indicate Feijtel1985!. Smith et al. �983! calculated that anequivalent of approximately 400 g C/m2/yr isevolved from the marsh surface as CO2 and CH4.While some CO2 is recycled back into the marshthrough uptake by photosynthesis, most of it islost to the atmosphere and combined with thecarbon accumulated as peat; very little of theproduction remains to be exported. However, thecontribution of belowground production by saltmarsh vegetation has yet to be determined, andthis fraction may supply much of the accumulatedpeat, as suggested by Day et al. �973!. Theamount of belowground macro-organic matter MOM! is several times the organic material inaboveground production. Buresh et al. �980!measured a ratio of MOM fractions to abovegroundmatter of 5.7 in Barataria salt marshes; however,little is known of turnover rates of belowgroundproduction.
Feijtel et al. �983! synthesized existingcarbon flux data and estimated carbon flow along asalinity gradient in Barataria Basin. Using a massbalance approach, they found an estuarine carbonsurplus of 130-230 g/m2/yr which originatedprimarily in the tidal salt marsh. Carbon export frommarshes to adjacent water bodies decreased withdistance upbasin from the Gulf of Mexico.
roots are found above 30 cm depth and as themarsh accretes, the N and P below this depthbecome unavailable as a source of nutrients to the
plants and are lost from the system.
Enrichment experiments Patrick andDeLaune 1976; Buresh et al. 1980; DeLaune et al.1983! have shown N to be limiting to the growth ofthe Q~ salt marshes. Despite the largereservoir of organic N in marsh soils, the short termsupply of N to plants is governed by the rate ofmineralization of organic N to the ammonium form.In these marshes this occurs at a rate of 25 gN/m /yr and represents 60/o of total inputs DeLaune et al. 1979!. Increases of up to 25'/o-30'/o in plant biomass and height have beenstimulated by the application of supplementalammonuim N to marsh sites. While supplemental Pwas rapidly taken up by marsh plants, there was noincrease in growth response, although luxuryconsumption increased tissue P concentrations by20'/o
Organic enriched mineral sedimentsprovide the greatest source of N to the lower basinmarshes, but a significant part of the N input iscontributed by the fixation of atmospheric N byheterotrophic bacteria associated with the marshplants Casselman et al. 1981!. Partitioningexperiments by Casselman et al. �981! indicatethat negligible fixation occurs in open water or onthe plants themselves only up to 0,2 g N/m2/yr!while marsh soil adjacent to plant roots is the site ofintense fixation: 4.5 g N/Wlyr in inland marshesand 15.4 g N/m2/yr in streamside marsh. In thecase of streamside marsh this represents almost40'/o of the total N budget of the Q~g plants.
In contrast to upper basin marshes, whichreceive almost no mineral sediment, the influx ofMississippi River water through the lower basinmouth provides mineral sediments for saltmarshes, augmenting the nutrient supply andenhancing sedimentation, The sediments makingup the wetland soils of the basin are largelyunweathered recent Mississippi River alluvium.For this reason the marshes usually containadequate supplies of most of the major plantnutrients such as phosphorus, potassium, calcium,and magnesium as well as essential micronutrients.Barataria Basin salt marshes have been shown toserve as great nutrient sinks DeLaune et al.1981!, accumulating N and P at rates of 21 g/m2and 1,79 g/re annually. The bulk of active plant
29
Tidal ftuxes provide the important linkbetween lower basin marshes and the open watersof the lower Barataria Basin, Caminada Bay, andBarataria Bay. Denitrification rates are low in themarsh approximately 3-4 g N/m2/yr; DeLaune et al.1983! and thus by far the major N loss from themarshes and consequently the important inputsto open waters! is due to tidal export of detritus. Itis estimated that approximately 40/o of the netaboveground Q~ fjgg N production �0-12 gN/m2/yr! is iost in this fashion, Because P is nottransformed into a gaseous phase in its nutrientcycle, all P export from the salt marsh �.0 gP/m2/yr or about 36%%d of production! is via tidalexport as well. Marsh vegetation is thus pumpingN, P, and C from sediment and atmospheric
reservoirs into the lower estuary in the form ofleachates and detritus. However, nutrient inputsfrom the accretion of mineral sediments, nitrogenfixation, rainfall, and the import of organic materialfrom the upper Barataria Basin far exceed theexport and loss of nutrients in the lower basin,making the Barataria salt marsh a net importer of Nand P,
3.5 SUMMARY
The Barataria Basin is a dynamic chemicallink between land and sea in coastal Louisiana. Inrecent years, its hydrology and chemistry havebeen altered by the natural abandonment of theriver channel and by human activity, and what oncewas a flow-through estuary dominated by riverineprocesses is now a swamp-lake system whosechemistry is largely controlled by precipitation andtides. Levees have isolated the basin. Canaldredging and increased pollution are creating asteep biological and chemical gradient along thebasin characterized by eutrophy and heterotrophyin the upper end and autotrophy in the saline end.Upper basin photosynthesis:respiration ratios P:R! of 0.71-0.76 Day et al, 1977! contrastsharply with lower basin P:R ratios of approximately1 Allen 1975!.
The upper basin may be experiencingproblems because natural controls of nutrientfluxes and primary production have been
compromised. Productivity and nutrient levels inthe upper basin are tied to the seasonal pattern ofprecipitation, upland nutrient runoff, and fertilizerapplication Stow et al. 1985!, Productivity in themiddle basin follows a more erratic, aseasonalpattern, and is dependent on water clarity anddepth Day et al. 1982!. Winds, river discharge,and tides control nutrient flux and primaryproductivity in the lower basin.
Sealing of the basin borders with leveeshas robbed the upper basin of its sedimentsource, essential to maintaining the wetlandagainst continuous loss to subsidence, In thelower basin, organic accumulation is supple-mented by Mississippi River sediment, depositedby tidal inundation. The salt marshes experiencean average of 160 major inundations per year,while fresh marshes experience only 20, With alack of new mineral sediment input, rnarshesthroughout the basin are deteriorating rapidly.
