of Changing Nutrient Loadsof Changing Nutrient Loads

educing nutrient loads, particularly nitrogen from the

Mississippi–Atchafalaya River Basin, will cause improvements

both in the hypoxic conditions in the Gulf and in water

quality and land-use conditions within the Basin. This chapter

presents potential effects over the next 10–20 years, in the Gulf

and the Basin:

� if there are no major changes in average nitrogen loads,

� if average nitrogen loads increase, and

� if nitrogen loads are reduced.

Effects if Average Loads Are Unchanged

Analyses supporting this assessment were based on data thatwere generally complete through the mid-1990s. Thus, the assess-ment of “current” activities is up to date only to about 1995 or1996. Current activities may be having effects for which data havenot yet been assessed.

Analyses presented previously in this assessment show thatMississippi–Atchafalaya River Basin nutrient concentrations andloads, particularly nitrogen, to the northern Gulf of Mexico havesignificantly increased over the twentieth century, especially dur-ing the last four decades. However, nitrogen loads appear to havereached a plateau of about 1.6 million metric tons per year, whichis the mean load over the period 1980–96. If nitrogen flux contin-ues at current levels, the present range of dimensions and severityof the hypoxic zone on the Louisiana shelf may be maintained.Hypoxia would vary, depending on physical factors in the Gulfand the Basin, such as spring and summer stratification caused byfresh river water flowing over marine waters, weather patterns,and temperature. However, even under these conditions, naturalvariability could produce new extremes of the extent and severityof hypoxia.

Agricultural nonpoint sources in the MARB constitute about 65percent of the nitrogen and about 74 percent of the nitrate load tothe Gulf. Nitrogen fertilizer use in the MARB appears to havereached a plateau beginning in the 1980s, while phosphorus fertil-izer use stabilized in 1980 and then dropped slightly. At the sametime, both the rise in river nitrate concentration and the fall in sil-icate concentration ceased.

Agricultural policies and practices have changed over the lasttwo decades so that both crop production and efficiency haveincreased. Average fertilizer use per acre of corn in the U.S. hasbeen essentially constant since the late 1970s; however, cornyields have continued to climb, resulting in an increase in the

Chapter 4 : The Effects � 29

bushels of corn produced per unit of fer-tilizer applied. According to commentsfrom the Fertilizer Institute, farmers havealso increased the amount of grain pro-duced from 0.76 bushel per pound ofnitrogen in the late 1970s to approxi-mately 1.0 bushel per pound of nitrogentoday—a 32 percent increase in effi-ciency. A mass balance of nitrogen inputsand outputs to the MARB indicates that

they have been approximately equaland at a steady state since about

1980 and suggests that levels ofnitrogen stored in the Basin

ecosystem are no longerincreasing as they didbetween 1955 and 1980(see Figure 4.1).

If there are no majorchanges in average nitro-

gen loads and the Gulf ofMexico continues to

experience a largezone of hypoxiaeach summer, the

area affected isexpected to continue to

suffer from annuallosses of biodiversity,abundance, and bio-

mass. The loss ofharvest from the

hypoxic regionmay, or may not, be

offset by harvests fromnonhypoxic waters. It is

unknown whether the northern Gulfcan sustain this level of impact withoutobservable reductions in overall fisheriesproduction.

Not all nutrients entering tributarieswithin the MARB are transported to theGulf. Substantial processing of nutrientsoccurs in the rivers and streams. Excessnutrients there can overstimulate primaryproduction, which commonly leads toimpaired water quality. Substantial num-bers of river miles in most states in theBasin suffer use impairment related tonutrient conditions, meaning that they donot fully support one or more resourceuses, including aquatic life, fish consump-tion, and swimming. This situation wouldmost likely remain the same without somechanges in nutrient loading.

