Reconsidering the physics of the Chesapeake Bay … the Physics of the...Estuaries Vol. 24, No. 5,...

Estuaries Vol. 24, No. 5, p. 655-669 October 2001

Reconsidering the Physics of the Chesapeake Bay Estuarine

Turbidity Maximum

LAWRENCE P. SANF(__)RD 1'*, STEVEN E. SUTTLES 1, a n d JEFFREY R HALICA 2

1 Unive~sity of Ma~'yland Center for Envhon~wntal Science, Horn Point Laboratory, P O. Box 775, Ca~nbridge, MaiTland 21613

Ma'r'yland Geolog'ical Su~we)!, 2300 St. Paul St'feet, Baltimore, Maryland 21218

ABSTRACT: A series of cruises was carried out in die ~stuarine turbidJty maximum (ETM) region of Chesapeake Bay in 1996 to examine physical and biological variability and dynamics. A large flood event in late January shifted die salinity structure of the upper Bay towards that of a salt wedge, but most of die massive sediment load delh,ered by die Susquehanna River appeared to bypass the ETM zone. In contrast, suspended sediments delivered during a f lood event in late October were t rapped very efficiendy in the ETM. The difference in sediment trapping appeared to be due to increa.s~s in particle settling speed f rom January to October, suggesting that the fate of sediments delivered during large events may depend on the season in which they occur. The ETM roughly tracked the liufit of salt (defined a.s the intersection of the 1 psu isohaline with the bottom) throughout die year, but it was often separated significantly f rom the limit of salt with die direction of separation unrelated to die phase of the tide. This was due to a lag of ETM sediment r~suspension and t ranspor t behind rapid meteorologically induced or river flow induced motion of the salt limit. Examination of detailed time series of salt, suspended sediment, and velocity col lected near die limit of salt, combined with odler indications~ led to the conclusion that the convergence of the ~stuarine circulation at die limit of salt is not die primary mechanism of particle trapping in the Chesapeake Bay ETM. This convergence and its a~sociated salinity structure contribute to strong tidal a.s.~nmetries in sed iment r~suspension and transport d~at col lect and maintain a resuspendable poo l of rapidly settling particles near the salt limit. Widmut tidal r~suspension and transport, die ETM would eidler not exist or be gready weakened. In spite of this repeated r~suspension, sedimentation is the ultimate fate of most terrigeuous material delivered to the Chesapeake Bay ETM. Sedimentation rates in die ETM channel are at least an order of magnitude greater allan on the adjacent shoals, probably due to focusing mechanisms dlat are poorly un- derstood.

I n a - o d u c f i o n

The estuarine turbidity mmximum (ETM) zone of upper Chesapeake Bay, a region of elevated sus- p e n d e d s e d i m e n t c o n c e n t r a t i o n s and r e d u c e d light availability near the limit of salt intrusion, was first studied in the late 1960s and early 1970s. Th e most extensive work was carried out by Schubel (1968a,b, 1971), Schubel and giggs (1969), and Schubel and Kana (1972), who mapped the spatial and seasonal variability of the ETM, examined tidal resuspension processes at its center, and carried out studies of suspended particles and agglomer- ates. giggs (1970) also considered the sediment budget of the uppe r Bay, while Nichols (1974, 1977) examined the dynamics of the ETM in the Rappahannock, a sou thern tributary of the Ches- apeake Bay.

These earl), studies provided good descriptions of ETMs in the partially mixed Chesapeake Bay and its tributaries, but they were limited in their ability to address dynamical questions for several

* Corresponding author; tele: 410/'228-8200; fax: 410/'221- 8490; e-mail: [email protected].

reasons. First, they were technolog~y" limited; Schu- bel (1968a) obtained a large data set on the distr ibution of suspended sediment in and near the Chesapeake ETM, but he repor ted just a few observations of currents, no salinity data, and no settling velocity data. Second, there were few other repor ted ETM studies for comparison, and there was a limited unders tanding of the dynamical com- plexities of ETM particle trapping. While Schubel (1968b) recognized the impor tance of tidal resuspension for maintaining high suspended sediment concentrat ions, he and o ther researchers (Festa and Hansen 1978; Officer 1980) at t r ibuted ETM particle t rapping primarily to the convergence of the estuarine circulation at the limit of salt intrusion combined with slow particle settling.

In the )rears since these initial studies of the Chesapeake Bay ETM, other investigators have shown that mechanisms of particle t rapping in ETMs are more complex than simple convergence of the estuarine circulation. Dyer (1988) and Dyer and Evans (1989) showed how a phase lag of sediment resuspension relative to near-bot tom currents can p roduce an ETM in the presence of

�9 2001 Estuarine Research Federation 655

6 5 6 L.P. 8anford et al.

asymmetr ical tidal currents wi thout convergence of the gravitational circulation. Geyer (199.3) showed that suppress ion of turbulent mixing by density stratification downst ream of the limit of salt intrusion ampli f ied t rapping efficiency for a range of particle settling velocities faster than those pro- posed by Festa and Hansen (1978). H a m b l i n (1989) demons t ra t ed that a combina t ion of ebb- flood asymmet ry in suspended sed iment t ranspor t and suppression of mixing by density stratification were likely responsible for particle t rapping in the St. Lawrence ETM. Jay and Musiak (1994) repor t - ed that s trong tidal assrmmetries in stratification and flow near the salt limit likely explain suspended-sediment t rapping in the Co lumbia River ETM. Uncles and S tephens (199S) demons t ra ted that the location of a pool of r esuspendab le particles def ined the location of the ETM in the Tamar e> tuary, which often was ups t ream of the salt limit. In contrast, Geyer et al. (1998) showed that lateral interact ions be tween topography and flow maintain a pool of resuspendable particles, hence an ETM, well downst ream of the salt limit in the Hud- son River estuary. Particle t rapping dynamics probably differ between estuaries or p r e d o m i n a t e at different times in the same estuary (e.g., spr ing tides versus neap tides).

In recen t }rears the ecological role of ETMs in suppor t ing a n a d r o m o u s fish r ec ru i tmen t also has been recognized (Dodson et al. 1989; Dauvin and Dodson 1990), and ETMs have been found to be m-eas of elevated zoop lank ton concentra t ions , es- pecially the calanoid copepod Eu'o~temora affinis (Simenstad et al. 1994; Morgan et al. 1997; Kim- mere r et al. 1998). It is believed that a b u n d a n t food in the fo rm of detritus, protozoa, and phy- toplankton, in addi t ion to the ETM particle trapping mechan i sms descr ibed above, suppor t the h igh z o o p l a n k t o n a b u n d a n c e s . I m m a t u r e a n d adul t Euryte'mora are impor t an t foods for larval and juveni le str iped bass and white pe rch in Chesa- peake Bay" and other estuarine systems (Setzler- Hami l ton 1991; Setzler-Hamil ton and Hall 1991) establishing a potent ia l t rophic link that suppor ts fish recru i tment .

With the potent ia l ecological role of ETMs and the new concepts of ETM particle t rapping in mind, an interdiscipl inary g roup of investigators unde r took a new study of the Chesapeake Bay ETM in 1996. U n d e r the aegis of the Nat ional Sci- ence F o u n d a t i o n L a n d M a r g i n Ecosys t em Re- search Program, Troph ic In terac t ions in Estuarine Systems (TIES), we conduc ted a series of cruises in u p p e r Chesapeake Bay to explore the ETM fea- ture and biological p roduc t ion a~ssociated with it. Pre l iminary repor ts on these studies (Boynton et al. 1997; R o m a n et al. 1997) describe impor t an t

linkages between physics, p lank ton biology, and p roduc t ion of young a n a d r o m o u s fish. More recent repor ts present addi t ional in fo rmat ion on a n a d r o m o u s fish r ec ru i tmen t (Nor th and H o u d e 2001) and zoop lank ton (Roman et al. 2001). This pape r presents an overview of the physics of the Chesapeake Bay ETM during 1996, with par t icular a t tent ion to its spatial and t empora l variability, the factors that control particle t rapping, and relationships between ETM processes and sed imenta t ion in u p p e r Chesapeake Bay.