New nutrient inputs each year total onlyone-fifth to one-half the nutrients regeneratedwithin the basin. The effect of these new inputs isto stimulate higher productivity and alos higherrecycling rates as more organic material is suppliedto benthic remineralizers. High nutrient loads havecreated an immensely productive heterotrophicupper basin, but the natural buffering capacity inthe middle basin has thus far protecteddownstream waters from eutrophy.
CHAPTER 4
VEGETATION' COMPOSITION AND PRODUCTION
by
W. H, Conner, J.W. Day, Jr., J. G. Gosselink,
C.S. I-topkinson, Jr., and W. C. Stowe
4.1 INTRODUCTION
The distribution and composition ofwetland plant communities within the BaratariaBasin has been described by a number ofresearchers Penfound and Hathaway 1938;O' Neil 1949; Chabreck 1970, 1972; Chabreck andLinscombe 1978; and Wicker 1980!. Primaryproductivity has been measured in the swamps Conner and Day 1976; Conner et al. 1981!, in themarshes Kirby and Gosselink 1976; Hopkinson etal. 1978, 1980; Sasser and Gosselink 1984!, in thebayous and other water bodies Allen 1975; Butter1975; Day et al. 1977; McNamara 1978;Hopkinson and Day 1979!, and in the Gulf ofMexico Sklar 1976!. In this chapter the majorwetland habitats of the Barataria Basin aredescribed, and the factors influencing speciesdistribution and productivity are discussed.
4.2 AQUATIC PRIMARY PRODUCTION
There is a distinct difference between thelower saline part of the basin and the upper freshwater zone in terms of aquatic productivity andcommunity metabolism. Waterbodies in the upperbasin have high levels of primary productivity, apronounced summer pulse, and stronglyheterotrophic characteristics, The lower basinaquatic community is less productive and lacks aconsistent seasonal trend, and respirationgenerally balances aquatic primary production.Waterbodies in the upper basin have high nutrientlevels leading to high primary productivity. In thelower basin, where light sometimes reaches thebottom, there can be significant benthic primaryproduction. Nutrient levels, water clarity, and water
depth are the parameters which most affectproduction. Production studies have beenconducted by Day et al. �973,1977!, Butler�975!, Allen �975!, Sklar �976!, McNamara�978!, and Hopkinson and Day �979!.
The uppermost lake in the basin, Lac desAllemands, is continuously fresh. It is surroundedby swamp and fresh marsh and receives drainagefrom a number of bayous and canals which receiverunoff from wetlands and uplands, principallyagricultural fields. Gross production in the watercolumn of Lac des Allemands is very high �,286 g02/m2/yr!, because of nutrient-enriched runoff Day et al. 1977!, There is no seasonal pattern ofprlrnary production in the bayous. In Day et al.'sstudy, Lac des Allemands was productive all year,but production was considerably higher from Aprilthrough September because of dense blue-greenalgal blooms. The bayous have a shallower watercolumn, less production �62 g 02/m2/y! due toshading by overhanging trees and floatingvegetation, and high turbidity caused byagricultural runoff. There is no measurablephytoplankton production in water overlying theswamp surface McNamara 1978!.
Both the lake and the bayous areheterotrophic Figure 22!. This reflects the highproportion of wetlands and uplands in the upperbasin and the export of organic matter to thewaterbodies. The highly eutrophic nature of Lacdes Allemands can also be demonstrated by thephytoplankton communtiy. Mean annual phyto-plankton density was 52,800 algal units/ml with a
g 02m> day-12m 2 day-1
902 d y19 02m day
10
a
8'Q
6
C4 50 4U!
3
J F IN A M J J A S 0 N D
MONTH
Figure 22. Aquatic productivity of Lac des Allemands and Bayou Chevreuil. NDPis net daytime productivity and NR is nighttime respiration reprinted, with permission,from Day et al. 1977, copyright Academic Press!.
4.2.2 ~tidal
32
range of 18,900 to 93,900 algal unitslml Day et al.1977!. Seventy percent of the phytopianktoncommunity was composed of blue-green algae.Filamentous ~A'~ma sp�h. affffi',, h. QOS=
r~ dominated during the late summer whilethe colonial g~hr g~~ Ltttg,~<;, Q. Jjiigg~e ' ' e.~nn~iim~ dominated during the spring.Unicellular and small colonial forms of the greenalgae were abundant year-round but reached theirmaximum in the late summer. The dominantorganisms in this peak were ~P~irgm
ki e.~rrteor t m emr~ri ~hm. The dietome, the othermajor component of the Lac des Allernandsphytoplankton community, exhibited a wintermaximum with a large number of small pennateforms. Generally the pennate to centric ratio was4:1, The most common diatom taxa were ~N' ~hisP., hL Paha, H. tl:XbhZZala, 5. dimiaaia. DdOiallag~z'n jiin�and Q. ~~~.
In the central part of the basin are threelakes that show a transition from fresh to saline
conditions. Lake Cataouatche is a slightly brackishlake �-2 ppt! with a maximum depth of about 2 m.It is bordered by fresh and intermediate marshes,predominantly bulltongue Qg~rja ~! andcattail ~ spp.!. It receives urban runoffdirectly from the New Orleans metropolitan area.Lake Salvador is also slightly brackish �-5 ppt! witha maximum depth of 3 m. It is bordered by cypressswamp and bulltongue marsh. It does not directlyreceive upland runoff, but receives drainagewaters from the upper basin via Bayou desAllemands and from I ake Cataouatche as well asfrom surrounding wetlands. Little Lake is a tidally-influenced brackish water lake �-10 ppt! with amaximum depth of less than 2 m, The surroundingmarsh is primarily @~in ~n. Little Lakereceives runoff from surrounding wetlands as wellas from the upper basin. In the past, Little Lakewas the only waterbody connecting the upper andlower basins. However, the construction of theBarataria Bay Waterway Figure 3! has caused ashift in the water flow in the basin. Much waterwhich would normally flow through Little Lake nowbypasses it and flows through the BaratariaWaterway Hopkinson and Day 1979!. Thus it isless affected by upper basin and upland runoffnow than in the past.