Effects if Average Loads Increase

Average nitrogen loads can increasebecause of anticipated growth in popula-tion, food production, and fertilizer use.Changes in climate may also tend to exac-erbate hypoxia in the Gulf. Increases inaverage nitrogen load might be offset bychanges that tend to mitigate hypoxia,such as ongoing improvements in farmingpractices, better targeting of riparian andwetland restoration, and improved river-flow management.

Current pressures that drive hypoxicconditions are likely to intensify. U.S.population and food demand are pro-jected to continue to grow. Over the pastdecade, the U.S. population has increasedby 10 percent (Bureau of the Census1999); middle series projections predict an8 percent increase by 2010 and a 17 per-cent increase by 2020. World populationis expected to grow at 1.2 percent peryear through 2010 according to theWorld Bank, and the U.S. has provided alarge percentage of the world’s food, par-ticularly grain.

The northern Gulf of Mexico is acoastal area that may experience increasedfreshwater runoff or greater extremes ofinflux as a result of global climate change.Streamflow was approximately 30 percenthigher during 1980–96 than during1955–70. Part of this increase may be dueto long-term climatic variation, and partmay be driven by shorter-term climaticcycles. The higher flows in the later halfof the twentieth century are attributed toincreased precipitation throughout theyear and, in particular, to warmer, wettersprings. During this same period, stream-flow has increased 30 percent, whilenitrate flux has increased 300 percent.

Higher precipitation and streamflow asoccurred in the later half of the twentiethcentury could influence nitrate flux in sev-eral ways. More nitrate would be trans-ported with the larger volume of flowunless concentrations decreased more read-ily than flow volume increased. The higherprecipitation could leach more accumu-lated nitrate from soils in the basin into tiledrains and ditches, resulting in highernitrate concentrations in streams. Higherstreamflow would decrease the residencetime of water in the river, reducing therates of denitrification, burial, and uptakeby aquatic plants and riparian vegetation.

30 � HYPOXIA IN THE NORTHERN GULF OF MEXICO

Reducingsources ofnutrients fromthe MARB mayhave as aprimary goaldecreasing thehypoxia problemin the Gulf ofMexico, butalso is expectedto affect waterquality conditionsin the Basinitself.

Chapter 4 : The Effects � 31

The resulting higher runoff in sum-mer would most likely affect watercolumn stability, surface productiv-ity, and oxygen cycling, leadingperhaps to an expanded extent ofthe hypoxic zone.

Understanding the effects ofhypoxia in other areas of the worldcould be useful in projectingeffects in the Gulf of Mexico.Experience with other hypoxiczones around the globe shows thatinitially the higher nutrient con-centrations lead to increased pro-duction of both organic matter andfisheries (see Figure 4.2). However,as organic matter productionincreases, changes occur in thefood web that lead to both theecological and the fisheries effectsbecoming progressively moresevere as hypoxia worsens. Thesechanges have been observed inmarine ecosystems around theglobe, resulting in serious ecologi-cal and economic consequencesfrom hypoxia.

Most notable among theseaffected marine systems are the

Kattegat, the Baltic and AdriaticSeas, and the northwestern shelf ofthe Black Sea. The consequencesrange from localized loss of targetfish catch and recruitment failure(low numbers joining a population,generally due to low reproductionrates) to complete system-wideloss of fishery species. Where oxy-gen depletion is severe, the foodweb that supports bottom feeders,such as shrimp and drum, is dis-rupted, as well as the natural pro-cessing of organic matter,nutrients, and pollutants.

In the Gulf of Mexico region,Louisiana leads in production andlandings of commercial and recre-ational marine fisheries. The fish-ery resources of the Gulf areamong the most valuable in theUnited States, generating $2.8 bil-lion annually. Although possiblydue to factors other than hypoxia,catch per unit of effort for brownshrimp, one of the most commer-cially valuable species in the Gulf,has trended down since the late1970s.