Study Site Earlier documenta t ions (cited above) d emo n -

strated that the ETM in Chesapeake Bay is con- tained within the u p p e r m o s t par t of the estuary (Fig. 1). Its location varies seasonally and at shor te r t ime scales, but it is a lmost always found between latitudes 39~ and $9~ a range of approximately 40 kin. This region of the Bay has a mean vo lume of approx imate ly 2.6S km s and a m e a n depth of approx imate ly 4.1 m (Cronin 1971), no t count ing the small tributaries. It is incised by a nar- row shipping channel main ta ined by dredging at a dep th of approx imate ly 12 m, which connects the Chesapeake and Delaware Canal with the Port of Bal t imore and serves as the p r imary pathway for up-Bay salt intrusion. T h e average as t ronomical tidal range increases nor thward f rom 0.$6 to 0.5 m, with typical m a x i m u m tidal cur ren t speeds of 0.5 m s -1 in the channe l and 0.S m s -1 over the broad shoals (Browne and Fisher 1988). Wind-forced water level f luctuat ions can be m u c h larger than the as t ronomical tide (up to 1 m in range) due to the 2 d quarter-wave seiche response of the Bay to the passage of weather systems (Chuang and Boicour t 1989; Boicour t 1990), with associated cur ren t fluctuations of up to 0.2 m s -I (Elliott et al. 1978). Net non-tidal gravitational circulation varies strongly in response to f luctuat ions in river flow (Elliott et al. 1978), and changes f rom a riverine, ba ro t rop ic downst ream flow above the limit of salt to a well- developed estuarine circulation below the limit of salt. Almost all of the freshwater flow enters f rom the Susquehanna River. At the average Susquehan- na River flow of 1,100 m s s 1 the freshwater re- p l a c e m e n t t ime of the ETM region of the Bay is approx imate ly 1 mo.

More than 80% of the sed imen t enter ing the u p p e r Bay comes down the Susquehanna River, with a lmost all of the rest result ing f rom shorel ine erosion (Biggs 1970). T h e bo t tom sediments be- came gradually finer southward of the broad, sandy delta known as Susquehanna Flats (not shown in Fig. 1 due to sampl ing l imitations), th rough short transit ion zones of silty sands and sandy silts, th rough the clayey silts that domina te

C h e s a p e a k e B a y E T M 657

39.60 -

3940 -

39.20-

39.00-

I I I I I

a) Havre De Grace ~ ~ ' L , . ~ . ~ . ~ . f ~

/,

i a a t m o r e | . , a ~ ( , 2 ~ "

", ~.~ SA Itr

10 0 10

Kilometers

i i I i 75180 7660 7640 7620 76.00

Longi tude [deg]

76.60 7640 7620 7600 7580

Longitude [deg]

Fig. 1. a) Sediment distribution lnap of upper Chesapeake Bay (not including tributaries) using Shepard's tertiary classification scheme (Kerhin et al. 1988). Blank areas in tertiary diagram indicate a sediment type not present, and blank areas on the map in the main Bay indicate no salnpling was done in that area. b) Map of upper Chesapeake Bay. Station locations for 1996: (41") TIES CTD survey. (A) ETM transect May 3-4, (O) ETM transect July 24-25, (m) ETM transect October 24-25. Buoy icon represents CBOS buov location off Howell Point. Depths greater than 7 m are shaded.

the ETM region, to the silty clays that dominate the broad lower reach of the upper Bay. The deep channels of the uppe r Bay sediment rapidly (Of- ricer et al. 1984) and require constant dredging to maintain navigable depths. Fringing sandy shelves in the broad lower reach reflect the impor tance of both wave-forced resuspension and shoreline erosion (Kerhin et al. 1988; Sanford 1994).

Suspended sediments are comprised of silts, clays, and aggregates thereof, with a low organic mat ter fi'action (Schubel 1968a; Biggs 1970; Schu- bel and Kana 1972). Total suspended sediment (TSS) concentra t ions in the entire upper Bay are elevated relative to the rest of tire estuary, with typical background concentra t ions of very slowly settling particles between 5-25 mg 1 1 (Schubel 1968a,b, 1971; Sanford et al. 1991; Sanford and Halka 1993; Sanford 1994). These background particles tend to be uniformly distributed th rough the water co lumn or slightly more concent ra ted in the lower water colunm, and they have a h igher organic fraction than the particles that are resus- p e n d e d f rom the bo t tom (Schubel and Biggs 1969). The ETM itself typically has TSS concentra- tions 20-100 mg 1 1 h igher than tile background, with the largest concentra t ions resulting fi'om tidal r e s u s p e n s i o n in n e a r - b o t t o m waters (Schube l 1968a,b). There is little spatial or temporal variation in dispersed (disaggregated) particle size dis- t r i bu t ions (Schube l 1968a; Schube l and Kana 1972), but aggregate sizes increase markedly at max imum tidal resuspension (Schubel 1971) and

decrease slightly dur ing periods of high riverflow (Schubel 1968a).

M e t h o d s

Five cruises were carried out in the tapper Bay between February 1 and October 27, 1996, including two early hydrographic surveys of oppor tuni ty and three 5-d interdisciplinary TIES cruises. A total of 8 hydrographic (CTD) surveys of tile upper Chesapeake Bay were made dur ing these cruises. All surveys included 8-11 stations along the axis of the main (eastern) channel f rom just nor th of the Bay Bridge (approximately 39~ to Turkey Point (approximately 39~ The February 1 survey used tire R /V Kerhin in conjunct ion with a Maryland Deparnnen t of Natural Resources (MD- DNR) water quality sampling cruise after a lateJan- uary flood event in the Susquehanna River basin. Tire March 15th survey used the R / V Orion and was t r a d e d by fire Maryland Sea Grant Program. Tire remaining 6 surveys were conduc ted as part of three seasonal TIES cruises in 1996 on the R / V Cape Hetdo/)en. Physical measurements dur ing the TIES ETM cruises consisted of an initial CTD survey (Fig. 1), a 25-h C T D / A D C P lateral transect at tire ETM location, a 25-h CTD/ADCP lateral n'ansect at a location down-Bay f rom the ETM (Gibson Island transect), and a final CTD survey.

A SBE 25 Sealogger CTD was used for the Feb- ruary 1 and March 15 hydrographic surveys. The SBE 25 was equipped with an auxiliary 5-cm pathlength transmissometer (SeaTech) as well as an

6 5 8 L.P. Sanford et al.

OB&3 (Doweling & Associates) backscatter sensor for measuring turbidity. The CTD was also outfit- ted with a well p u m p located at the same level as the turbidity sensors for collecting water samples in order to calibrate turbidity (NTUs) to TSS (rag 1-1). Additional auxiliary sensors included a PAR sensor, a dissolved oxygen (DO) sensor, and a fluo- rometer. The surveys began with a lateral transect near the Bay Bridge (8 stations) and then contin- ued up the eastern channel of the Bay to the last station near Turkey Point. There were a total of 10 stations in these surveys, 8 of which were axial.

On the TIES cruises we used the R /V Cape Hen [open Neil Brown CTD moun ted on a General Oceanics rosette, equipped with a 5-cm pathlength SeaTech transmissometer, a DO sensor, and a fluo- rometer. A well p u m p for collecting water samples was again attached to the CTD for calibration of the turbidity sensors. The first survey of each cruise (May 2,July 21, and October 92) began with a series of three lateral transects stm-ting just nor th of the Bay Bridge and cont inued up-Bay to Turkey Point (Fig. 1). These surveys consisted of 18 total stations, 11 of which were axial. The location of the ETM was de termined from an axial salinit}r and turbidity con tour map of the initial CTD survey for each cruise, p roduced immediately after the survey was completed. The final survey" of each cruise (May 6, July 25-26, and October 27) began at the no r the rnmos t station and proceeded down-Bay, omitting most of the lateral stations.

