Estuaries
Vol. 24, No. 5, p. 655-669
October 2001
Reconsidering the Physics of the Chesapeake Bay Estuarine Turbidity Maximum LAWRENCE P. SANF(__)RD1'*, STEVEN E. SUTTLES 1, a n d JEFFREY R HALICA2
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 o u t in die ~stuarine turbidJty m a x i m u m (ETM) region of Chesapeake Bay in 1996 to examine physical and biological variability and dynamics. A large flood event in late J a n u a r y shifted die salinity structure of the upper Bay towards that of a salt wedge, but most of die massive s e d i m e n t load delh,ered by die Susquehanna River appeared to bypass the ETM zone. In contrast, s u s p e n d e d sediments delivered during a flood event in late October were t r a p p e d very efficiendy in the ETM. T h e difference in sediment trapping a p p e a r e d to be due to increa.s~s in particle settling s p e e d f r o m J a n u a r y to October, suggesting that the fate of sediments delivered during large events may d e p e n d 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) t h r o u g h o u t die year, but it was often separated significantly f r o m 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 r a n s p o r 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 collected near die limit of salt, c o m b i n e d 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 sediment r~suspension and transport d~at collect and maintain a resuspendable p o o l of rapidly settling particles near the salt limit. W i d m u t tidal r~suspension and transport, die ETM w o u l d eidler not exist or be gready weakened. In spite of this repeated r~suspension, sedimentation is the ultimate fate of m o s t terrigeuous material delivered to the Chesapeake Bay ETM. Sedimentation rates in die ETM channel are at least an o r d e r of magnitude greater allan on the adjacent shoals, probably due to focusing mechanisms dlat are poorly understood.
reasons. First, they were technolog~y" limited; Schubel (1968a) obtained a large data set on the distribution of suspended sediment in and near the Chesapeake ETM, but h e r e p o r t e d just a few observations of currents, no salinity data, and no settling velocity data. Second, there were few other r e p o r t e d ETM studies for comparison, and there was a limited u n d e r s t a n d i n g of the dynamical complexities of ETM particle trapping. While Schubel (1968b) recognized the i m p o r t a n c e of tidal resuspension for maintaining high suspended s e d i m e n t concentrations, he and o t h e r researchers (Festa and H a n s e n 1978; Officer 1980) attributed ETM particle trapping primarily to the convergence of the estuarine circulation at the limit of salt intrusion c o m b i n e d with slow particle settling. In the )rears since these initial studies of the Chesapeake Bay ETM, other investigators have shown that mechanisms of particle trapping in ETMs are m o r e 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-bottom currents can p r o d u c e an ETM in the presence of
Ina-oducfion
T h e estuarine turbidity mmximum (ETM) zone of u p p e r Chesapeake Bay, a region of elevated suspended sediment concentrations and reduced light availability near the limit of salt intrusion, was first studied in the late 1960s and early 1970s. T h e most extensive work was carried out by Schubel (1968a,b, 1971), Schubel and giggs (1969), and Schubel and Kana (1972), who m a p p e d the spatial and seasonal variability of the ETM, examined tidal resuspension processes at its center, and carried out studies of suspended particles and agglomerates. giggs (1970) also considered the s e d i m e n t budget of the u p p e r Bay, while Nichols (1974, 1977) examined the dynamics of the ETM in the R a p p a h a n n o c k , a s o u t h e r n tributary of the Chesapeake 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 * C o r r e s p o n d i n g author; tele: 410/'228-8200; fax: 410/'2218490; e-mail:
[email protected]. 9 2001 Estuarine Research Federation
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asymmetrical tidal currents without c o n v e r g e n c e of the gravitational circulation. Geyer (199.3) showed that suppression of t u r b u l e n t mixing by density stratification d o w n s t r e a m of the limit of salt intrusion amplified t r a p p i n g efficiency for a r a n g e of particle settling velocities faster than those proposed by Festa a n d H a n s e n (1978). H a m b l i n (1989) d e m o n s t r a t e d that a c o m b i n a t i o n of ebbflood a s y m m e t r y in s u s p e n d e d s e d i m e n t t r a n s p o r t a n d suppression of mixing by density stratification were likely responsible for particle t r a p p i n g in the St. Lawrence ETM. Jay and Musiak (1994) r e p o r t ed that strong tidal assrmmetries in stratification a n d flow n e a r the salt limit likely explain suspende d - s e d i m e n t t r a p p i n g in the C o l u m b i a River ETM. Uncles and S t e p h e n s (199S) d e m o n s t r a t e d that the location of a pool of r e s u s p e n d a b l e particles defined the location of the ETM in the T a m a r e> tuary, which often was u p s t r e a m of the salt limit. In contrast, Geyer et al. (1998) showed that lateral interactions b e t w e e n t o p o g r a p h y a n d flow maintain a p o o l of r e s u s p e n d a b l e particles, h e n c e an ETM, well d o w n s t r e a m of the salt limit in the H u d son River estuary. Particle t r a p p i n g dynamics p r o b ably differ between estuaries or p r e d o m i n a t e at different times in the same estuary (e.g., spring tides versus n e a p tides). In r e c e n t }rears the ecological role of ETMs in s u p p o r t i n g a n a d r o m o u s fish r e c r u i t m e n t also has b e e n recognized ( D o d s o n et al. 1989; Dauvin a n d D o d s o n 1990), a n d ETMs have b e e n f o u n d to be m-eas of elevated z o o p l a n k t o n concentrations, especially the