Productivity patterns in the three lakesreflect the degree to which uplancl runoff affectsthe lakes Figure 23!. Gross production washighest in Lake Cataouatche �,222 g 02/m2/y!, aresult of high nutrient loading from upland runoff Hopkinson and Day 1979!. Mean annual chloro-phyll in the lake was about 50 mg/m3. Grossproduction in the other two lakes was significantly
PRODUCTIONeke Cataou etc heake Salvadorittle Lake
14
12
10CII'E S
0 6
4
AL MEAN
liDJ FMAM JJASONDJF
CTION
5
4ev'E
0 2
1
DJ FMAM J JASON DJF
ATION
5sra
EtO 2cls
10
DJ FMAM JJASOtrlDJFMONTHS
Figure 23. Community gross production, netdaytime photosynthesis, and nighttime respirationin three Barataria lakes reprinted, with permission,from Hopkinson and Day 1979, copyright PlenumPress!
organisms in Lake Salvador were Chlam domonassp., Qh~r~ ~i~r~,~men hiniana, ~Di ioneus atlittLtrt, G mnodini mf~m, ~hg~mn~ sp., P~r'~II~ ~in ~m,
' h.and coccoid blue-greens. Dominant within LittleLake were Qh~rgg~ ~i~r~, Qht~m-
*, rlower; however, Little Lake was higher than LakeSalvador �307 g 02 as compared to 1058!. Meanannual chlorophyll in Lake Salvador and Little Lakewas 12 and 10 mg/m3, respectively. Nutrientlevels in the two lakes were about the same. The
higher production in Little Lake seems to becaused by the production by benthic algae. Theshallow depth and clear water allow light topenetrate to the bottom at times.
The Lake Cataouatche phytoplanktoncommunity reflects eutrophic conditions but atlower levels than Lac des Allemands, Mean annualphytoplankton density for Lake Cataouatche was25,100 algal units/ml with a range of 16,560 to31,200 algal units/ml, which is much lower than Lacdes Allemands. The dominant blue-greens werethe colonial and single coccoid forms. The coccoidforms which were never satisfactorily identified!were common in all samples. Numbers of coloniespeaked in early November and May, The most
sp and Qllg~m sp Unicellular and smallcolonial greens were second in prominence with afall maximum. The most common taxa were
uru sr r" ua" ' rr ~hI ~rll ~vitri . The diatoms, while conspic-uous, were less significant producers than in Lacdes Allemands, The dominant diatom taxa
.u - 'I',urN. iinearis, and ~urirett ~robust were most oom-mon during the winter.
Phytoplankton densities in Little Lake andLake Salvador are lower than Lake Cataouatche
with mean annual population densities of 6,500and 9,200 algal units/ml respectively. Densityranged from 1,500 to 12,300 algal units/ml forLittle Lake and from 5,400 to 13,300 algal units/mlfor Lake Salvador. In both lakes, diatoms, greenalgae, and flagellates were more important thanblue-green algae. Blue-green algae dominatedthe winter months, while diatoms were dominantduring the spring and fall seasons. The dominant
33
Table 5. Comparative aquatic productivity g 02/m2/yr! and mean chlorophyll g concentrations mg/m3! in the Barataria Basin from freshwater bayous to the offshore zone from Day et al. 1982!.NDP = net daytime photosynthesis, NR = nighttime respiration, GP = gross production, and NCP= net community production
NDP NR GP NCP Chla Reference
BayousLac des AllemandsLake CataouatcheLake SalvadorLittle Lake
Brackish-sa!ine
Day et al. 1977Day et al. 1977Hopkinson and Day 1979Hopkinson and Day 1979Hopkinson and Day 1979Allen 1975
Day et al. 1973Happ et al 1977;Sklar and Turner 1981
316 446 � -1301,41 8 1,868 3,286 -450
876 1,205 2,222 -350402 602 1,058 -198639 753 1,307 -117940 910 1,850 0 to
+54732
25
65
5012
10'IO
Offshore
34
sp., C~lirgtroth~o tjjsiformi , gy"knell sp.,Qt~mn~in~i sp., Q~~~ sp., andPerittiniomtteenta on m.
Hopkinson and Day �979! found that inLittle Lake monthly change in secchi depths i,e,,water clarity! was strongly correlated to themagnitude of production, When water trans-parency increased February and April, forinstance!, significantly higher production tookplace Figure 23!. Lake Salvador, by comparison,has similar water transparency but is deeper andlight never reaches the bottom, AII three lakeswere heterotrophic, ranging from -350 g 02/m2/yfor Cataouatche to -117 for Little Lake Table 5!,We believe that in the absence of direct uplandrunoff, waterbodies would have two patterns ofproductivity. Shallow lakes throughout the basinwould have seasonal patterns like Little Lake andAirplane Lake. Deeper lakes would be similar toLake Salvador.
Seasonal patterns in the three lakes alsoreflect the impact of nutrient loading, water clarity,and depth. The upper basin is characterized byclear seasonal patterns while the lower basin isoscillatory. Lake Cataouatche with high nutrientsand turbidity is similar to Lac des Allemands, Thereis high productivity from May through September.Lake Salvador also shows a distinct seasonalpattern, but summer production levels areconsiderably less than in Lake Cataouatche.There is no consistent seasonal pattern in LittleLake, Here water clarity seems to the major factor
determining production patterns. LakeCataouatche probably was similar to Little Lakebefore the introduction of upland runoff, both interms of total production and seasonal patterns.