All CTD surveys followed the same protocols, paying particular attention to the turbidity sensors because of their impor tance for this stud?,. Top to bot tom CTD profiles were made at each station, making a special effort to sample as close to the bot tom as possible (usually within 0.5 m). The lens- es on the t r a n s m i s s o m e t e r were t h o r o u g h l y cleaned with detergent and DI water prior to each hydrographic survey and open air and blocked path voltages were recorded for calibration. Water samples f iom the well pump were collected during CTD upcasts at locations and depths selected to cover the full range of turbidities encountered , and the times of the p u m p e d samples noted for later calculation of turbidity calibration values from the transmissometer. Samples were collected in 250 nil Nalgene bottles and were kept refriger- ated for laboratory TSS analysis.

TSS values (mg 1 1) were calculated from turbid- it}, using a two step process. The voltage output of each transmissometer was calibrated to NTUs using a well-mixed laboratory Formazin turbidity standard. NTU values calculated from the field mea~surements were then converted to TSS by means of a calibration relationship derived for each cruise. The calibration relationship was de-

rived by linear regression (sometimes in several parts) of water sample TSS values against NTUs from the transmissometer on the CTD. Water sample TSS analysis was pe r fo rmed by the Analytical Services Depar tmen t at H o r n Point Laboratory using standard methods (APHA 1975). Prewashed and weighed Whatman 04 mm (9S4-AH) filters, with 1.5 p~m nominal pore size, were used for all TSS samples.

All CTD data were processed by bin averaging over 0.95 m depth bins using Seasoft software (Sea- bird Electronics, v. 4.916), correct ing raw depths for instrumental and atmospheric pressure ofs Salinity and TSS for individual transects were grid- ded and contoured using Surfer (Golden Software, v. 6.0). The data were gr idded using the kriging algorithm with more weight given to points in the horizontal direction due to the asymmetric distri- but ion of the data. The resulting distributions of salinit}r and TSS were used to estimate the positions of the limit of salt (defined as the intersection of the 1 psu isohaline with the bot tom) and the center of the ETM (defined as the center of the region with near-bottom TSS concentrat ions more than 90 mg 1 1 greater than background TSS concentra- tions). These position estimates are repor ted with a resolution of 1 km, accurate to approximately + 2 km. This accuracy, which is better than the station spacing of 6 km, wa~s derived by trying several different estimation techniques and not ing the differences in position estimates.

Lateral transect time series stations were occu- pied for between 22-27 h during each cruise, run- ning alternating Acoustic Doppler Current Profiler (ADCP) and CTD transects across the shipping channel near the center of the ETM. The ADCP (RD I n s t r u m e n t s , 1.2 MHz B r o a d b a n d ) was moun ted on an a]uminum mast fixed to the port side of the vessel, with the transducer head sub- merged approximately 70 cm below the water surface. Cur ren t velocity profiles were collected in 50 cm bins from o m below the surface to within 1 m of the bottom. Ferrous metal on the ship can cause a directionally dependen t ofs in the internal ADCP compa~ss, such that a heading correct ion for the ADCP was derived by comparing the heading from the ADCP compass to the ship's g-yro at 10 ~ increments a round the compass. This heading cor- rection had the form of a cosine funct ion and was applied during post-cruise data processing.

A modified Owen settling tube (Valeport) was used to measure settling velocities of relatwely un- disturbed particles collected just above the bot tom during each of the ETM lateral transects. The tube is approximately 5 cm in diameter by 1 m long. It is deployed horizontally" in the water co lumn at the desired depth and is oriented into the flow by" a

vane. A messenger is sent to close the ends of the tube, which is then raised back to the surface. On deck, the tube is placed vertically inside a water jacket and samples are withdrawn from the bot tom for TSS analysis at specified, geometrically increasing time intervals. The water .jacket, flushed with water at or very near the same tempera tu re as the sample, is necessary to prevent convective cells f rom forming inside the settling tube. The se- quence of bottom-withdrawal TSS values is ana- lyzed using a spreadsheet implementa t ion of the p rocedure described by Owen (1976). The prod- uct of this p rocedure is a settling velocity distri- but ion of relatively undis turbed particles and aggregates.

Wind and salinity data were obtained from the n o r t h e r n bay Chesapeake Bay Observing System (CBOS) Buoy loca ted of f Howel l Po in t at $9~ 76~ Susquehanna River discharge data f rom the gauging station at Conowingo Dam were obtained from the U.S. Geological Survey. Cor responding monthly and annual sed iment load estimates were obtained from U.S. Geological Sur- vey a s well. These were derived using a new esti- mator model that optimizes estimates of average sediment load from average river discharge (Yo- chum 9000). Tidal height data for the Tolchester Beach, Maryland Station were obtained from the National Oceanic and Atmospher ic Administra- tion, National Ocean Service. Hour ly tide data were $4 h low-pass filtered using a bu t te rwor th filter to reveal subtidal variability" more clearly.

R e s u l t s

CALIBRATION OF TURBIDITY TO T S S

Derived calibrations between turbidity and TSS are shown in Fig. 2. An example of a single cruise calibration from July is shown in the upper panel, where the necessity for a two-part calibration is obvious. The slope of the calibration line at low concentrat ions is much lower than the slope at high concentrat ions. This is consistent w-ith a two-pm-t particle popula t ion consisting of fine, slowly settling pm-ticles that are nearly always in suspension and relatively large aggregates that are resuspend- ed from the bot tom and settle out of suspension rapidly. References cited above indicate that these two particle populat ions are always present in upper Chesapeake Bay. T h e increase in slope at 12 mg 1 -I (approximately the background TSS concentrat ion in July) is consistent with the response of transmissometers to larger particles (Baker and Lavelle 1984; no te that their calibration plots have axes reversed relative to ours). A qualitatively" similar calibration response was repor ted by Sanford (1994) for TSS collected in Sep tember 1999 in up-

300

200

100

L-

s 0 5 300 - -

200

100

0

Chesapeake Bay ETM

TSS Calibration- July21 -26, 1996

659

ii!i!5 ' " ' / / / / / / - :

---- Oc{. 2 ~ . / . ' / " .

0 50 100 150

Turbidity, [NTU]

Fig. 2. Calibration of turbidity (NTU) ~'om 5 cm pa th l eng th t r anmis somete r to TSS (rag 1 1) f rom collected ~-ater samples. U p p e r pane l shows example calibration curve f rom the July TIES cruise with fi t ted l inen- regression line (two pro't). Lower pane l shows NTU to TSS regress ions for all five 1996 cruises. Symbols indicate m e a n TSS concent ra t ion in O~ven Tube sm~> pies f rom May (O),July (IlL and Oc tober (A) TIES cruises.

per Chesapeake Bay. Two-part turbidity" calibrations also were repor ted by Pak et al. (1988) and Downing and Beach (1989) in very different en- vironments.

The lower panel of Fig. 2 shows the NTU-TSS calibration curves derived for all 5 ETM cruises in 1996. The calibration for the February 1 cruise is markedly different f rom the others. One line represents the entire calibration, with a slope similar to the low concentra t ion slopes of the other calibrations. This implies that the suspended particles during the February cruise were relatively unag- gregated fines. The May calibration slope was 270% higher than the February calibration slope at high concentrat ions, but only 70-80% as high as the slopes for the remaining cruises in the same range. At the very high TSS concentra t ions en- counte red near the bot tom in March and October , third pieces of the calibration diagrams had to be

660 L.P. Sanford et al.

2000 120

1500

1000

t 91

lOO

80

~ 1 7 6

r~ ,-- 40

500 ~ 20 g

0 0

000 | g ~ 760 �9 ~ 200

3000 ~3 ~ 160

2000 ~ 120

8O 1000

40

0 ~ , 0 Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec

[] Flow 1996 [] Sediment Load

Fig. 3. Discharge and sed imen t load en te r ing Chesapeake Bay f rom the S u s q u e h a n n a at the Conowingo Dam gaug ing station. Uppe r pane l shows annua l m e a n s for individual ca lendar years f rom 1991 t tn 'ough 1999; m e a n s of discharge ( ) and s e d i m e n t load ( ~ ) for enth-e 9-yr p m i o d are indicated. Low- er pane l shows m o n t h l y m e a n s for 1996 (bars) and m o n t h l y m e a n s for the 9-yr pe r iod (lines).

added with even s teeper slopes. T h e calibrations for each cruise were used to calculate TSS h-om turbidity for that cruise alone.