calanoid c o p e p o d Eu'o~temora affinis (Simenstad et al. 1994; M o r g a n et al. 1997; Kimm e r e r et al. 1998). It is believed that a b u n d a n t food in the f o r m of detritus, protozoa, and phyt o p l a n k t o n , in addition to the ETM particle trapping m e c h a n i s m s described above, s u p p o r t the high zooplankton abundances. Immature and adult Euryte'mora are i m p o r t a n t foods for larval a n d juvenile striped bass a n d white p e r c h in Chesap e a k e Bay" and other estuarine systems (SetzlerH a m i l t o n 1991; Setzler-Hamilton a n d Hall 1991) establishing a potential trophic link that s u p p o r t s fish r e c r u i t m e n t . With the potential ecological role of ETMs a n d the new concepts of ETM particle t r a p p i n g in mind, an interdisciplinary g r o u p of investigators u n d e r t o o k a new study of the C h e s a p e a k e Bay ETM in 1996. U n d e r the aegis of the National Scie n c e F o u n d a t i o n L a n d M a r g i n E c o s y s t e m Research P r o g r a m , T r o p h i c I n t e r a c t i o n s in Estuarine Systems (TIES), we c o n d u c t e d a series of cruises in u p p e r C h e s a p e a k e Bay to explore the ETM feature and biological p r o d u c t i o n a~ssociated with it. Preliminary r e p o r t s on these studies (Boynton et al. 1997; R o m a n et al. 1997) describe i m p o r t a n t
linkages between physics, p l a n k t o n biology, a n d p r o d u c t i o n of y o u n g a n a d r o m o u s fish. M o r e recent r e p o r t s p r e s e n t additional i n f o r m a t i o n on a n a d r o m o u s fish r e c r u i t m e n t (North and H o u d e 2001) a n d z o o p l a n k t o n ( R o m a n et al. 2001). This p a p e r presents an overview of the physics of the C h e s a p e a k e Bay ETM during 1996, with particular attention to its spatial and t e m p o r a l variability, the factors that control particle trapping, a n d relationships between ETM processes and s e d i m e n t a t i o n in u p p e r C h e s a p e a k e Bay.
Study Site Earlier d o c u m e n t a t i o n s (cited above) d e m o n strated that the ETM in C h e s a p e a k e Bay is contained within the u p p e r m o s t p a r t of the estuary (Fig. 1). Its location varies seasonally a n d at s h o r t e r time scales, but it is almost always f o u n d between latitudes 39~ and $9~ a r a n g e of a p p r o x imately 40 kin. This region of the Bay has a m e a n v o l u m e of a p p r o x i m a t e l y 2.6S km s a n d a m e a n d e p t h of a p p r o x i m a t e l y 4.1 m (Cronin 1971), n o t counting the small tributaries. It is incised by a narrow s h i p p i n g c h a n n e l m a i n t a i n e d by d r e d g i n g at a d e p t h of a p p r o x i m a t e l y 12 m, which connects the C h e s a p e a k e a n d Delaware Canal with the Port of Baltimore a n d serves as the p r i m a r y pathway for up-Bay salt intrusion. T h e average a s t r o n o m i c a l tidal r a n g e increases n o r t h w a r d from 0.$6 to 0.5 m, with typical m a x i m u m tidal c u r r e n t speeds of 0.5 m s -1 in the c h a n n e l a n d 0.S m s -1 over the b r o a d shoals (Browne and Fisher 1988). Wind-forced water level fluctuations can be m u c h larger than the a s t r o n o m i c a l tide (up to 1 m in range) due to the 2 d quarter-wave seiche r e s p o n s e of the Bay to the passage of weather systems ( C h u a n g a n d Boicourt 1989; Boicourt 1990), with associated c u r r e n t fluctuations of up to 0.2 m s -I (Elliott et al. 1978). N e t non-tidal gravitational circulation varies strongly in r e s p o n s e to fluctuations in river flow (Elliott et al. 1978), and changes from a riverine, b a r o t r o p i c d o w n s t r e a m flow above the limit of salt to a welldeveloped estuarine circulation below the limit of salt. Almost all of the freshwater flow enters f r o m the S u s q u e h a n n a River. At the average Susquehanna River flow of 1,100 m s s 1 the freshwater rep l a c e m e n t time of the ETM region of the Bay is a p p r o x i m a t e l y 1 mo. M o r e t h a n 80% of the s e d i m e n t entering the u p p e r Bay c o m e s down the S u s q u e h a n n a River, with almost all of the rest resulting f r o m shoreline erosion (Biggs 1970). T h e b o t t o m sediments became gradually finer southward of the broad, sandy delta k n o w n as S u s q u e h a n n a Flats (not shown in Fig. 1 due to s a m p l i n g limitations), t h r o u g h short transition zones of silty sands a n d sandy silts, t h r o u g h the clayey silts that d o m i n a t e
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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. Buoyicon represents CBOS buov location off Howell Point. Depths greater than 7 m are shaded. the ETM region, to the silty clays that d o m i n a t e the b r o a d lower reach of the u p p e r Bay. T h e deep channels of the u p p e r Bay sediment rapidly (Ofricer et al. 1984) and require constant d r e d g i n g to maintain navigable depths. Fringing sandy shelves in the b r o a d lower reach reflect the i m p o r t a n c e of b o t h wave-forced resuspension and shoreline erosion (Kerhin et al. 1988; Sanford 1994). S u s p e n d e d sediments are comprised of silts, clays, a n d aggregates thereof, with a low organic m a t t e r fi'action (Schubel 1968a; Biggs 1970; Schubel a n d Kana 1972). Total s u s p e n d e d sediment (TSS) c o n c e n t r a t i o n s in the entire u p p e r Bay are elevated relative to the rest of tire estuary, with typical b a c k g r o u n d c o n c e n t r a t i o n s of very slowly settling particles between 5-25 m g 1 1 (Schubel 1968a,b, 1971; Sanford et al. 1991; Sanford and Halka 1993; Sanford 1994). These b a c k g r o u n d particles tend to be uniformly distributed t h r o u g h the water c o l u m n or slightly m o r e c o n c e n t r a t e d in the lower water colunm, and they have a h i g h e r organic fraction than the particles that are resusp e n d e d f r o m the b o t t o m (Schubel and Biggs 1969). T h e ETM itself typically has TSS concentrations 20-100 m g 1 1 h i g h e r than tile b a c k g r o u n d , with the largest c o n c e n t r a t i o n s 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 w a t e r s ( S c h u b e l 1968a,b). T h e r e is little spatial or temporal variation in dispersed (disaggregated) particle size dist r i b u t i o n s ( S c h u b e l 1968a; S c h u b e l a n d K a n a 1972), but aggregate sizes increase markedly at m a x i m u m tidal resuspension (Schubel 1971) and