Aquatic production in the saline waters ofthe lower Barataria Basin Day et al. 1973, Table 5!was somewhat higher than in the nonenrichedwaters of Little Lake. With the exception ofperiodic summer blooms, there was not a strikingproductivity difference between summer andwinter Figure 23!, Allen �975! measured aquaticproductivity at four sites in saline and brackishwaters in the Terrebonne Basin west of theBarataria Basin!. These waters received very littleupland runoff, Both the levels of production andthe seasonal patterns were similar to those foundin Little Lake and the lower Barataria Basin. Theresults of these two studies are plotted with thoseof Nixon and Oviatt �973! to emphasize thedifferences between the subtropical coast ofLouisiana and a north temperate area in NewEngland Figure 24!. Productivity levels aregenerally similar from April through September, butproduction is much lower in New England duringthe rest of the year,
In a study of Airplane I ake, Day et al.�973! separated water column and benthicproduction. Production by benthic diatoms andalgae was about 20'1o higher than byphytoplankton. Thus the results from both LittleLake and Airplane Lake indicated that bottomproduction is significant, In both of these areas,
4.2.313
12
10
0
E4
35
J F M A M J J A S 0 N D
MONTHS
Figure 24. Seasonal gross production curves forAirplane I ake Barataria Basin!, Terrebonne Basin,and New England salt marshes Allen 1975!.
the shallowness of the water was critical in allowinglight penetration to the bottom. This happenedoften enough so that there was significantproduction. In Airplane Lake the dominant benthicforms were diatoms Day et al. 1973!.
Airplane Lake phytoplankters are domi-nated by diatoms and dinoflagellates. Thesephytoplankters have a mean annual density of16,500 algal units/ml with a range of 7,740 to28,100 algal units/ml. Dominance is very seasonal;the diatoms predominate during the winter whilethe dinoflagellates reach their maximum during thesummer. The dominant phytoplankters are
~l, ~A~rn ~ll ggg~ni, ~Bi elhi ~rir" ' I . ~ii: xcentralis, C, radiatus, Ceratium ~hircu,Qym~ningirrt ~rvi Qgn~i~lx sp., ~rr~n~rm~mican, P. maximum,and P.~curn r gaum.
These results also indicate that the factorscontrolling productivity change from the upper tothe lower basin. In the upper basin, nutrientloading from upland runoff clearly controls bothseasonal pattern and the magnitude of production.In the lower basin a combination of water clarity anddepth is important.
The nearshore area off of Barataria Bay isstrongly influenced by the discharge of theMississippi River. This fresh water, being lessdense than the salty gulf waters, floats on thesurface and moves in variable directionsdepending on winds, tidal currents, and oceaniccurrents. Sometimes the plume forms a giant gyrethat sweeps in a clockwise direction and directlyimpinges on the Barataria area. Surface salinities atthe coast then drop to low brackish levels.
Light levels, salinity, nutrient concen-trations, and productivity are directly related to theinflux of Mississippi River water Sklar 1976!.Maximum surface productivity occurred in Aprilduring maximum river discharge, whereas theminimum occurred in September when riverdischarge was low Figure 25!. Surface measure-ments of annual net productivity were generallyhigher in turbid coastal waters brown water! offBarataria Bay than in relatively clear gulf waters green water! further offshore. Total annual pro-duction of 266 g C/m2 was estimated for thenearshore area off of Barataria Bay. Happ et al.�977! measured a mean chlorophyll g concen-tration of 7.6 mg/rn3 in these offshore waters. Theimpact of estuarine outwelling on the nearshorephytoplankton is unknown. However, Sklar andTurner �981! found that during the winter monthswhen northerly winds decrease the water levels inthe marsh, there was evidence that the BaratartiaBay exported nutrients, and this increased theprimary production of the coastal waters Figure25,January!.
4.3 WETLANDS OF THEBARATARIA BASIN
Two major aspects of wetland compositionfrom swamp to saline zones are decreasingdiversity and increasing consistency of communitycomposition Table 6!. The total number ofspecies identified for each wetland type is:swamp, over 200; fresh marsh, 154; brackishmarsh, 23; and saline marsh, 25 Conner et al.1986!. Stands consistently dominated by,'3i~i~ina
rnifl~r are common in both the salt andbrackish marsh zones. In the fresh marsh,p~ni ~m Q~mi m~n covers over 41% of the totalarea; however, there are large expanses where it isa minor component or absent. In the swamp forest
aterm 3/yr
ater/m~/yr
200
MONTHS
1>000
800
600
400
20O
153 O
10
200
A S 0 N 0 J F M A M J1974 1975
MONTHS
Figure 25. Average primary productivity of offshore surface waters A! andaverage annual variation of Mississippi River discharge and rainfall B!. Ihe termsgreen water and brown water referto water masses differing in amounts of sus-pended material, Brown water contains large amounts of rnatter, probably frommarsh inputs and turbulent mixing Sklar 1976!.
~Tx~~i dL'Qi~ and ~ ~m dominatein various proportions.
Marsh vegetation zones have beendelineated by Chabreck �972! and Chabreck andLinscombe �978!. None of the zones has aunique flora and many species occur in more thanone zone. No species occurs in all marsh types,and only three species, ~ii~hli g~i, /@gain g~n, and ~l~hri spp., compose more than
!12aa
OD
1ooa0K
800
QnC
L o 600
CII4oo
Ill
LLI
1% coverage in three zones. Q. ~n and D~ic~t are dominant in the brackish zone, and Q~lr~ifl I~r and ~rI m Eyer'~n reach highestcover in the saline marsh.
The swamp community in the Barataria Baywatershed is strongly affected by water level anddrainage. Baldcypress and water tupelo are
Table 6, Plant species composition of marshes in the Barataria Basin iIafter Chabreck f 972!.
Specific Name/
Common Name
Percent
BrackishFresh Salt
~Pni pm~hmi ~nM aide ncane
Sa~iri f~lcatBulltongue
Alte~rn n~h~r ghif<~xr !~id
41,4
17.4
Alligator weed~Th sp.
Cattail
~E~h'n Q~l ~wlWalter's millet
P.i~h~rni ~~Water hyacinth
~pc i num ap.Smartweed
~Decod n~vWater willow
~in ~r~nDeer pea
g~ii >g~i ~mili'~tfaGiant cutgrass
Qg~LD~nni r'Water hyssop
~Guru ~od r~aCyperus
ale i~hri,> sp.Spike rush
~ gqiqyb~rCamphorweed
Jaamaaa mKt.MMorning glory
Qgg,~in ~Saltmeadow cordgrass
~Di tii~hli gjiiggSaltgrass
~El ~nn ~arvuiDwarf spikerush
~i~~inOlney's three-corner grass
g~rni~fl rSaltmarsh cordgrass
~n~~~ri nBlack rush
~Bti ~mri~imSaltwort
vandalic >~rni virnrrini aG lasswort
Othe rsa
3.4
2.2
2.0
1.6
1.2
1.4
1.8 12.0
3,2 2.7
12.3 1.8
8.4
0.7
7.8
1 6.1 10.1
2,8
1.7
62.84,5
1.7 14.9
3.1
'l,2
0,213.7 2.6
alncludes plants making up less than 1% of the species composition.