SUSQUEHANNA RIVER F L O W AND ETM PARTICLE

TRAPPINO

Calendar year averaged 1996 Susquehanna River flow and sed iment load were the highest of the 9 years plot ted in Fig. ga. T h e 1996 average flow of 1,800 m ~ s -1 exceeded the 9-yr average by 60% and the 1996 average sed iment load exceeded the 9-yr average by 176%. T h e high annual average flow and sed imen t load were due to several factors. T h e most i m p o r t a n t by far was an e n o r m o u s f lood event dur ing the last week of January, result ing f rom a record snowf:all followed by a d rench ing rain. This flood resulted in approx imate ly 9 • 109 m s of freshwater and 1.5 • 109 kg of sed iment en- tering the Bay f rom the Susquehanna River in ap- proximate ly 9 wk (Zynjuk and Majedi 1996). T h e highest flow rates were approx imate ly 10 times the J a n u a r y average flow and the total sed iment loading was approx ima te ly 17 times the total for an

g

cD E3

S F S F SF SF SF S F S F E

_ . . . . = _ . , q _ . . * . ~ _ - - - . .

Salinity [PSU] Feb 1 1996 ".

2 4 ' ! -q . . . . . . . . . . . . , -

1 6 :

TSS (mg I d) Feb 1; 1996

2 4 I I ........ = r - - 10 20 30 40 50 60 70 80

Distance from Havre De Grace, [km]

Fig. 4. Salinity a n d TSS con tour plots of axial CTD sm'veys on February 1. U p p e r pane l shows salinity (psu) con tours and lower pane l s show filled TSS (rag 1- 0 contom-s. Distance fl 'om h e a d of bay at Havre De Grace (1,an) is shown on the abscissa. CFD data po in t s (+) and phase of tide (F = flood, E = ebb, SF slack before flood, a n d SE slack before ebb) are indicated on the salinity contom- plots.

average January. Flow and sed iment loading during the rest of the w-inter, spring, and most of the s u m m e r were not remarkably different f rom average condit ions, with 20% h igher freshwater flow and 20% lower sed imen t loading (Fig. Sb). A difference in 1996 was that the winter-spring h-eshet was spread out un i formly dur ing the mon ths of February-May, ra ther than peaking in March and April as usual. T h e last 4 mo of 1996 were m u c h wetter than usual w-ith ] 40 % higher h-eshwater flow and 370% higher sed iment loads than average.

Figure Sa and b are plot ted such that flow and sed iment loading bars of equal he ight would cor- r espond to a month ly average TSS concent ra t ion of 60 mg 1-1 in inflowing Susquehanna River waters. T h e bars are se ldom of equal height , however, reflecting the non l inea r re la t ionship between flow and sed imen t loading of the Susquehanna basin. Low flows yield very low sed iment loads, and high flows yield very high sed iment loads. Inflow-ing Sus- q u e h a n n a TSS concent ra t ions averaged 219 mg 1 1 in J anua ry 1996, 20 mg 1-1 in March, 32 mg 1-1 in May, 11 mg 1 -I inJuly, and 42 mg 1 -I in October . T h e average for 1996 was 65 mg 1 1.

Axial distributions of salinity and TSS observed on February 1, at the tail end of t h e J a n u a r y flood, are sho~m in Fig. 4. These r ep resen t the most ex- t reme TSS distributions observed in 1996. O n Feb- ruary 1, the J a n u a r y flood a p p e a r e d to have changed the u p p e r gay f rom a partially mixed estuary into a salt wedge estuary. Freshwater was be-

Chesapeake Bay ETM 661

Fig. 5.

SE F F F F F F F F F SF E E E E E E E SF F F F . . . . . . . I I I ' . ~ ; I . I

. . . . . . . . .... I iit . • -~ : . + ~ . ~ ~ . - * ~ - _ $ , , ' " + . ; . _ . - . . _

- i . . . . ~ : ~ " . . . . . . . . . . . . . , . ' ~ . I

16 I S&lioity, [PSIJ] Oct. ;'2, 1996 " ~ _ : i 1 Sal ini ty [PSU] Oct . 27, 1996 : ' (

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a ,.! .. _._-. -eo :/" '*o :.: .... _- ' - = 7 . =n " " : . . . . . . E

16 TSS (m 9 14) Oct. 22, 1996

24. I ' 10 20 30 40 5'0 60 70 80

Sal in i ty a n d TSS c o n t o u r p lo t s o f ax ia l CTD surveys

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�9 TSS (mg 14) Oct. 27, 1996

I " I ' ! i 10 20 30 40 5'0 Bo ro 80

Distance from Havre De Grace. [km]

on O c t o b e r 22 (left) a n d O c t o b e r 27 ( r igh t ) , as in Fig. 4.

ing advec ted seaward in a th in lens overlying a very sharp pycnoc l i ne at 4 -6 m depth . TSS concen t r a - t ions o f 80 -60 m g 1-1 were be ing carr ied seaward in this f resh sur face layer. T h e initial surge o f the f lood f r o m J a n u a r y 20 -22 may have p u s h e d salt ou t o f the u p p e r Bay, bu t if so the salinity s t ruc tu re had r e b o u n d e d s t rongly by F e b r u a r y 1. T h e resul t was an in t rus ion of salt water m o r e p r o n o u n c e d than any o the r we obse rved in 1996. T h e lower layer was relatively clem-, excep t for a weak ETM- like nero--bottom TSS m a x i m u m be tween ki lome- ters 14-80 n e a r the l imit of salt. T h e major i ty o f the f lood sed imen t s a p p e a r to have b e e n escaping the u p p e r Bay, at least over the channe l . T h e r e were no c u r r e n t data col lected du r ing this pe r iod , so d i rec t est imates of s e d i m e n t t r anspor t rate were no t possible. T h e turbid surface layer was d e e p e r than the average d e p t h of the ETM zone , such that s e d i m e n t may have b e e n depos i ted over the shal-

70

6O

C =. 50 g g 40 m

._c 30

20

=o 10 8

0

<0.02 0.02 0.06

;~

. . . . . . l i ] .... 0.14 0.27 0.53 1.26

Settling Velocity, m m s-"

U �9 2.97 8.84 15.40

~ O c t o b e r , 193mg 14 ~ July, 224mg I~ F ] May, 88mg 14

Fig. 6. Settling velocity distributions from modified Owen Tube casts on May, July, and October TIES cruises. All casts were taken near bottom, in tile area of the ETM, dm-ing tile ETM lateral transect. Slightly negative values at high settling velodties indicate errors in sample collection or analysis, or the accu- muiated error of the nm~erical analysis.

low shoal areas ad jacen t to the channe l . S e d i m e n t cores were col lec ted in 8 d i f ferent loca t ions on F e b r u a r y 1 and March 13, a nd they Failed to iden- tify any clear ev idence of newly depos i t ed f lood sed imen t s (Ha lka u n p u b l i s h e d data) .

In cont ras t to the fate o f the J a n u a r y f lood, the O c t o b e r 22 and 27 surveys p r e s e n t e d in Fig. 5 show a case of what appea r s to be very eff icient ETM s e d i m e n t t rapping. Heavy rains in the u p p e r pa r t o f the S u s q u e h a n n a watershed several days p r io r to the first survey resul ted in a large pulse o f freshwater flow that p e a k e d at jus t over 5,000 m s s -1 on O c t o b e r 21 and 22. T h e O c t o b e r 22 survey caugh t this inf low and its associated s e d i m e n t load as the}, e n t e r e d the u p p e r pa r t o f the ETM zone. A l t h o u g h a weak ETM was c e n t e r e d at km 50,.just above the limit of salt, the h ighes t TSS c o n c e n t r a t i o n s were d is t r ibuted near ly u n i f o r m l y t h r o u g h the water col- u m n above km 20 in the inf lowing freshwater. O n O c t o b e r 27, the salt c o n t e n t o f the u p p e r Bay was marked ly lower, t h o u g h the l imit of salt was at al- mos t the same loca t ion as on O c t o b e r 22. TSS concen t ra t ions above km 20 had fallen to b a c k g r o u n d lm, els, bu t a p r o n o u n c e d ETM was c e n t e r e d at km 41 with nea r -bo t t om TSS c o n c e n t r a t i o n s > 500 m g 1 1. It is qui te likely that this large pool o f suspended solids in the lower layer of the c h a n n e l resul ted f r o m di rec t t r app ing of the h igh s e d i m e n t loads en t e r ing the Bay a few days before .