Other areas that have experi-enced severe hypoxia, with near-anoxic conditions, experiencedgreater mortality and had fewerspecies and lower biomass thanareas with intermittent or lesssevere hypoxia. If experiences inother coastal and marine systemsare applicable to the Gulf ofMexico, then the potential impactof worsening hypoxic conditionscould be the decline (perhaps pre-cipitous) of ecologically and com-mercially important fisheries.

The degree of ecological andeconomic effects related tohypoxia varies from system to sys-tem. For example, both ecologicaland economic effects of the com-bined problems of eutrophicationand hypoxia have been seen inthe Black and Baltic Seas, wheredemersal trawl fisheries have beeneither eliminated or severelystressed. Initially in the Kattegat,hypoxia caused mass mortality ofcommercial and noncommercialspecies. Now large-scale migra-tions and/or mortality among

Nitrogen inputs to and outputs from the Basin have been approximately equal since about 1980, and the residuals (differencebetween inputs and outputs) have been variable. The cumulative residual is at a steady state, indicating that levels of nitrogenstored in the Basin are no longer increasing as they did between 1955 and 1980.

F IG. 4. 1

Source: Figure 6.4B of the Topic 3 Report for the Integrated Assessment.

Cumulat ive Res idual f rom e Res idual f rom Nit rogen Mass Balance: 1955Nit rogen Mass Balance: 1955–97

100

90

80

70

60

50

40

30

20

10

01950 1960 1970 1980 19901955 1965 1975 1985 1995 2000

Millions

of

Metric To

ns

c umula t ive res idual ,exclud ing ni t rogen outpu tin Miss i s s ipp i River discharge

cumula t ive res idual ,inc lud ing ni t rogen outpu tin Miss i s s ipp i River discharge

32 � HYPOXIA IN THE NORTHERN GULF OF MEXICO

demersal fish and the Norwaylobster (Nephrops) continue, result-ing in a changed species composi-tion and reduced growth andbiomass. Hypoxia in the Kattegatis believed to be partly responsi-ble for the overall decline in stocksize, recruitment, and landings ofcommercial fish over the last twodecades.

Louisiana’s commercial andrecreational fisheries depend onspecies that spend part of theirlife cycles within shallow conti-nental shelf waters that oftenoverlap the hypoxic zone. This is

true not only for shrimp but alsofor the benthic organisms thatform the forage base for valuablecommercial and recreationalspecies. Because shrimp mustmove from inshore wetland nurs-eries to offshore feeding andspawning grounds, hypoxia canblock their migration. Spawninggrounds, migratory pathways,feeding habitats, and fishinggrounds of important species areaffected by the extent and sever-ity of hypoxic waters. Expansionof the hypoxic zone could lead tofurther decreases in productivity

at higher levels of the food weband loss of essential habitats.

Increasing nitrogen loads havedecreased the silica-to-nitrogenratio. Decreased silica availabilityin relation to increasing availabil-ity of other nutrients could stimu-late blooms of harmful algae, suchas dinoflagellates. Pseudo-nitzschiaspp., a group of diatoms, includ-ing some toxic forms, that requiresilica, have increased in abun-dance in spite of silica decreases,most likely in response to increas-ing nitrogen inputs from theMississippi River.

Although higher nutrient concentrations initially increase the productivity of fisheries, ecological systems worldwide shownegative effects as nutrient loading increases and hypoxic or anoxic conditions develop. Each generic curve in the lower halfof the figure represents the reaction of a species guild to increasing nutrient supplies. The top half of the figure illustratestrends in various marine systems around the world. Reversals show that trends toward over-enrichment have been turnedaround in several areas.

ComparaComparat ive Evaluat ion o f F i shery Response to Nuo Nutr ients

Produc

tion and

Fish

ery Yield

Nu t r ients Loading

SE North Sea

N. Gulf o f Mexico

N. Adr ia t ic Sea

Great Lakes

Kattegat

Bal t ic Sea

Seto In land Sea

Yel low Sea

NW Black Sea

Chesapeake Bay

Meso trophic Eu t rophic Dyst rophicMeso t rophic(meduim productivity)(meduim productivity)

Eu t rophic(h igh product iv i ty)

Dyst rophic(no product iv i ty)

Ol igo t rophic( low product iv i ty)w product iv i ty)

Otherzoop

lankton

feeder

s (e .g. med

usae)

Zooplank

tontivoro

us f i sh

Benthos-fee

dingf i sh

Benthos

Seasonal Permanent bottom anoxia

FIG. 4.2F IG. 4.2

Source: Redrawn from Caddy 1993.