37
4,3,2 Fresh Marah
38
characteristic of poorly drained and frequentlyflooded areas while bottomland hardwoods are
found on slightly higher, better drained areas.Brown �972! stated that a 15-cm difference inwetland elevation is more significant in changingplant communities in Louisiana than 30 crn inmountains. In the Barataria Basin, swamp landsmake up 16'/. of the total area Table 1!.
Of the habitats in the basin, plant diversityis greatest in the swamp forest. Over 200 speciesof plants have been noted in this area Conner etal. 1975!. Baldcypress and water tupelo are thedominant trees in the Barataria swamp. Drummondred maple Acer rubrum var. ~mm~~nii!, ash ~Fr <~in,> sp.!, and a number of woody shrubssuch as Virginia willow tttaa ~vtr inicat andbuttonbush ~hl nh ~gji;I~nil ! are alsorelatively dominant. In the slightly drier areas,species like cottonwood ~PI ~g ~he er t~h11a,black willow ~i gjiir!, hackberry ~liiagviggtaJ, locust ~Gtaditst sp.!, oak tgue~rcuspp,!, and hickories ~ sp.! are found.
Cypress lumbering thrived in Louisianabetween 1880 and 1925, Unfortunately there areno accurate records to verify how much was cut{Norgress 1936; Mancil 1972!. However, theimportance of the southeastern Louisiana cypressforests is reflected in the fact that the Louisiana
Cypress Company of Harvey and the I utcher andMoore Cypress Company of Lutcher both in ornear the basin! were the two largest cypress mills inthe world, The extensive network of loggingcanals that show up on old aerial photographs stillseen on many recent photographs! is also anindication that most if not all of the swamp forestswere logged. Only a few virgin trees remain.
The fresh marsh zone begins around Lacdes Allemands and extends south to the Gulf
Intracoastal Waterway Figure 3!, ln all there are155,030 ha of fresh marsh habitat includingwaterbodies! composing about 25/o of the basin.Waterbodies constitute a higher proportion of thisarea than in the swamp forest, but less so than inthe brackish and saline marshes Table 1!.
Water levels in freshwater wetlands are
controlled more by freshwater inflow, rainfall, andthe direction of prevailing winds than by tidal
effects. The total annual inundation time does not
vary much across different marsh habitats, but thefrequency of inundation a measure of marshflushing! is lowest in freshwater areas Figure 26!.As a result, much of the production of theemergent plants accumulates in place. This oftengives rise to floatant or floating marsh. Floatantmarsh consists of a dense mat of vegetationsupported by detritus several feet thick, which isheld together by a matrix of living roots andextends outward from the true shoreline. In
theory, as the bottom and floating layer accumulatemore material, they merge, forming a newshoreline. However, in the Lake Boeuf area this isnot happening Sasser and Gosselink 1984!.
One of the most obvious differencesbetween swamp forest wetlands and fresh marshwetlands is the increase in organic matter contentof the soils in the fresh marsh. Much of the detritusdeposited on the surface of the fresh marsh is notexported. This added to root production results ina buildup of peat, The organic content of freshmarsh soils is approximately 65/o, double that ofswamp soils.
Maidencane ~~ ~~ILL!!, orwpaille fine" as the French-speaking natives call it,is the dominant plant species in fresh marsh Table6!, and it is seldom found in other wetland habitats.Bulltongue and spikerush ~EI ahri spp.! arealso common. Fresh marsh is characterized bymore plant species and groups of associatedspecies than any other marsh type. The plantassociation common to this habitat is the
maidencane association which typically includeswater hyacinth gjiih ~rni ~r~i!, duckweed ~mn spp.!, water lettuce ~PI i g~ri ~,smartweed ~PI ~nrn ~num, bulltongue,bulrush ~r~ spp.!, and cattail as minorcomponents.
Brackish and salt marshes are dominated
by perennials which form stable communities thatchange relatively little from year to year, Incontrast, fresh marshes support a large number ofannual grasses which contribute to the increasedplant species diversity seen here. The seeds ofsome species germinate in the spring and othersin the fall. The dominant annual at a given locationoften changes from season to season or year toyear depending on competition and localenvironmental conditions.
48
e200200
40
18036
16032rce
28LLj
24
140
120
200
16g
12
100
80
60
40
20
0 0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8WATER INCREIIIIENTS II ABOVE MSL!
0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.6 2.0WATER INCREMENTS R ABOVE MSL!
Figure 26. Percent of the year one can expect the water level to be above a specified height A!and the number of times during the year one can expect the water to exceed a specified height B! Byrne et al. 1976!.
4.3.3
4.3.4 ~lt Marish
39
A broad band of brackish marsh exists inthe Barataria Basin. This marsh zone is the most
extensive and productive of all wetland types. Italso seems to be the most vulnerable to loss since
the brackish marsh is disappearing at a rate higherthan any other Craig et al. 1979!. This zonerepresents the first vegetative unit in the salinitygradient to be strongly influenced by tidal action.Salinity averages between 2 to 10 ppt, but can varyfrom fresh conditions to almost ocean levels.
Salinity iis strongly influenced by runoff of rainwater from the upper basin and by movement ofgulf waters up the basin during high tides or stormsurges. Storm surges periodically raise waterlevels and increase salinity. Sustained winds,however, are probably the most important factor inmarsh flooding; northerly winds tend to depresswater levels, while easterly and south-southeasterly winds tend to increase water levelsby forcing water up into the estuary against theslight surface slope. During periods of heavyrainfall these marshes are flushed with freshwater.
The most common plant in the brackishmarsh is saltmeadow cordgrass Spain ~n!.