T h e obvious ques t ion is why the massive J a n u a r y f lood s e d i m e n t load appea r s to have largely escaped the ETM, but the O c t o b e r s to rm s e d i m e n t load a ppe a r s to have b e e n efficiently t rapped . T h e likely answer is shown in Fig. 6, which summar i ze s the results of sett l ing tube e x p e r i m e n t s carr ied ou t du r ing each of the th ree TIES cruises. T h e s e re- suits are p r e sen t ed in terms of the TSS concen t r a - t ion in each of 10 sett l ing veloci ty classes. All th ree sett l ing samples were drawn f r o m relatively h igh

662 L.P. Sanford et al.

60

5 0 : -

- r '~ 4 0 -

SE

SF

E

E

F

T S F i

1 0 , �9 ' 01-Feb 15-Mar02-May06-May 21-Jul 25-Jul 22-Oct 27-Oct

Survey Date in 1996

~ 1 t contour ~ Center of ETM psu sa

Fig. 7. Rela t ive p o s i t i o n o f t h e 1 p s u i s o h a l i n e a n d t h e cen - t e r o f tl~e E T M f o r tl~e e i g h t sm 'veys m a d e in 1996. P h a s e o f t ide (F f l o o d , E e b b , SF s l ack b e f o r e f l o o d , a n d SE s l ack b e f o r e e b b ) a t s t a t i on c loses t to t h e E T M is i n d i c a t e d .

TSS concent ra t ion near -bo t tom water at the center of the ETM, and all three had approx imate ly the same concent ra t ion of background TSS (settling slower than 0.02 m m s-l) . Discount ing this very slowly settling material , the May and July samples had approx imate ly the same med ian settling speed (0.S m m s 1), while the Oc tobe r sample had an order of magn i tude h igher med ian settling speed ( g m m s-l) . Referr ing to Fig. 2, at the concentration of the settling tube samples the slopes of the calibration lines increased slightly f rom May to July and m o r e than doubled to October . T h e implica- tion is that the O c t o b e r sample conta ined larger particles that settled at m u c h higher rates than ei- ther the May orJu ly sample. By extension, the Feb- ruary TSS must have been relatively fine material , settling at m u c h slower rates than in any of the other surveys. We infer that t h e J a n u a r y f lood sed- imen t load escaped the ETIVl because it settled too slowly and the Oc tobe r s torm sed imen t load was efficiently t rapped because it settled quite rapidly (but not rapidly enough to deposi t before it r eached the ETM).

COVARIABILITY OF THE ETM AND THE LIMIT OF S A L T

From the example salinity-TSS distr ibutions presented in Figs. 4 and 5, it is appa ren t that the ETM in u p p e r Chesapeake Bay tended to be associated with the limit of salt intrusion. It is also a p p a r e n t that this association was not perfect . T h e center of the ETM was approx imate ly 7 km up-Bay of the limit of salt on Oc tobe r 22, but 6 km dovm-Bay of the limit of salt on Oc tobe r 97. This canno t be expla ined by the tidal resuspens ion lag discussed

E E E L L L L L ,~ff 8 ~ E E E E E E SF F F F

' ~ ~ ~ 7 . _ ? . . - ' : ' 7 ~ i : . - , . . . . . . . . ~: . . . .

' $~1 n ty. [PSUI Jlliy 2t , 1~6 " ]

"% z4 s

a : '.a0 " -2~-.- % - . a ' ... . s0- ,~ ; '~ : .2m.~ 'q - ' r ." _ ~ .

1 6 IS8 (rag I 1 ) July 21,1996 i l i S (m~ I 1 ) July 25-26,19N

24 10 80 10 20 30 410 5'c 50 70 80 2o 3a 43 50 ,60 vO

Distance Iic~,= Hawe t)e Cra(;~, IkmJ

Fig. 8. Sa l in i ty a n d TSS c o n t o u r p l o t s o f ax ia l C T D sm'veys o n . J u l y 21 (left) a n d . J u l y 2 5 - 2 6 ( r i g h t ) , as in Fig. 4.

by Dyer and Evans (1989) for which the limit of salt would lead the ETM in the direct ion of tidal flo~: In the presen t examples, the opposi te was true; the limit of salt appears to have lagged behind the ETM in the direct ion of tidal flow in bo th s u r v e y s ,

In order to investigate the covariability of the ETM and the limit of salt further, we est imated the posit ion of both for all 8 axial surveys carried out in 1996 (Fig. 7). T h e results show that the two tended to covary on a seasonal basis. O n average, the limit of salt was 3 km ups t ream of the ETM. T h e limit of salt r e sponded as would be predic ted based on the pa t t e rn of month ly averaged freshwater flows (Fig. 3). It was fur ther down-Bay in high flow mon ths and fur ther up-Bay in low flow months , with the except ion of the February 1 survey after the J a n u a r y flood (discussed above). T h e center of the ETM and limit of salt were usually not coincident . T h e average difference between the two was 7 kin, approx imate ly the same as a typical tidal excursion in the u p p e r gay, but the direct ion of the difference was unre la ted to the phase of the tide dur ing the survey. If the ETM was caused by the convergence of the gravitational circulation alone, it would always be coincident with the limit of salt. As stated above, a tidal resuspens ion lag a lone would result in the salt l imit always leading the ETM in the direct ion of the tidal flow. Ne i the r of these predic t ions is b o r n e out by the data, but why?

T h e answer is likely related to daily or weekly variabilitjr of ETM dynamics caused by f luctuat ions in freshwater flow, s ed imen t loading, and a tmo- spheric forcing, variability that was imperfect ly re- solved by our sampl ing scheme. T h e beg inn ing and ending surveys f rom the July cruise (Fig. 8) illustrate the consequences of this variability. On July 21 the center of the ETM and the salt limit were nem-ly coincident at km g0 on an ebb tide. On July" 95-96 the limit of salt was in a lmost the same location, but the center of the ETM was 6 km fur ther ups t ream, again on an ebb tide.

Fortunately, a CBOS moor ing was in place on

' f .... i t .... ..... .... o.5 looo % d

O[ - - waterlevel . . . . . . ] 5 0 0 a=~) / . . . . . discharge / g

-0 .5" . . . . . . . . . . . . . . /o -5

~"~" lO[ . . N . . . . . . . . . .

~ - 1 o ~ . . . . . . . . . . . . . . .

1 D 0.8 (,3 P-- 0.6

% 0.2 1/1

o 20

2 . 5 m . . . . . 10.5m , !',

i I

/i~ tl I

21 22 23 24 25 26 27 28 July 1996, [days]

Fig. 9. Tide, river disd~m-ge, wind, and salinity time sm%s for July cruise. Upper panel: 84 h low-pass filtered water level, in meters above MLLW, for hom'ly tide data fl-om Tolchester Beach, Maryland. The daily discharge f rom the Susquehanna River through the Conowingo Dam is also shown. Middle panel: vector plot of winds (oceanographic convention) fi'om CBOS buoy near Howell Point. Lower panel: salinity thne series fi'om the uppe r (2.5 m) and lower (10.5 m) cm'rent meters at CBOS buoy site; t imes of passage of initial and final CTD surveys m-e indicated by (1), and (2), respectively.

the ea~stern side of the channel at km 20 throughout this time period, with current meters and T / C sensors at 2.5 and 10.5 m depth and an ane- momete r at S m above the water surface. The salinity time series from this moor ing, wind, low-pass filtered water level at Tolchester, and daily Susque- hanna River flow are shown in Fig. 9. Between the two surveys it appears that the limit of salt had moved upstream to almost km 20 (note the salinity of 0.8 at 10.5 m depth on July 25) and was in the process of moving back downstream when it was observed early on the m orn ing of July 26. The ex- act cause of this short term intrusion and retreat is not obvious. It may have been a delayed depth- dependen t response to southward winds before July 24 followed by northward winds from July 24- 26. It may have resulted from the barotropic filling and emptying of the upper Bay apparen t as the low frequency rise in water level that peaked on July 25 and began falling on July 26. It may have been associated with the decreasing Susquehanna River flow starting on July 99. It may have been some combinat ion of all of the above. It is apparen t that the limit of salt moved up the channel and back approximately 10 km over a 4-6 d period. Signifi-


cant non-tidal current fluctuations in this same period band were repor ted by Elliott et al. (1978) from a moor ing at the same location.