decrease slightly d u r i n g periods of high riverflow (Schubel 1968a). Methods
Five cruises were carried out in the tapper Bay between F e b r u a r y 1 a n d O c t o b e r 27, 1996, including two early h y d r o g r a p h i c surveys of o p p o r t u n i t y and three 5-d interdisciplinary TIES cruises. A total of 8 h y d r o g r a p h i c (CTD) surveys of tile u p p e r Chesapeake Bay were m a d e d u r i n g these cruises. All surveys included 8-11 stations along the axis of the main (eastern) c h a n n e l f r o m just n o r t h of the Bay Bridge (approximately 39~ to Turkey Point (approximately 39~ The February 1 survey used tire R / V Kerhin in c o n j u n c t i o n with a Maryland D e p a r n n e n t of Natural Resources (MDDNR) water quality sampling cruise after a lateJanuary flood event in the S u s q u e h a n n a 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 r e m a i n i n g 6 surveys were c o n d u c t e d as part of three seasonal TIES cruises in 1996 on the R / V Cape Hetdo/)en. Physical m e a s u r e m e n t s d u r i n g 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 C T D / A D C P lateral n'ansect at a location down-Bay f r o m the ETM (Gibson Island transect), and a final CTD survey. A SBE 25 Sealogger CTD was used for the Febr u a r y 1 and March 15 h y d r o g r a p h i c surveys. T h e SBE 25 was e q u i p p e d with an auxiliary 5-cm pathlength transmissometer (SeaTech) as well as an
658
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OB&3 (Doweling & Associates) backscatter sensor for measuring turbidity. T h e CTD was also outfitted 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 fluorometer. T h e surveys began with a lateral transect near the Bay Bridge (8 stations) and then continued up the eastern c h a n n e l of the Bay to the last station n e a r Turkey Point. T h e r e were a total of 10 stations in these surveys, 8 of which were axial. O n the TIES cruises we used the R / V Cape Hen [open Neil Brown CTD m o u n t e d on a General Oceanics rosette, e q u i p p e d with a 5-cm p a t h l e n g t h SeaTech transmissometer, a DO sensor, and a fluorometer. A well p u m p for collecting water samples was again attached to the CTD for calibration of the turbidity sensors. T h e first survey of each cruise (May 2,July 21, and O c t o b e r 92) began with a series of three lateral transects stm-ting just n o r t h of the Bay Bridge and c o n t i n u e d up-Bay to Turkey Point (Fig. 1). These surveys consisted of 18 total stations, 11 of which were axial. T h e location of the ETM was d e t e r m i n e d from an axial salinit}r and turbidity c o n t o u r map of the initial CTD survey for each cruise, p r o d u c e d immediately after the survey was completed. T h e final survey" of each cruise (May 6, July 25-26, and O c t o b e r 27) b e g a n at the n o r t h e r n m o s t station and p r o c e e d e d 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 i m p o r t a n c e for this stud?,. Top to b o t t o m CTD profiles were m a d e at each station, making a special effort to sample as close to the b o t t o m as possible (usually within 0.5 m). T h e lenses 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 d e t e r g e n t and DI water prior to each h y d r o g r a p h i c survey and o p e n air and blocked path voltages were r e c o r d e d for calibration. Water samples fiom the well p u m p were collected during CTD upcasts at locations and depths selected to cover the full range of turbidities e n c o u n t e r e d , and the times of the p u m p e d samples n o t e d for later calculation of turbidity calibration values from the transmissometer. Samples were collected in 250 nil Nalgene bottles and were kept refrigerated for laboratory TSS analysis. TSS values (mg 1 1) were calculated from turbidit}, using a two step process. T h e voltage o u t p u t of each transmissometer was calibrated to NTUs using a well-mixed laboratory Formazin turbidity standard. N T U values calculated from the field mea~surements were then converted to TSS by m e a n s of a calibration relationship derived for each cruise. T h e 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 p e r f o r m e d by the Analytical Services D e p a r t m e n t at H o r n Point L a b o r a t o r y using standard m e t h o d s (APHA 1975). Prewashed and weighed W h a t m a n 04 m m (9S4-AH) filters, with 1.5 p~m n o m i n a l pore size, were used for all TSS samples. All CTD data were processed by bin averaging over 0.95 m d e p t h bins using Seasoft software (Seabird Electronics, v. 4.916), correcting raw depths for instrumental and atmospheric pressure ofs Salinity and TSS for individual transects were gridded and c o n t o u r e d using Surfer (Golden Software, v. 6.0). T h e data were gridded using the kriging algorithm with m o r e weight given to points in the horizontal direction due to the asymmetric distribution of the data. T h e 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 bottom) and the center of the ETM (defined as the center of the region with near-bottom TSS concentrations m o r e than 90 mg 1 1 greater than b a c k g r o u n d TSS concentrations). These position estimates are r e p o r t e d 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 n o t i n g the differences in position