Other major macrophytee are ~Disti hii a~icata,~nc~ r rmj~ri n, and ~ir ~ spp. Althoughfreshwater and brackish water bayous are similar inmany aspects, the latter differ because of thealternating current patterns. Because of thesalinity, floating aquatic plants like Lemna minorand ~Ei hh >~rni ~r~jiigZare not prominent.
Salt marshes have been extensivelystudied and are the best understood marsh type inthe Barataria Basin. Salt marshes are for the most
part higher energy habitats than other marshzones, Though water levels on the marsh aregenerally shallow, tidal inundation is frequent Figure 26!. Salinities vary seasonally andsometimes daily, depending on climatic factors.Salinities are highest during summer when surplusrainfall is low and gulf water levels are high.Conversely, during spring floods water from theMississippi River and from the northern part of thebasin causes the salinity to drop,
Qg~i.i ~lrn~ifl r is the dominantspecies of the salt marsh. The saltworts, Batis
A!
I-tcE
0.00
est
0 tD0 J F M A
7 � 7 971
MONTHS
OttccD
I-sc
E 99
K
0-10 10-20 20-30EDGE
HE I G HT 0 N ST E M cm!
LOCATION
4.4.1
40
~mjrrirgg and ~li ~rni ~mini Et, occur to asignificant extent only in the saline marsh. Aninteresting feature of /gawain ~lm~if rg stands isthe occurrence of distinct height forms along agradient frolrn streamside to inland. Along the tidalcreeks, QggrLi~n plants are approximately 1 rn tallwhile further inland they are <50 cm tall DeLauneet al. 1979!.
Even though emergent plants producethe bulk of the energy fixed in salt marshes,epiphytic and benthic algae are also abundant.While epiphytic production is dwarfed by that ofthe marsh rnacrophytes, they are highly significantfor the quality of their productivity Mason andBryant 1975!.
In the lower Barataria Bay $ggrLIn~~lrrrif~lr, the dominant emergent plant, servesas a host substrate for four genera of macroscopicalgal epiphytes; ~limni sp. and ~B<r~hisp. dominate during the summer while ~Ectoca us
sp, dominate during thewinter Figure 27A!, These epiphytes on Q.y!~trnifl|rrt are restricted to a horizontal band nowider than 70 cm from the shoreline and usuallyless than 10 crn wide Stowe 1972!, Themicroscopic algal community is dominated bydiatoms with occasional species of the blue-greengenus Qgiil~in being found. The diatoms occurin densities of about 105/cm2 culm surface areaand decrease in density with distance from theshoreline and with elevation on the culm Stowe1982; Figure 27B!, The dominant diatom taxon isDenticula ~subtili which exhibits an inverse relationwith height, increasing in relative abundance withelevation Stowe 1980!. Stowe �982! gave adetailed analysis of the diatom communityassociated with 5.;~lrn~ifl r .
4.4 WETLAND PRODUCTIVITY
Functionally, the swamp forest is similar tothe rnarshes Seasonal flooding provides optimumconditions for growth Conner and Day 1976;Conner et al. 1981!. Flooding, however, variesfrom area to area within the swamp. This isillustrated for three sites in the Barataria Basin Figure 28!. Water levels fluctuate dependingmainly upon rainfall, which causes the water levelto rise during and after rainfall and fall between
� ~I~I and ggjllsi~hott E9 9 Sstsmr res
M J J A S 0 N D J~1 � 7979 ~
Figure 27. The seasonal biomass cycles of themajor macroscopic algae in the salt marsh A! anddensity and occurrence of microscopic epiphytes B!. "Inland" in the lower graph represents all themarsh area over 70 cm from the lake edge Stowe1972!,
storms, In a controlled system like the crayfishfarm, however, water levels are kept at about 40 crnthrough the winter and spring. During the summerthe area is normally dry,
Stern productivities and litterfall have beenmeasured in the three areas for 3 years Table 7!.Even though the stem growth of individualbaldcypress and water tupelo in the permanentlyflooded area Figure 29! is greater than in the otherareas presumably the result of reducedcompetition and greater sunlight!, the fewernumber of trees in that area results in lower areal
productivity Conner et al. 1981!. Averageaboveground net production values of 1344, 906,and 1915 g dry wt/m2/yr have been measured inthe seasonally flooded, permanently flooded, andcrayfish farm, respectively.
A! CRAYFISH POND
504Q
O 30
2010
crt Q
LU J F MA M J J A SO N DZ
8! PERMANENTLY FLOODED
50
Q 4030
20o 10
0
U J F MA M J J A SO N D
! SEASONALLY F
I-
Table 7. Aboveground net primary production NPP!in three swamp sites.
NP P g dry wt/m2/yr!Species orcomponent
19771978 1978 1979 1980
- - Seasonally flooded--646.0 858.3 839.957.9 120.6 53,944.9 62.5 60.4
417.4 ~447. 4~17.1,166.2 1,489.2 1,372.0
mPr in
BaldcypressWater tupeloOthers
LitterfailTOTAL
- - Permanently flooded --209.9 212.8 256.2149,1 202.5 153.648.6 62.2 60.0
150.8 176,6 176.4
BaldcypressWater tupeloAsh
Others
C504030
2010
0
~Li ~r~filTOTAL
$28 7 ~71. ~27.1887.1 925.9 903.3
- -Crayfishfarm--
387.8 523.6 628.4
57.8 63.8 59.6453,1 468.1 360.8
332.4 390.4 380.1
mPr iJ F MA M J J A SO N DBaldcypressWater tupeloAsh
Others
Figure 28. Water level fluctuations in threeswamp areas reprinted, with permission, fromConner et al. 1981, copyright American Journal ofBotany!, Water depth in this case refers to depthabove the forest floor.
~Li r~fllTOTAL
~14, ~71 4 ~1.71,845.1 2017.3 1947.6
TUPELO 1426
t4E26
ttr t6K
%12roIrtz
GtatzZ 6Vtrlr6 .-4K
12
10
FLOODING REGIME~ Seasonally flooded~ Crayfish pond
Permanently tlooded
J FMAMJJ AS
Figure 29, Seasonal stem growth patterns for three major tree species in the Barataria swamp forest reprinted, with permission, from Conner et al. 1981, copyright American Journal of Botany!.