This rapid non-tidal movemen t of the salt front, when added to or subtracted from regular tidal movement , most likely led to the observed sepa- rations of the ETM and the salt limit. The mechanism was most probably a suspended sediment transport lag, consisting of both a resuspension lag (due to the finite bot tom stress required to erode deposited sediments and the finite upward mixing time of eroded sediments) and a transport lag (due to the preferential concentra t ion of rapidly settling ETM particles near the bot tom in slower moving water). The salt limit and the ETM started out together on July 21, The salt limit moved rapidly up the Bay from July 92-95 and the ETM followed behind. The salt limit subsequently began to move back down-Bay to its July 21 location, once again lemring the ETM behind but now in an up- Bay direction. Our surveys caught two snapshots of a continuously and rapidly changing environment , rather than two steady-state distributions,

TEMPORAL VARIABILITY AT THE ETM GENTER

The full spatial and temporal variabilities of the 25-h lateral transect time series observations during each of the TIES cruises are too complex to discuss in this overview paper, Suffice it to say here that we observed significant lateral variability a~s- sociated with the changing phases of the tide and the steep channel-shoal topography, that undoubt - edly play a role in channel-shoal exchange processes, We believe that axial dynamics in the channel dominate particle trapping processes in the Chesapeake Bay ETM because the ETM is so clearly associated with the limit of salt and because salt t ransport is primarily confined to the axial channel. We present and discuss only the time series from the center channel station of the lateral transect carried out on October $4-25, 1996 (Fig. 10). This transect was chosen because, on average, it was situated almost exactly at the limit of salt.

The time series began as ebb tide was decreasing towards slack before flood. The water column was unstratified and nem-ly fi-esh, since the salt limit was almost at its maximum down-Bay excursion. As the tide turned and flooded, the salt limit was advected past the transect location as a sharp near- bot tom front. At slack before ebb near-bottom sa- linities were > 4 in a thin bot tom mixed layer be- neath a sharp pycnocline. The upper water c o h m n remained unstratified and nearly fresh. The salt limit was advected down-Bay past the transect location on the succeeding ebb tide. The entire se- quence was repeated one more time before the end of the time series.

6 6 4 L . P . , S a n f o r d e t al.

Salinity O.{ ~ I ;_ : ...... I , _ . . . ~ . i , i . : . ~

+ § ~ ?+ i- .i :~ - - =-T. -' ' _ 2{._: ::_ I 44 ;

1 2 5 . . . . . . ~ " " " I ~ . . . . r " ' ' r - , i - - T ' ' I [ "

TSS (mg I " 1 )

0 [ " " , ' . , , I . , I I . . . . . . . , " -

4 �9 : ':' ,- ' ' . " w , , ! , : , ~ , . ~ o . ~ , , ' . - x : 6 1 . . , , ~ . - ' , , , !

Axial Velocity (cm s -~)

/ ' , . ' ,1[' ~ I 2 " ' , | " ' : I

4 I . , / ' l i , i l ' ' / ' " i ' , I I

/ ~ , ~ ,, , * a , g '. N o N ~ ' I " 6 ! , , o ~ . ,. i , I ~. . . . .

8~, i I

1 2 1 " [ . . . . . " ~ I " i , = I " '

14 16 18 20 22 0 2 4 6 8 10 12 14 16

Time of Day, [hours, ED-I] Fig . 10. T i m e - d e p t h c o n t o u r s o f s a l i n i t y , T S S , a n d a l o n g -

d ~ a n n e l c u r r e n t s f r o m fl~e E T M d ~ a n n e l s t a t i o n o n O c t o b e r 24- 25. U p p e r p a n e l : s a l i n i t y c o n t o m ' s ( p s u ) . M i d d l e p a n e l : f i l l e d T S S c o n t o m - s ( r a g 1-~). S a l i n i t y a n d T S S m-e f r o m C T D c a s t s

t a k e n a t t h e c e n t e r c h a n n e l s t a t i o n d u r i n g t h e E T M l a t e r a l t r a n - sec t . C T D d a t a p o i n t s a r e i n d i c a t e d b y ( + ) . L o w e r p a n e l : a l o n g

c h a n n e l c u n ' e n t s ( p r o j e c t e d a n g l e 195 ~ c m s 1) f r o m s u c c e s - s ive A D C P p a s s a g e s a c r o s s t h e c h a n n e l s t a t i o n . P o s i t i v e v e l o c i t i e s

( ) a r e e b b a n d n e g a t i v e v e l o c i t i e s ( ~ ) a r e f l o o d .

T h e stratification variat ions imposed by the passage of the salt limit had clear consequences for sed iment resuspension and velocity shear. Local resuspension is mos t clearly indicated by near-bottom increases and decreases of TSS in phase with the near -bo t tom velocity. Local resuspension was quite p r o n o u n c e d at m a x i m u m flood tide and m u c h weaker at m a x i m u m ebb tide. Upward mixing a p p e a r e d to have been limited by the pycnocline. T h o u g h it is possible that some of the TSS high in the water co lumn at m a x i m u m ebb resulted f rom upward mixing th rough the pycnocline, it is m o r e likely that this mater ial resul ted f rom re-

suspension ups t ream, rapid downst ream advect ion by the strongly sheared ebb currents, and rapid diffusion th rough the well-mixed u p p e r water column. T h e slightly elevated above-pycnocl ine TSS concent ra t ions on flood are probably the r e m n a n t of this material . Stratification and gravitational circulation nea r the salt limit affected the observed velocity s t ructure by increasing vertical shear and raising the height of the velocity m a x i m u m on ebb, while decreasing vertical shear and lowering the height of the velocity m a x i m u m on flood. This pat- tern is mos t obvious in the ebb at 00:00 on Oc tobe r 95 and the f lood at 06:00 on Oc tobe r 95. T h e gravitational circulation also caused the nero--bottom currents to turn to flood sooner and to ebb later than the near-surface currents.

D i s c u s s i o n

While the results presented he re conf i rm the ba- sic descript ions of the Chesapeake gay ETM of- fered by Schubel (1968a, b) and Schubel and Prit- chard (1986), they refute the idea that the convergence of the gravitational circulation is pr imari ly responsible for its fo rmat ion , with tidal resuspension mere ly modula t ing TSS concent ra t ions (Schu- bel 1968a,b; Festa and Hansen 1978; Officer 1980). If this earlier explanat ion was correct , then the ETM would not b e c o m e separa ted f rom the limit of salt, as we observed. T h e range of part icle settling speeds t rapped by the gravitational circulation in the mode l of Festa and Hansen (1978) was 10-100 times smaller than the settling speeds we observed. T h e highest settling speeds t rapped in Geyer 's (1995) improved ETM mode l combin ing gravitational circulation with stratification d a m p e d mixing (but no tidal currents) are at the low end of the range of our observat ions as well.