estimates. Lateral transect time series stations were occupied for between 22-27 h during each cruise, running alternating Acoustic D o p p l e r C u r r e n t Profiler (ADCP) and CTD transects across the shipping channel near the center of the ETM. T h e ADCP (RD I n s t r u m e n t s , 1.2 M H z B r o a d b a n d ) was m o u n t e d on an a ] u m i n u m mast fixed to the port side of the vessel, with the transducer head subm e r g e d approximately 70 cm below the water surface. C u r r e n 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 d e p e n d e n t ofs in the internal ADCP compa~ss, such that a h e a d i n g correction for the ADCP was derived by c o m p a r i n g the h e a d i n g from the ADCP compass to the ship's g-yro at 10 ~ increments a r o u n d the compass. This h e a d i n g correction had the form of a cosine function and was applied during post-cruise data processing. A modified O w e n settling tube (Valeport) was used to measure settling velocities of relatwely undisturbed particles collected just above the b o t t o m during each of the ETM lateral transects. T h e tube is approximately 5 cm in diameter by 1 m long. It is deployed horizontally" in the water c o l u m n at the desired depth and is oriented into the flow by" a
Chesapeake Bay ETM
vane. A messenger is sent to close the ends of the tube, which is then raised back to the surface. O n deck, the tube is placed vertically inside a water j a c k e t and samples are withdrawn from the b o t t o m for TSS analysis at specified, geometrically increasing time intervals. T h e water .jacket, flushed with water at or very near the same t e m p e r a t u r e as the sample, is necessary to prevent convective cells from f o r m i n g inside the settling tube. T h e seq u e n c e of bottom-withdrawal TSS values is analyzed using a spreadsheet i m p l e m e n t a t i o n of the p r o c e d u r e described by Owen (1976). T h e product of this p r o c e d u r e is a settling velocity distribution of relatively undisturbed 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 l o c a t e d o f f H o w e l l P o i n t at $9~ 76~ S u s q u e h a n n a River discharge data from the gauging station at Conowingo Dam were obtained from the U.S. Geological Survey. C o r r e s p o n d i n g monthly and annual s e d i m e n t load estimates were obtained from U.S. Geological Survey a s well. T h e s e were derived using a new estim a t o r m o d e l that optimizes estimates of average sediment load from average river discharge (Yoc h u m 9000). Tidal height data for the Tolchester Beach, Maryland Station were obtained from the National Oceanic and Atmospheric Administration, National O c e a n Service. H o u r l y tide data were $4 h low-pass filtered using a b u t t e r w o r t h filter to reveal subtidal variability" m o r e clearly. Results
CALIBRATION OF T U R B I D I T Y T O 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 u p p e r panel, where the necessity for a two-part calibration is obvious. T h e slope of the calibration line at low concentrations is m u c h lower than the slope at high concentrations. This is consistent w-ith a two-pm-t particle population consisting of fine, slowly settling pm-ticles that are nearly always in suspension and relatively large aggregates that are resuspended from the b o t t o m and settle out of suspension rapidly. References cited above indicate that these two particle populations are always present in upper Chesapeake Bay. T h e increase in slope at 12 mg 1-I (approximately the b a c k g r o u n d TSS concentration in July) is consistent with the response of transmissometers to larger particles (Baker and Lavelle 1984; n o t e that their calibration plots have axes reversed relative to ours). A qualitatively" similar calibration response was r e p o r t e d by Sanford (1994) for TSS collected in S e p t e m b e r 1999 in up-
659
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Turbidity, [NTU] Fig. 2. Calibration of turbidity (NTU) ~'om 5 cm p a t h l e n g t h t r a n m i s s o m e t e r to TSS (rag 1 1) f r o m collected ~-ater samples. U p p e r p a n e l shows e x a m p l e calibration curve f r o m the July TIES cruise with fitted linen- regression line (two pro't). Lower p a n e l shows N T U to TSS regressions for all five 1996 cruises. Symbols indicate m e a n TSS c o n c e n t r a t i o n in O~ven T u b e sm~> pies f r o m May (O),July (IlL a n d O c t o b e r (A) TIES cruises.
per Chesapeake Bay. Two-part turbidity" calibrations also were r e p o r t e d by Pak et al. (1988) and Downing and Beach (1989) in very different environments. T h e lower panel of Fig. 2 shows the NTU-TSS calibration curves derived for all 5 ETM cruises in 1996. T h e calibration for the F e b r u a r y 1 cruise is markedly different from the others. O n e line represents the entire calibration, with a slope similar to the low concentration slopes of the other calibrations. This implies that the suspended particles during the F e b r u a r y cruise were relatively unaggregated fines. T h e May calibration slope was 270% h i g h e r than the F e b r u a r y calibration slope at high concentrations, but only 70-80% as high as the slopes for the r e m a i n i n g cruises in the same range. At the very high TSS concentrations enc o u n t e r e d near the b o t t o m in March and October, third pieces of the calibration diagrams had to be
660
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Distance from Havre De Grace, [km] Fig. 4. Salinity a n d TSS c o n t o u r plots of axial C T D sm'veys on F e b r u a r y 1. U p p e r p a n e l shows salinity (psu) c o n t o u r s a n d lower p a n e 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 s h o w n on the abscissa. CFD data p o i n t s (+) a n d p h a s e of tide (F = flood, E = ebb, SF slack before flood, a n d SE slack before ebb) are indicated on the salinity contom- plots.