41
4,4.2 Q+~h
Table 8. Estimated aboveground net primary productivity for each marsh habitat.
Contribution
Coverage to total NPP '/o! g dry wt/m2/yr!
NPP
g dry wt/m2/yr!Species
1,501b3 140c1,420d
~<~ittari ~fl~lr ~~~h'~~~~Tha sp,OtherTotal NPP
17.4
3A
2.6
76.6
261
107
37
1,953
6 043b3,2372,658~3 416b
~Sa~in t ~tnxiii;~ gjii;gg
~Oj~r
45.8
29,0
9,03.3
12.9
2,768939239113
4,554
gr!~mrsOtherseTotal NPP
1,450b3,4163,2376,043
/@~~in gl~rnif~lr 62.8
14.910.1
7,8
911
509327
2,218
Juamroameriaaus.~Di i~hli gjiigfg
uataaaTotal NPP
aChabreck �972!.~Hopkinson et al. �978!.c oyd�969!
Whigham et al. 1978.eProductivity assumed to be equal to the average of other species in the habitat.Kirby and Gosselink �976! assume 70/o inland area and 30'/o streamside area and the salt marshacreage was divided up using these percentages in order to calculate this figure,
42
At first glance, these results indicate thatthe productivity of the permanently flooded area isnearly as high as for the seasonaHy floodedswamp, However, the marketable forest species inthe flooded area are declining because of a lack ofrecruitment. Only 943 trees/ha are found in thepermanently flooded area as compared to 1,303trees/ha in the seasonally flooded swamp and1,564 trees/ha in the crayfish farm. A significantportion of the productivity of the permanentlyflooded area is due to small shrubs likebuttonbush, snowbell, and maple which arebecoming dominant as the tupelo, baldcypress,and ash die or are blown over,
We estimated the average productivity ofeach marsh type using measured and estimatedproductivity values for species which occur there Table 8!. For the brackish and saline marshes, wewere able to account for a large percentage of theproduction with production values measured inLouisiana; therefore the average values should befairly accurate. For the fresh marsh, however, ahigh percentage of the over-all average is basedon estimates. Thus, these values must be con-sidered tentative. We will consider reasons for thispattern in the next section.
Brackish marshes have the highest overallaboveground net primary productivity of all marshtypes in the Barataria Basin, followed by saline andfresh rnarshes Table 8!. This results from highproduction values reported for 5. ~~ thedominant species in this marsh type, For thosespecies whose productivity has been measured inthe Barataria Basin, Q. ~ is apparently themost productive, followed by J, ~rme~rian, D.~i~,+~rifi r, and/,~ Table 8!,
The productivity of two freshwater marshspecies has been studied in coastal Louisiana.Hopkinson et al. �978! measured the productivityof Qgj~ ~ at 1,501 g dry wt/m2/yr Table8!. ~Phr-ggg~i aggfr~li was slightly moreproductive at 2,318 g dry wt/m2/yr. Production ofthese two species was seasonal, with the highestrates in summer and lowest rates in winter. Peak
live biomass ocurred in June for Q. /afar and inlate summer for Phrg<~mit Figure 30!. Minimumvalues of winter live biomass were very low for bothspecies. Dead biomass exhibited the oppositepattern for both species. This type of seasonalproduction is common in the fresh rnarshesbecause of the abundance of annual species.Though fresh rnarshes have not been studied asmuch as salt rnarshes, Whigham et al. �978!showed in a summary of productivity data fromfresh marshes along the Atlantic coast that theycanbe as productive as salt marshes.
The production of @~in ~n and~Di ti~hli atttcata reaches a maximum in thebrackish marshes Payonk 1975; Wopkinson et al.1978; Cramer et al, 1981!, Net primary productionwas 3,237 g dry wt/rn2/yr for D. g~igg and 6,043for 5. ~. Both species had considerable livestanding biomass throughout the year. There wasa seasonal trend for both live and dead biomass for9, @~i~, but there was no clear trend for Q.~tens Figure 30!.
Production of salt marsh grasses isgenerally high. Net aboveground production of Q.+igr~nifl r was measured by Kirby and Gosselink�976! and Hopkinson et al. �978!. Themeasurements for streamside productivity wereclose; 2,645 and 2,658 g dry wt/m2/yr,respectively. Both researchers reported clearseasonal patterns for both live and dead biomass Figures 30 and 31!, Peak live biomass occurred inlate summer while dead biomass reached a peak inmid winter and a minimum in late summer. Loss of
dead grass is due to decomposition and physicalflushing of the marshes by tides. Loss rates arelow in winter due to low water levels and lowtemperatures which inhibit decomposition. Lossrates are highest in the spring because of highertemperatures, more frequent flooding, and a highbiomass of dead in . Production 50 m inlandwas 1,323 g dry wt/rn yr Kirby and Gosselink1976!, one-half of streamside productivity,
Hopkinson et al. �978! also measured theproduction of Jinn~ r~m~ri n a andgyno~sr<jii<~ as 3,416 and 1,355 g dry wt/m2/yr,respectively. There was a clear seasonal patternfor live biomass for Q, ~ngggg>~le but not so fordead biomass Figure 30! or for dead or livestanding crop for Jim's~, A significant differencein biomass between Barataria Bay and other areasof the U.S. is the low proportion of live to deadvegetation. On an average annual basis, there isalways more dead material than living, with live todead ratios in the Barataria area ranging from 0.21to 0.91, Using end-of-season biomass maxima, alive to dead ratio of 0.7 to 1.0 was obtained. This ismuch less than 5.3 from Georgia Smalley 1958! or2.1 for Maryland Keefe and Boynton 1973!,Turner and Gosselink �975! reported ratios inTexas averaging 1.5, slightly higher than in theBarataria Bay. These low ratios may be a reflectionof high turnover rate of live vegetation and low tidalenergy of the Louisiana coast Hopkinson etal,1978!.