Our data indicate that asymmetr ical tidal resuspens ion and asymmetr ical tidal t ranspor t of rapidly settling aggregates are primari ly responsible for the Chesapeake Bay ETM, as illustrated schemati- cally in Fig. 11. Tidal suspended sed iment trans- por t is biased in a net downst ream direct ion above the limit of salt and in a ne t ups t ream direct ion below the limit of salt, leading to the fo rmat ion of a pool of resuspendable particles near the limit of salt. T h e axial convergence of the gravitational circulation and the associated salinity s t ructure near the limit of salt are the major causes of these tidal asymmetr ies , but without tidal resuspens ion the rapidly settling aggregates would likely r emain on the bo t t om where the), were initially deposited, the c o n c e n t r a t e d p o o l of r e s u s p e n d a b l e pa r t i c l e s would not be fo rmed , and the ETM as such would ei ther not exist or be greatly weakened. T h e resus- pendab le particle pool lags beh ind the mot ion of the salt limit because of the resuspension lag de-

a) Full Ebb Tide

~54S{~!g'- ~ . .r ~ "~ } ~ . ~ r ~"~ ~ : : . ~ , . ~ : : �9

b) Slack before Flood

a,

~ ' ~ ~:.~La,'t~J ' ~., ,a~

c) Full Flood Tide

~ , �9 ~ : ~ ... : . .

~ n ~ ' [ ! . ~a,, , ...~....~....:.:.,.!~,~;..

d) Slack before Ebb I I I I I I I n l l l

e) Flood Tide + Seaward Wind

Fig. 11. Conceptual diagram of particle ti-appmg in the Ca~esapeake Bay ETM. The river is to the left and the sea is to the right. The hear T curved line represents the 1 psu isohalme, the arrows represent cm'rent vectors, and the shading represents suspended sediment. The strength of the gravitational ci> culation (GC) is exaggerated for pro-poses of illusn'ation, a) At full ebb tide, the GC enhances ebb cm'rents upsti-eam of the salt front, resulting in resuspension of sediments and zooplankton high into the unstratJSed water column and advection downstream above the pycnocline. Some settling through the pycnodine occm's. The GC opposes ebb cm'rents below the salt front, such that only settling occurs, b) At slack before flood,


scr ibed by Dyer (1988) a n d because the near -bot - tom cen t e r of mass of the rapidly se t t l ing su spend - ed par t ic les moves m o r e slowly t h a n the water above it. T h e wind- forced s epa ra t i on of the ETM a n d salt l imit sho~m in Fig. 11 is s imi lar to the f reshwater flow lags of the ETM b e h i n d the salt l imi t obse rved in the Tamm- a n d Weser es tuar ies by G r a b e m a n n et al. (1997).

Th i s e x p l a n a t i o n of the C h e s a p e a k e Bay ETM is also m o r e in l ine with r e c e n t e x p l a n a t i o n s of ETM f o r m a t i o n in o t he r estuaries. In a lmos t every case, a spatially l imi ted poo l of r e s u s p e n d a b l e par t ic les is the key factor. T h e m e c h a n i s m s r e spons ib l e for f o r m a t i o n of this par t ic le pool vary f rom es tuary to estuary, s o m e t i m e s d e p e n d i n g on n o n l i n e a r tidal p u m p i n g (Jay a n d Musiak 1994; Unc le s et al. 1998; g r e n o n a n d Le Hi r 1999; G u e z e n n e c et al. 1999), s o m e t i m e s on a t idal r e s u s p e n s i o n lag (Dyer 1988; Dt, er a n d Evans 1989; H u g h e s et al. 1998), s o m e t i m e s on tidally i n d u c e d t o p o g r a p h i c trap- p i n g or tidally va ry ing c h a n n e l - s h o a l e x c h a n g e (Geyer et al. 1998), etc. In par t ia l ly m i x e d es tuar ies like C h e s a p e a k e Bay, a symmet r i c t idal t r an spo r t n e a r the l imi t of salt is of ten the r e spons ib l e mech- a n i sm ( H a m b l i n et al. 1988; g u r c h a r d a n d gau - i ne r t 1998), bu t the c o n v e r g e n t c i rcu la t ion and as- socia ted salinity s t r uc t u r e tha t cause the tidal as3,m- me t r i e s a re specific c o n t r i b u t i n g factors, n o t un i - versal r e q u i r e m e n t s for ETM f o r m a t i o n . I n d e e d , the 1999 C h e s a p e a k e Bay ETM s t r uc t u r e r e p o r t e d by N o r t h a nd H o u d e (2001) is be low the l imi t of salt a n d a ppe a r s to be closely associated with rapidly c h a n g i n g t o p o g r a p h y n e a r the u p p e r end of the ETM zone .

Given ou r emphas i s on tidal asymmetry, a discuss ion of its effects on our survey data a n d ETM pos i t ion est imates is a p p r o p r i a t e . T h e r e are essen- tially th ree effects. First, the phase of the tide shou ld affect the a m o u n t a n d loca t ion of resus- p e n d e d s e d i m e n t in the water c o l u m n , as i l lustrat-

only the GC rema ins and previously r e suspended sed iments have set t led to fl~e bot tom. Stl"atification is m a x i m m ~ below the salt front, c) At full f lood tide, the GC enhances f lood cm'rents below the salt f ron t and causes resuspens ion of sediments , bu t only to the h e i g h t of the pycnod ine . The GC opposes f lood tidal cmTents above fl3e salt front, with no resuspension, d) At slack before ebb, only the GC remains and previously resus- p e n d e d sediments have set t led to the bot tom. Stratification is m i n i m u m , e) A seaward x,vind circulat ion added to f lood tidal currents and the GC fllrther enhances resuspension below the salt front, but it also moves the toe of the salt fl'ont landward faster than the ETM and results in a temporm'y sepal"ation between salt and suspended sediments. The enhanced lower layer drculation is lagged behind the onset of the wind by the time required to establish an opposing sm'face slope m the upper Bay.


ed in Fig. 11. Second, tidal straining of the density s t ructure should result in changes in the slope of the salt front, also as illustrated in Fig. 11. Third , tidal resuspension lags should result in the ETM lagging the salt front, r emain ing slightly seaward at the end of flood and slightly landward at the end of ebb. T h e r e are some indications of these effects in the surveys p resen ted here. T h e ebb tide surveys of July 91 (Fig. 8) and O c t o b e r 97 (Fig. 5) show higher concent ra t ions distr ibuted t h r o u g h o u t the water co lumn ups t ream of the salt f ront and ex- tended slightly downst ream in the surface layer, as in panel a of Fig. 11, a l though the expected intratidal dif ferences are overwhelmed by differences in freshwater flow. sed iment loading, wind forcing, and tidal cu r ren t s t rength between the surveys. In fact, in five out of the eight cases shown in Fig. 7, correct ing for the expected tidal lags would increase the separa t ion between the ETM and the salt limit ra ther than decrease it. Tidal effects are impor t an t at intratidal t ime scales (Fig. 10), but longer- term, larger scale effects domina te over times scales of days to months .

T h e res idence time of terrestrial particles in the ETM pool must be finite, or ETM TSS concentra- tions would continually increase. In some estuaries (e.g., the Co lumbia River), the ETM represents a delay be tween the delivery of terrestrial mater ial f rom the river and its ejection to the coastal ocean, with little ne t accumula t ion in the bo t t om sediments (C rump and Baross 1996). In others, accumula t ion in the sed iments and accret ion of the bo t tom is the ul t imate fate of terrestrial material . T h e r e is a b u n d a n t evidence that sed imenta ry accumula t ion dominates in Chesapeake Bay. T h e upper Bay is shoaling at a m u c h h igher rate than the mid-Bay (Officer et al. 1984; Co lman et al. 1992). Estimates range between 0.S-1.2 cm yr -1 (Officer et al. 1984; Kerhin et al. 1988; D o n o g h u e et al. 1989; Halka unpubl i shed data). O t h e r estimates indicate that the ETM region of Chesapeake Bay per- manent ly traps between 70% (Biggs 1970; Schubel and Pri tchard 1986) and 100% ( D o n o g h u e et al. 1989) of the terrestrial mater ial delivered f rom the Susquehanna River. Exactly when and how per- m a n e n t sed imenta t ion occurs is not known, but we speculate that spr ing-neap variability in tidal currents may be an impor t an t factor in the ETM channel. All of the 1996 TIES ETM cruises were de- signed to be centered a round spring tides, when tidal currents and tidal resuspension were at a max imum. Neap tidal currents in the u p p e r Bay are approx imate ly 30% weaker than spring tidal currents. This may be jus t enough of a decrease in energy to allow a substantial por t ion of the resus- pendab le particle pool to consolidate, resist sub- sequent erosion, and be bur ied unde r newly deliv-

ered material . Over the shoals, net sed imenta t ion is m o r e likely control led by the t iming between delivery events and wave-forced erosion events (San- ford 1994).