[] Sediment Load
Fig. 3. Discharge a n d s e d i m e n t load e n t e r i n g C h e s a p e a k e Bay f r o m the S u s q u e h a n n a at the C o n o w i n g o D a m g a u g i n g station. U p p e r p a n e l shows a n n u a l m e a n s for individual calendar years f r o m 1991 ttn'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. Lower p a n e l shows m o n t h l y m e a n s for 1996 (bars) a n d m o n t h l y m e a n s for the 9-yr p e r i o d (lines).
a d d e d with even steeper slopes. T h e calibrations for each cruise were used to calculate TSS h-om turbidity for that cruise alone. S U S Q U E H A N N A RIVER F L O W AND TRAPPINO
Feb 1; 1996
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C a l e n d a r year averaged 1996 S u s q u e h a n n a River flow and s e d i m e n t load were the highest of the 9 years plotted in Fig. ga. T h e 1996 average flow of 1,800 m ~ s -1 e x c e e d e d the 9-yr average by 60% a n d the 1996 average s e d i m e n t load e x c e e d e d the 9-yr average by 176%. T h e high a n n u a l average flow a n d s e d i m e n t load were due to several factors. T h e m o s t i m p o r t a n t by far was an e n o r m o u s flood event during the last week of January, resulting from a record snowf:all followed by a d r e n c h i n g rain. This flood resulted in a p p r o x i m a t e l y 9 • 109 m s of freshwater a n d 1.5 • 109 kg of s e d i m e n t entering the Bay f r o m the S u s q u e h a n n a River in app r o x i m a t e l y 9 wk (Zynjuk and Majedi 1996). T h e highest flow rates were a p p r o x i m a t e l y 10 times the J a n u a r y average flow and the total s e d i m e n t loading was a p p r o x i m a t e l y 17 times the total for an
average January. Flow a n d s e d i m e n t loading during the rest of the w-inter, spring, and m o s t of the s u m m e r were n o t r e m a r k a b l y different f r o m average conditions, with 20% h i g h e r freshwater flow a n d 20% lower s e d i m e n t loading (Fig. Sb). A difference in 1996 was that the winter-spring h-eshet was s p r e a d out u n i f o r m l y during the m o n t h s of February-May, rather than p e a k i n g in M a r c h a n d April as usual. T h e last 4 m o of 1996 were m u c h wetter than usual w-ith ] 40 % higher h-eshwater flow a n d 370% h i g h e r s e d i m e n t loads than average. Figure Sa and b are plotted such that flow a n d s e d i m e n t loading bars of equal h e i g h t would corr e s p o n d to a m o n t h l y average TSS c o n c e n t r a t i o n of 60 mg 1-1 in inflowing S u s q u e h a n n a River waters. T h e bars are s e l d o m of equal height, however, reflecting the n o n l i n e a r relationship between flow a n d s e d i m e n t loading of the S u s q u e h a n n a basin. Low flows yield very low s e d i m e n t loads, a n d high flows yield very high s e d i m e n t loads. Inflow-ing Susq u e h a n n a TSS c o n c e n t r a t i o n s averaged 219 m g 1 1 in J a n u a r y 1996, 20 m g 1-1 in March, 32 mg 1-1 in May, 11 m g 1-I inJuly, a n d 42 m g 1-I in October. T h e average for 1996 was 65 mg 1 1. Axial distributions of salinity and TSS observed on F e b r u a r y 1, at the tail end of t h e J a n u a r y flood, are sho~m in Fig. 4. T h e s e r e p r e s e n t the m o s t ext r e m e TSS distributions observed in 1996. O n Febr u a r y 1, the J a n u a r y flood a p p e a r e d to have c h a n g e d the u p p e r gay from a partially m i x e d estuary into a salt wedge estuary. Freshwater was be-
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S a l i n i t y a n d TSS c o n t o u r p l o t s o f a x i a l CTD surveys o n O c t o b e r 22 (left) a n d O c t o b e r 27 ( r i g h t ) , as i n Fig. 4.
ing a d v e c t e d seaward in a thin lens overlying a v e r y s h a r p p y c n o c l i n e at 4 - 6 m d e p t h . TSS c o n c e n t r a tions o f 8 0 - 6 0 m g 1-1 were b e i n g carried seaward in this fresh s u r f a c e layer. T h e initial surge o f the f l o o d f r o m J a n u a r y 2 0 - 2 2 m a y have p u s h e d salt o u t o f the u p p e r Bay, b u t if so the salinity s t r u c t u r e h a d r e b o u n d e d strongly by F e b r u a r y 1. T h e result was an i n t r u s i o n of salt water m o r e p r o n o u n c e d t h a n any o t h e r we o b s e r v e d in 1996. T h e lower layer was relatively clem-, e x c e p t for a weak ETMlike nero--bottom TSS m a x i m u m b e t w e e n kilometers 14-80 n e a r the limit of salt. T h e majority o f the f l o o d s e d i m e n t s a p p e a r to have b e e n e s c a p i n g the u p p e r Bay, at least over the c h a n n e l . T h e r e were no c u r r e n t data collected d u r i n g this p e r i o d , so direct estimates of s e d i m e n t t r a n s p o r t rate were n o t possible. T h e turbid surface layer was d e e p e r t h a n the average d e p t h of the E T M z o n e , such that s e d i m e n t m a y have b e e n d e p o s i t e d over the shal70 6O
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664
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suspension u p s t r e a m , rapid d o w n s t r e a m advection by the strongly sheared ebb currents, a n d rapid diffusion t h r o u g h the well-mixed u p p e r water colu m n . T h e slightly elevated above-pycnocline TSS c o n c e n t r a t i o n s on flood are p r o b a b l y the r e m n a n t of this material. Stratification and gravitational circulation n e a r the salt limit affected the observed velocity structure by increasing vertical shear a n d 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 pattern is m o s t obvious in the ebb at 00:00 on O c t o b e r 95 and the flood at 06:00 on O c t o b e r 95. T h e gravitational circulation also caused the nero--bottom currents to turn to flood s o o n e r a n d to ebb later than the near-surface currents.
! Discussion
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Time of Day, [hours, ED-I] F i g . 10. T i m e - d e p t h contours of salinity, TSS, and alongd ~ 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 2425. 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 taken at the center channel station during the ETM lateral transect. CTD data points are indicated by (+). Lower panel: along channel cun'ents (projected angle 1 9 5 ~ c m s 1) f r o m s u c c e s sive ADCP passages across the channel station. Positive velocities ( ) are ebb and negative velocities ( ~ ) are flood.