NO

AA

NA

TIO

NA

L M

AR

INE

FIS

HE

RIE

S S

ER

VIC

E

Chapter 4 : The Effects � 33

Effects if Average Loads Are Reduced

Reducing sources of nutrientsfrom the MARB may have as aprimary goal decreasing thehypoxia problem in the Gulf ofMexico but also is expected toaffect water quality conditions inthe Basin itself. Although thereare distinct differences betweenthe nature of the responses tosuch reductions in the two sys-tems, not surprisingly, there arealso substantial similarities. Inboth the Gulf and the Basin,response variables of interestinclude various nutrient forms,chlorophyll, dissolved oxygen,water clarity, planktonic and ben-thic biota, and higher organisms.However, the response variable ofprimary interest for hypoxia is dis-solved oxygen in the bottomwaters of the Gulf of Mexico.

Depending on the suite oftechniques used to reduce loads(i.e., the mix of source reductions,wetland restoration, and river

management), a number of costswould be incurred, includingthose associated with highermaterial, equipment, and manage-ment costs of improved farmingpractices; loss of productive farm-land; and increased treatmentcosts for municipalities and indus-try (air and wastewater controls).In addition to reducing risks ofprecipitous decline in Gulf fish-eries from hypoxic/anoxic condi-tions, these costs would be offsetby a number of benefits withinthe Basin, including improvedwater and habitat quality, reducedsoil erosion, reduced contamina-tion of drinking-water sources bynitrates, improved water qualityfor recreational use, improvedrecreational fisheries and wildlife,cost-effective flood damage reduc-tion, and restoration of criticalcoastal wetlands in Louisiana.

Changes in the Gulf of MexicoSimulations with a quantitative

water quality model (Bierman et al.1994) indicate that dissolved oxy-

gen and chlorophyll concentra-tions on the Louisiana continentalshelf are likely to be responsive toreductions in nutrient loads fromthe MARB. However, there arelarge uncertainties in the magni-tudes of these responses for a givenload reduction. For nutrient loadreductions of 20–30 percent,bottom-water dissolved oxygenconcentrations were estimated toincrease by 15–50 percent, andsurface chlorophyll concentrationswere estimated to decrease by5–10 percent. The ranges corre-spond to different assumptions forsediment responses and large-scaleGulf of Mexico water quality, andto different hydrometeorologicalconditions among different years.Although both nitrogen and phos-phorus load reductions producedresponses in the model, theresponse, particularly for dissolvedoxygen, was somewhat greater fornitrogen reductions.

There has been no statisticallysignificant long-term trend inphosphorus flux to the Gulf for

Thousands of small shrimp boats trawl the northern Gulf of Mexico. Important commercial species in the Gulf, such asshrimp, avoid hypoxic areas, where they would otherwise spend part of their life cycle. Benthic organisms that providefood for other sought-after species are also disrupted in the hypoxic zone.

KE

VIN

F. D

EN

NE

HY,

U.S

. GE

OLO

GIC

AL

SU

RV

EY

34 � HYPOXIA IN THE NORTHERN GULF OF MEXICO

the period 1972–96. Over thepast half century, phosphorus flux,a large portion of which is associ-ated with sediments, has probablydecreased. Nitrogen is the onlycomponent of nutrient flux thathas increased. For nitrogen loadreductions of 30–50 percent(returning to the levels thatexisted in 1955–70), the modelestimates that dissolved oxygenconcentrations would increase by20–75 percent.