Sed imenta t ion rates in the ETM shipping channels are remarkably high compared to the adjacent shoals, where all of the u p p e r Bay sed imenta t ion rates cited jus t above were collected. It is not possible to estimate sed imen ta t ion rates in d redged sh ipping channels by convent ional means, but the dredging records themselves may be used to obtain an a p p r o x i m a t e value. Annua l ma in t enance dredging records f rom the u p p e r 48 km of the shipping channel leading f rom Bal t imore to the Chesapeake and Delaware Canal (27 km of which fo rm the channel of the u p p e r par t of the ETM zone) have been compi led by the Maryland Geological Survey since 1986 (Panageo tou et al. 1998). T h e autho- rized width of the channel is 1S7 m, which gives a total s ed imen t surface area of 6.6 • 100 m 2. T h e annual average dredged v o h m e is 1.1 • 10 ~ m s, which gives an annua l m~erage sed imenta t ion of 17 cm, 14-55 times the annual sed imenta t ion rate on the adjacent shoals. This focusing of sed imenta t ion into the channel may" be due to redis t r ibut ion fi-om the shoals to the channe l by s torm events (Sanford 1994), focusing of initial deposi t ion into the channel by unknown lateral t ranspor t processes, or up- Bay t ranspor t of mater ial f rom below the ETM zone. We also note that there is an appa ren t cor- relat ion between annual f luctuat ions in sed iment loading f rom the Susquehanna and m a i n t e n a n c e dredging vo lume dur ing the following winter (not shown), implying that the focusing process may occur quickly'.

Two of our impor t an t results are related to sed- imen t settling velocities; settling velocities of resus- p e n d e d TSS f rom the Chesapeake Bay ETM were high relative to the settling velocities of disaggregated silt and clay particles, and settling velocities of ETM particles increased significantly f rom Feb- ruary th rough May and July" to October . Both results were based primari ly on settling velocity distr ibutions measured using a modif ied Owen settling tube, in combina t ion with changes in the turbidity-TSS calibration relationships. T h e r e have been quest ions raised in the l i terature abou t the accuracy and repeatabil i ty of the settling tube tech- nique, which we would be remiss to ignore. Dear- naley (1997) showed that floc break-up, re-floccu- lation, and internal circulat ion within a settling tube had the net result of reduc ing settling speed estimates relative to direct video techniques. Dyer et al. (1906) also indicated that, relative to video techniques, settling tubes t ended to underes t ima te settling velocities, t hough careful control of sampling protocols gave comparab l e results between

different tube designs. Hill and Milligan (1999) ar- gued that the artii:acts induced by floc breakup and reflocculat ion can explain the apparen t concentration de pe nde nce of settling velocity as observed in settling tube measurements (Burt 198(3).

While we cannot state categorically that these problems did not affect our measurements , there are several factors that indicate such problems may not have been as serious in our case, or at least that the settling effects that we observed were qualitatively correct. First, we observed internal circu- lations in our settling tube exper iments in previous trials, that were related to convection caused by tempera ture differences between the sample and the external environment . We devised our temper- ature control water jacket to minimize this prob- lem, and did not use any data fi-om exper iments with visible convection cells. Internal circulation tends to homogenize the sediment suspension inside the tube, lowering the median settling velocity estimate, which may explain some of the effect described by Dyer et al. (1996). It is also possible that the video techniques that give h igher median settling speeds do not resolve the fine, slowly settling fi-action of the particle populat ion, such that vi- deographic median settling speeds are biased upward. We did not observe a concentra t ion depen- dence of settling speed in our samples; two samples at almost the same concentra t ion (October and July) had very different settling velocity distributions, while two samples at very different concentrat ions (May and July) had very similar median settling velocities. The relative differences between our samples cannot be due to a bias intro- duced by different initial TSS concentrat ions. Th e relative settling speeds of our samples are consistent with the relative slopes of the accompanying turbidity-TSS calibration lines, assuming that more rapidly settling particles represent larger aggregates of similar composit ion. We have found in previous model ing work that a settling velocity of approximately 1 m m s -I is requi red to match observed resuspension-deposi t ion cycles in upper Chesapeake Bay (Sanford and Halka 1998; Sanford and Chang 1997), very close to the average of the three settling velocities quoted here.

We believe that the median settling velocities de- t e rmined Dom our settling tube measurements are reasonable, that the relative increase in settling velocity f rom May to October is real, and that the implied lower settling velocity of the January flood material is real. Such seasonal changes in settling speed have impor tan t consequences. They may explain why there was very little evidence of sediment trapping in the Chesapeake Bay ETM following the January 1996 flood, while Hur r i cane Agnes in June 1972 left approximately 75% of its sediment load


in the ETM zone in a layer approximately 0.9 m thick (Schubel and Pri tchard 1986). Th e relatively efficient t rapping after Agnes may have been because particles were more highly aggregated and settled more rapidly. We will never know if this is true, but it is plausible and implies that the sedi- mentological consequences of major events may depend on when the}, occur.

Seasonal changes in settling speed are most probably due to biogenic changes in the stickiness or packaging of the aggregates. Schubel and Kana (1979) found that zooplankton fecal pellets were impor tan t agents of particle agglomerat ion in upper Chesapeake Bay, and zooplankton activity is clearly seasonal. Jahmlich et al. (1999) also found increasing aggregate size related to t ransparent ex- opolymer particles and bacterial cell abundance in the bot tom bounda ry layer of a bight of the Baltic Sea, and Z immermann-Timm et al. (1998) found that the largest aggregates in the Elbe Estuary oc- curred in spring and summer in association with high particle-attached microbial activity. Data obtained at the mou th of the Susquehanna River for the U.S. Environmental Protect ion Agency Chesa- peake Bay Monitor ing Program indicate seasonal increases of approximately 50% in DOC concen- trations between the winter /ear ly spring and the summer / fa l l during 1994 and 1995. Increased organic loading and increased microbial activity might both lead to increased particle stickiness and increased aggregation.

We acknowledge that our results are tantalizing but preliminary. We have a t tempted to construct a plausible scenario for the controll ing physics of the Chesapeake Bay ETM based on data that is intrigu- ing but limited. We have suggested and discussed specific physical mechanisms that are consistent with the data and more in keeping with recent un- derstanding than previous ideas. We have not tried to prove these suggested mechanisms or explore them quantitatively----to do so would have been to push the data set beyond its limits.

ACKNOVCLED GMENTS

We grateffflly acknowledge the suppor t of the National Sci- ence Founda t ion tl~-ough Gran t No. DEB-9412113. We also t hank W. C. Boicom't for tlae Howell Poin t CBOS buoy data, S. Yochum for the S u s q u e h a n n a s e d i m e n t load data, a n d J . Hagy for providing S u s q u e h a n n a dissolved organic carbon concentration data culled f rom the U.S. Env i ronmenta l Protect ion Agency C~hesapeake gay Program data base. We thank two a n o n ~ o u s reviewers for tlaeir he lpfu l sugges t ions for improv ing tlae paper. Finally, ou r thanks go to the crews of tlae R /Vs Kerhir~, Orior~, a n d Gape He'~*l@e'n for tl~eir he lp d i n i n g tl~e field work. This is University of Maryland Center for Env i ronmenta l Science publication no. 3469.

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Received fc~r cor~side~ration, l!4ay 26, 2000 Acce'pted for publication, lt/la.rch 8, 2001

Reconsidering the physics of the Chesapeake Bay … the Physics of the...Estuaries Vol. 24, No. 5,...

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