T h e stratification variations i m p o s e d by the passage of the salt limit had clear c o n s e q u e n c e s for s e d i m e n t resuspension and velocity shear. Local resuspension is m o s t clearly indicated by near-bottom increases and decreases of TSS in p h a s e with the n e a r - b o t t o m 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 a n d m u c h weaker at m a x i m u m e b b tide. U p w a r d mixing a p p e a r e d to have b e e n limited by the pycnocline. T h o u g h it is possible that s o m e of the TSS high in the water c o l u m n at m a x i m u m ebb resulted f r o m upward mixing t h r o u g h the pycnocline, it is m o r e likely that this material resulted f r o m re-
While the results p r e s e n t e d h e r e confirm the basic descriptions of the C h e s a p e a k e gay ETM offered by Schubel (1968a, b) a n d Schubel a n d Pritchard (1986), they refute the idea that the converg e n c e of the gravitational circulation is primarily responsible for its f o r m a t i o n , with tidal resuspension m e r e l y m o d u l a t i n g TSS c o n c e n t r a t i o n s (Schubel 1968a,b; Festa and H a n s e n 1978; Officer 1980). If this earlier e x p l a n a t i o n was correct, t h e n the ETM would not b e c o m e separated f r o m the limit of salt, as we observed. T h e r a n g e of particle settling speeds t r a p p e d by the gravitational circulation in the m o d e l of Festa and H a n s e n (1978) was 10-100 times smaller than the settling speeds we observed. T h e highest settling speeds t r a p p e d in Geyer's (1995) i m p r o v e d ETM m o d e l c o m b i n i n g gravitational circulation with stratification d a m p e d mixing (but no tidal currents) are at the low end of the r a n g e of our observations as well. O u r data indicate that asymmetrical tidal resusp e n s i o n and asymmetrical tidal t r a n s p o r t of rapidly settling aggregates are primarily responsible for the C h e s a p e a k e Bay ETM, as illustrated schematically in Fig. 11. Tidal s u s p e n d e d s e d i m e n t transp o r t is biased in a net d o w n s t r e a m direction above the limit of salt a n d in a n e t u p s t r e a m direction below the limit of salt, leading to the f o r m a t i o n of a p o o l of r e s u s p e n d a b l e particles n e a r the limit of salt. T h e axial c o n v e r g e n c e of the gravitational circulation a n d the associated salinity structure n e a r the limit of salt are the m a j o r causes of these tidal asymmetries, but without tidal r e s u s p e n s i o n the rapidly settling aggregates would likely r e m a i n on the b o t t o m w h e r e 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 p a r t i c l e s would n o t be f o r m e d , a n d the ETM as such would either n o t exist or be greatly weakened. T h e resusp e n d a b l e particle p o o l lags b e h i n d the m o t i o n of the salt limit because of the resuspension lag de-
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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 hearT 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,
665
s c r i b e d by Dyer (1988) a n d b e c a u s e t h e n e a r - b o t t o m c e n t e r of mass of t h e r a p i d l y s e t t l i n g s u s p e n d ed p a r t i c l e s m o v e s m o r e slowly t h a n the water a b o v e it. T h e w i n d - f o r c e d s e p a r a t i o n of the E T M a n d salt l i m i t s h o ~ m i n Fig. 11 is s i m i l a r to the f r e s h w a t e r flow lags of the E T M b e h i n d the salt l i m i t o b s e r v e d i n the Tamm- a n d Weser e s t u a r i e s by G r a b e m a n n et al. (1997). T h 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 E T M is also m o r e i n l i n e with r e c e n t e x p l a n a t i o n s of E T M f o r m a t i o n i n o t h e r estuaries. I n a l m o s t every case, a spatially l i m i t e d p o o l of r e s u s p e n d a b l e p a r t i c l e s is the key factor. T h e m e c h a n i s m s r e s p o n s i b l e for f o r m a t i o n of this p a r t i c l e p o o l vary f r o m e s t u a r y to estuary, s o m e t i m e s d e p e n d i n g o n n o n l i n e a r tidal p u m p i n g (Jay a n d M u s i a k 1994; U n c l e s et al. 1998; g r e n o n a n d Le H i r 1999; G u e z e n n e c et al. 1999), s o m e t i m e s o n a tidal 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 o n tidally i n d u c e d t o p o g r a p h i c trapp i n g or tidally v a r y i n g 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. I n partially m i x e d e s t u a r i e s like C h e s a p e a k e Bay, a s y m m e t r i c tidal t r a n s p o r t n e a r the l i m i t of salt is o f t e n the r e s p o n s i b l e m e c h a n i s m ( H a m b l i n et al. 1988; g u r c h a r d a n d g a u i n e r t 1998), b u t the c o n v e r g e n t c i r c u l a t i o n a n d ass o c i a t e d salinity s t r u c t u r e t h a t cause the tidal as3,mm e t r i e s a r e specific c o n t r i b u t i n g factors, n o t u n i versal r e q u i r e m e n t s for E T M 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 E T M s t r u c t u r e r e p o r t e d by N o r t h a n d H o u d e (2001) is b e l o w t h e l i m i t of salt a n d a p p e a r s to b e 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 e n d of the E T M z o n e . G i v e n o u r e m p h a s i s o n tidal asymmetry, a disc u s s i o n of its effects o n o u r survey d a t a a n d E T M p o s i t i o n estimates is a p p r o p r i a t e . T h e r e are essentially t h r e e effects. First, t h e p h a s e of the tide s h o u l d affect t h e a m o u n t a n d l o c a t i o n of resusp e n d e d s e d i m e n t i n the water c o l u m n , as illustrat-
only the GC r e m a i n s a n d previously r e s u s p e n d e d s e d i m e n t s have settled to fl~e bottom. Stl"atification is m a x i m m ~ below the salt front, c) At full f l o o d tide, the GC e n h a n c e s f l o o d cm'rents below the salt f r o n t a n d causes r e s u s p e n s i o n of sediments, b u t only to the h e i g h t of the p y c n o d i n e . T h e GC opposes flood tidal cmTents above fl3e salt front, with n o r e s u s p e n s i o n , d) At slack before ebb, only the GC r e m a i n s a n d previously resusp e n d e d s e d i m e n t s have settled to the b o t t o m . Stratification is m i n i m u m , e) A seaward x,vind circulation a d d e d to f l o o d 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.