There appears to be no evi-dence that the ecological changesrelated to hypoxia in the northern

Gulf of Mexico at this point areirreversible. Experience hasdemonstrated that ecosystems canrespond positively to nutrientreductions. Examples includeChesapeake, Tampa, and SarasotaBays. Reductions in loads toTampa Bay are attributed toimprovements in wastewater treat-ment and in fertilizer loadingoperations in the Port of Tampa.In Sarasota Bay, reductionsoccurred as a result of improve-ments in wastewater treatmentand agricultural re-use, and theuse of deep-injection wells for

wet-weather effluent disposal(Johansson and Greening 1999).There are many examples ofsmall-scale hypoxia reversals asso-ciated with improvements in treat-ment of sewage and pulp milleffluents. In the U.S., theimproved water quality in LakeErie is a good example of positiveresponse to nutrient reductionsbut is also a demonstration thatthe time interval for achievingnoticeable improvements may belong—on the order of five to tenyears after nutrients are reduced.Substantial nutrient reduction tar-gets are being proposed for thedrainage area of the Baltic Sea inorder to control eutrophication(Jansson and Dahlberg 1999).

Changes in the WatershedThe most direct effect of

actions to reduce nutrient load-ing to surface waters in theMARB will be to decrease con-centrations and to shift the com-position of nutrients in its riversand streams. The changes innutrient concentrations and com-position will induce otherchanges in trophic conditionsand should result in Basin-wideimprovements in surface-waterquality. The extent of reductionsachieved (and subsequently theeffects) will depend on site-spe-cific characteristics (climate,soils, cropping history), the typesof improvements in management,and the baseline conditions towhich the management improve-ments are being compared.

Aquatic macrophytes (such asduckweed, water lilies, andsedges) have important effects onwater quality in shallow watersthroughout the Basin. If reduc-tions in nitrogen and phosphoruslevels cause an increase in under-water light, submerged aquaticmacrophyte distribution would beexpected to expand in the upperMississippi River. The effects onwater quality will be beneficial,both locally and system-wide.Increased macrophyte abundancemay enhance nutrient retention

Public water supplies are treated, if necessary, to ensure that nitrate levelsare below the drinking-water standard of 10 mg/l. People drinking groundwater from shallow wells in vulnerable geologic settings (sand, gravel, orkarst) in rural agricultural areas, however, are exposed to risks that could bereduced through reductions in nitrogen loading.

Chapter 4 : The Effects � 35

significantly, leading to lowerdelivery rates of nutrients to theGulf of Mexico than would other-wise be predicted for direct effectsof source reductions.

Reductions in nutrient concen-trations would not be expected tostrongly affect sport fisheriesbecause the rivers and streams inthe Basin would most likely con-tinue to be highly productive sys-tems. Any decline that mightoccur in total biomass productionwould likely be more than offsetby habitat and other improve-ments that would promote gamefish over rough fish populations.

All the nutrient-reductionapproaches considered—includ-

ing steps to reduce nutrient lossesfrom farmlands, reduce nutrientdischarges from point sources,and increase natural processes toremove excess nutrients from sur-face waters—are expected to pro-duce environmental benefitswithin the Basin. These includethe values associated withrestored wetlands, reduced soilerosion, reduced nitrogen con-tamination of drinking water,reduced vulnerability to floods,better wildlife habitat, andimproved recreational water qual-ity. Other potential benefits ofnutrient-reduction activitiesinclude more efficient use oforganic and inorganic fertilizers

and the energy associated withthem, lower overall fertilizercosts, decreased health risk fromcontamination of public and pri-vate drinking-water supplies, andimproved aquatic habitat instreams, lakes, rivers, and estuar-ies. All the approaches consideredwill also have associated costs,including costs for changes inland use; introduction of newtechnologies, such as precisionagriculture and biological nitro-gen removal; and implementationof improved management prac-tices. The estimated economiccosts of the various approaches toreduce nutrients are described inthe next chapter.