4.5 FACTORS AFFECTINGMARSH PRODUCTIVITy
Studies in Louisiana and elsewheresuggest that nutrients and hydrology are importantin determining production levels. These twofactors are highly interrelated and do not actseparately. Patrick and DeLaune �976! reportedthat additions of nitrogen, but not phosphorus,stimulated short Qgartiit growth in Baratariamarshes. The standing crop of tall /@~in wasnot affected by the application of fertilizer.
This finding is consistent with otherenvironmental studies from the Atlantic coast Sullivan and Daiber 1974; Valiela and Teal 1974;Broome et al. 1975; Gallagher 1975; Mendelssohn1979!. Although this seems to indicate that theshort form is nitrogen limited, other factors may beinvolved. Mendelssohn �978,1979! suggeststhat intense soil anaerobiosis resulting from poor
43
Smarting atternifinra S~aittaria faicata800
800
600 600
400E
200
0
15001400cL'
g 1200IZI
1000
400
200
0 JFMAMJJA SON DJFMAMJJA SON D
600
400800
200600
400AS 0 ND
~atensS~artina1600
1400
12001000
8001000
600
400E
200
'Cl 0
1600
14002Q 1200
1000
800
600
400
3100
2600
2100
1600800
1100600
400 600ASONDS ON D
Figure 30. Seasonal changes in live and dead biomass of some of the more dominant marsh plants
Juncus roemerianusDistichlis a~teats
1000 L'15001400800
1200600
1000400
2006
0a'Zl
1600
N1400
|2 1200III
800
600
400 J F MA M J J AS ON D
1400
1200
1000
8001000600
800400300
J F MA M J J A S ON D
1100
900
700
500cv
300
1000
4000U!CO
X3000
0
200073
1000J F MAM
45
reprinted, with permission, from Hopkinson et al. 197B, copyright Ecological Society of America!.
STREAMSIDE Y= 431,6-250 4X+ 147 1X -201X + 0 79X2 3 4
R2 098Y = 413 5- 369 4X + 165 4X - 20.9X + 0 79X4
R ~ 0.8820
! NLAND
1000
800
600E
400
200
0 M A M J J A S 0 N 0 J F M ALEGEND
0 STREAMSIDE - RAW DATAI3 NLAND - RAW DATA
STREAM SIDE - CURVE-FIT DATA~ INLAND- CURVE-FITDATA
1600
1400
1200
E1000
800
M A M J J A S 0 N 0 J F M A
MONTH
Figure 31. Live and dead standing crop of streamside and inlancl @~in Zltgr~ifl ~r reprinted, withpermission, from Kirby and Gosselink 1976, copyright Ecological Society of America!.
areas receive less sediment and are primarilyanaerobic.
46
soil drainage may inhibit nitrogen uptake directly bydecreasing the amount of oxygen available foractive uptake or indirectly by generatingsubstances which are potentially toxic to activenutrient uptake. This inhibition is overcome withthe application of nitrogen fertilizer because theconcentration gradient of available nitrogen intothe root is increased, The lack of an increase in
standing crop of the tall form after fertilization maybe due to greater input of nutrient enrichedsuspended sediments DeLaune et al. 1979!, andto the fact that these areas are primarily aerobic Mendelssohn et al. 1981! whereas the inland
Several studies have shown that alteration
of normal hydrological conditions can affectstructure and productivity of wetlands.Mendelssohn et al. �981! reported thathydrological modifications of salt marshes thatcause increased waterlogging may affect plantproductivity. As an example, the Leeville oil fieldlies on the western boundary of the Barataria Basinin Qgg~i ~fI~ marsh. There is a densenetwork of canals dug for access to drilling sites,
60
$0
40Ol
30O
20
10
30
20
10
CV0
O- 10
Ede
J J A S 0 N
30
EN 20
NO
10E
0J J
MONTH
47
Spoil disposal levees line many of the canals.Allen �975! reported that estimated standing liveQi~rtin biomass was 50% lower in marshessurrounded by spoil banks than in a comparablecontrol site. In the same area, the density ofnatural tidal channels was inversely related to thedensity of artificial canals Craig et al. 1979!. On abroader level, the erosion rate of wetlands wascorrelated with the density of canals Craig et al.1979!, Erosion is part of the deltaic cycle, bIjt thepresent rate is 3-4 times higher than would occurnaturally Craig et al, 1979!.
The pattern of highest marsh productivityin the mid-Barataria Basin Table 8! may be causedby a combination of hydrologic factors and nutrientlevels. The upper basin is characterized by highernutrient levels and the lower basin by much morefrequent flushing of marshes by tidal action. Wehave shown that both of these factors are relatedto marsh productivity. It may be that the optimumconditions for marsh growth occur where there areenriched waters and enough tidal action so thatmarshes are flooded regularly Schelske andOdum 1961!.
4.6 SALT MARSHEPIPHYTIC ALGAE
Epiphytic algae have been studied only inthe salt marshes Stowe 1972, 1980, 1982!. Ingeneral,. the productivity is low compared to that ofthe vascular plants. Production dynamics are areflection of several interacting factors: shading bythe at~in canopy, the seasonal dominantmacroscopic algal form, water level, salinity,frequency of flooding, and temperature. Stowe�972! studied the structure and productivity ofmicroalgal communities on the shore of AirplaneLake and 1.5 m into the marsh, The shorelinecommunity receives direct sunlight all year andmaintains a higher level of production.
in terms of their relative productivities, theshoreline and inland microalgal communities arevery distinct Figure 32A,B!. The shorelinecommunity exhibited maximum rates of productionduring the periods of ~B+~hi I and ~Pigli~si heidominance. The inland community was alwaysvery low. Net productivity of the inland community
was negative during most of the year. Only duringperiods of thinning /~~in and low temperatureswas the inland community a positive net producer.Both communities have very high respiration rates Figure 32C!. Integration of the area under thecurves for gross production and respiration showthe shoreline community to contribute a net of 16g C/m2/yr to the water column while the inlandcommunity required an additional input equivalentto 63 g C /&lyr for maintenance to.account formeasured levels of respiration.
J J A S 0 N D J F M A M
Figure 32. Metabolic rates of the epiphyticcommunity associated with QggrLIII ~ln~ifI r Stowe 1972!.