666
L.P. Sanford et al.
ed in Fig. 11. Second, tidal straining of the density structure 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 e m a i n i n g slightly seaward at the end of flood and slightly landward at the end of ebb. T h e r e are s o m e indications of these effects in the surveys p r e s e n t e d here. T h e ebb tide surveys of July 91 (Fig. 8) a n d O c t o b e r 97 (Fig. 5) show higher c o n c e n t r a t i o n s distributed t h r o u g h o u t the water c o l u m n u p s t r e a m of the salt f r o n t and ext e n d e d slightly d o w n s t r e a m in the surface layer, as in panel a of Fig. 11, a l t h o u g h the e x p e c t e d intratidal differences are o v e r w h e l m e d by differences in freshwater flow. s e d i m e n t loading, wind forcing, a n d tidal c u r r e n t strength between the surveys. In fact, in five out of the eight cases shown in Fig. 7, correcting for the e x p e c t e d tidal lags would increase the separation between the ETM and the salt limit r a t h e r than decrease it. Tidal effects are i m p o r t a n t at intratidal time scales (Fig. 10), but longer-term, larger scale effects d o m i n a t e over times scales of days to months. T h e residence time of terrestrial particles in the ETM pool must be finite, or ETM TSS concentrations would continually increase. In s o m e estuaries (e.g., the C o l u m b i a River), the ETM r e p r e s e n t s a delay b e t w e e n the delivery of terrestrial material from the river a n d its ejection to the coastal ocean, with little n e t a c c u m u l a t i o n in the b o t t o m sedim e n t s ( C r u m p a n d Baross 1996). In others, accum u l a t i o n in the s e d i m e n t s a n d accretion of the b o t t o m is the ultimate fate of terrestrial material. T h e r e is a b u n d a n t evidence that s e d i m e n t a r y acc u m u l a t i o n d o m i n a t e s in C h e s a p e a k e Bay. T h e upp e r Bay is shoaling at a m u c h h i g h e r rate than the mid-Bay (Officer et al. 1984; C o l m a n et al. 1992). Estimates r a n g e 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; H a l k a u n p u b l i s h e d data). O t h e r estimates indicate that the ETM region of C h e s a p e a k e Bay perm a n e n t l y traps between 70% (Biggs 1970; Schubel a n d Pritchard 1986) and 100% ( D o n o g h u e et al. 1989) of the terrestrial material delivered f r o m the S u s q u e h a n n a River. Exactly w h e n and how perm a n e n t s e d i m e n t a t i o n occurs is not known, but we speculate that spring-neap variability in tidal currents may be an i m p o r t a n t factor in the ETM channel. All of the 1996 TIES ETM cruises were designed to be c e n t e r e d a r o u n d spring tides, w h e n tidal currents a n d tidal resuspension were at a m a x i m u m . N e a p tidal currents in the u p p e r Bay are a p p r o x i m a t e l y 30% weaker than spring tidal currents. This may be just e n o u g h of a decrease in energy to allow a substantial p o r t i o n of the resusp e n d a b l e particle p o o l to consolidate, resist subs e q u e n t erosion, and be buried u n d e r newly deliv-
ered material. Over the shoals, net s e d i m e n t a t i o n is m o r e likely controlled by the timing between delivery events and wave-forced erosion events (Sanford 1994). S e d i m e n t a t i o n rates in the ETM shipping channels are r e m a r k a b l y high c o m p a r e d to the adjacent shoals, where all of the u p p e r Bay s e d i m e n t a t i o n rates cited just above were collected. It is not possible to estimate s e d i m e n t a t i o n rates in d r e d g e d s h i p p i n g channels by conventional means, but the d r e d g i n g records themselves may be used to obtain an a p p r o x i m a t e value. A n n u a l m a i n t e n a n c e dredging records f r o m the u p p e r 48 km of the shipping channel leading f r o m Baltimore to the C h e s a p e a k e a n d Delaware Canal (27 km of which f o r m the channel of the u p p e r p a r t of the ETM zone) have b e e n c o m p i l e d by the M a r y l a n d Geological Survey since 1986 ( P a n a g e o t o u et al. 1998). T h e authorized width of the c h a n n e l is 1S7 m, which gives a total s e d i m e n t surface area of 6.6 • 100 m 2. T h e a n n u a l average d r e d g e d v o h m e is 1.1 • 10 ~ m s, which gives an a n n u a l m~erage s e d i m e n t a t i o n of 17 cm, 14-55 times the a n n u a l s e d i m e n t a t i o n rate on the adjacent shoals. This focusing of s e d i m e n t a t i o n into the c h a n n e l may" be due to redistribution fi-om the shoals to the c h a n n e l by s t o r m events (Sanford 1994), focusing of initial deposition into the channel by u n k n o w n lateral t r a n s p o r t processes, or upBay t r a n s p o r t of material f r o m below the ETM zone. We also n o t e that there is an a p p a r e n t correlation between a n n u a l fluctuations in s e d i m e n t loading f r o m the S u s q u e h a n n a a n d m a i n t e n a n c e d r e d g i n g v o l u m e during the following winter (not shown), implying that the focusing process may occur quickly'. Two of our i m p o r t a n t results are related to sedi m e n t settling velocities; settling velocities of resusp e n d e d TSS f r o m the C h e s a p e a k e Bay ETM were high relative to the settling velocities of disaggregated silt and clay particles, a n d settling velocities of ETM particles increased significantly f r o m Febr u a r y t h r o u g h May a n d July" to October. Both results were based primarily on settling velocity distributions m e a s u r e d using a m o d i f i e d O w e n settling tube, in c o m b i n a t i o n with changes in the turbidity-TSS calibration relationships. T h e r e have b e e n questions raised in the literature a b o u t the accuracy a n d repeatability of the settling tube technique, which we would be remiss to ignore. Dearnaley (1997) showed that floc break-up, re-flocculation, and internal circulation within a settling tube had the net result of r e d u c i n g settling s p e e d estimates relative to direct video techniques. Dyer et al. (1906) also indicated that, relative to video techniques, settling tubes t e n d e d to u n d e r e s t i m a t e settling velocities, t h o u g h careful control of sampling protocols gave c o m p a r a b l e results between
Chesapeake Bay ETM
different tube designs. Hill and Milligan (1999) argued that the artii:acts i n d u c e d by floc breakup and reflocculation can explain the a p p a r e n t concentration d e p e n d e n c e of settling velocity as observed in settling tube m e a s u r e m e n t s (Burt 198(3). While we c a n n o t state categorically that these problems did not affect our measurements, there are several factors that indicate such problems may not have b e e n as serious in our case, or at least that the settling effects that we observed were qualitatively correct. First, we observed internal circulations in our settling tube experiments in previous trials, that were related to convection caused by t e m p e r a t u r e differences between the sample and the external environment. We devised our temperature control water jacket to minimize this problem, and did not use any data fi-om experiments with visible convection cells. Internal circulation tends to h o m o g e n i z e the sediment suspension inside the tube, lowering the m e d i a n 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 i g h e r m e d i a n settling speeds do not resolve the fine, slowly settling fi-action of the particle population, such that vid e o g r a p h i c m e d i a n settling speeds are biased upward. We did not observe a concentration dependence of settling speed in our samples; two samples at almost the same concentration ( O c t o b e r and July) had very different settling velocity distributions, while two samples at very different concentrations (May and July) had very similar median settling velocities. T h e relative differences between our samples c a n n o t be due to a bias introduced by different initial TSS concentrations. T h e relative settling speeds of our samples are consistent with the relative slopes of the accompanying turbidity-TSS calibration lines, assuming that m o r e rapidly settling particles r e p r e s e n t larger aggregates of similar composition. We have found in previous modeling work that a settling velocity of approximately 1 m m s -I is r e q u i r e d to match observed resuspension-deposition cycles in u p p e r Chesapeake Bay (Sanford and Halka 1998; Sanford and Chang 1997), very close to the average of the three settling velocities q u o t e d here. We believe that the m e d i a n settling velocities det e r m i n e d Dom our settling tube m e a s u r e m e n t s are reasonable, that the relative increase in settling velocity from May to O c t o b e r is real, and that the implied lower settling velocity of the J a n u a r y flood material is real. Such seasonal changes in settling speed have i m p o r t a n t consequences. T h e y may explain why there was very little evidence of s e d i m e n t trapping in the Chesapeake Bay ETM following the J a n u a r y 1996 flood, while H u r r i c a n e Agnes in J u n e 1972 left approximately 75% of its sediment load
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in the ETM zone in a layer approximately 0.9 m thick (Schubel and Pritchard 1986). T h e relatively efficient trapping after Agnes may have been because particles were m o r e highly aggregated and settled m o r e rapidly. We will never know if this is true, but it is plausible and implies that the sedimentological consequences of major events may d e p e n d 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) f o u n d that z o o p l a n k t o n fecal pellets were i m p o r t a n t agents of particle agglomeration in upper Chesapeake Bay, and z o o p l a n k t o n activity is clearly seasonal. Jahmlich et al. (1999) also f o u n d increasing aggregate size related to transparent exopolymer particles and bacterial cell a b u n d a n c e in the bottom b o u n d a r y layer of a bight of the Baltic Sea, and Z i m m e r m a n n - T i m m et al. (1998) f o u n d that the largest aggregates in the Elbe Estuary occurred in spring and s u m m e r in association with high particle-attached microbial activity. Data obtained at the m o u t h of the S u s q u e h a n n a River for the U.S. Environmental Protection Agency Chesapeake Bay Monitoring Program indicate seasonal increases of approximately 50% in DOC concentrations between the winter/early spring and the s u m m e r / f a 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 t e m p t e d to construct a plausible scenario for the controlling physics of the Chesapeake Bay ETM based on data that is intriguing but limited. We have suggested and discussed specific physical mechanisms that are consistent with the data and m o r e in keeping with r e c e n t understanding than previous ideas. We have not tried to prove these suggested mechanisms or explore them quantitatively----to do so would have b e e n to push the data set b e y o n d its limits. ACKNOVCLED GMENTS We grateffflly acknowledge the s u p p o r t of the National Science F o u n d a t i o n tl~-ough G r a n t No. DEB-9412113. We also t h a n k W. C. Boicom't for tlae Howell P o i n t CBOS b u o y data, S. Y o c h u m 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 r o m the U.S. E n v i r o n m e n t a l Protection Agency C~hesapeake gay P r o g r a m data base. We t h a n k two a n o n ~ o u s reviewers for tlaeir h e l p f u l s u g g e s t i o n s for i m p r o v i n g tlae paper. Finally, o u r t h a n k s go to the crews of tlae R / V s Kerhir~, Orior~, a n d Gape He'~*l@e'nfor tl~eir help d i n i n g tl~e field work. This is University of Maryland C e n t e r for E n v i r o n m e n t a l Science publication no. 3469. LITERATURE C I T E D APHA. 1975. S t a n d a r d M e t h o d s for E x a m i n a t i o n of Water a n d Wastewater. A m e r i c a n Public H e a l t h Association, W a s h i n g t o n , D.C.
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