Effects of sediment resuspension on organic matter processing in coastal environments: A simulation model

a, * 9

a Institute b

ine and Coastal Sciences, Rutgers Unioersity, osyterns Center, Marine Biology Laboratory,

o/e,

Received 13 July 1995; accepted 17 July

A model, constructed using STELLATM, was used to simulate changes in standing stocks and flows of organic resulting from sediment resuspension in shallow coastal environments. Preiious studies suggested t determine the sites and rates of organic matter mineralization in shallow environments (Hopkinson, 1985, studies predicted that resuspended organic material could exert an enhanced demand on dissolved oxygen. Our model results support this hypothesis. Total system metabolism receives increasing contributions from the water column as settli decreaszs. Water column respiratioq also increases relative to henthic respiration as the f~e~~~~cy resuspension events increases. This it; driven by higher specific degradation rates in the watts ~01~~~1~ t environment. Furthermore, overall respiration (benthic + pelagic) increases in response to resuspension. Kepnrds:

biogeochemisrry; carbon; rcsgiration; photosynthesis; boundary layer; ecosystems; shdiow-water cavironmrut

Sediment resuspension is a ubiquitous phenomenon in both marine and freshwater systems. Resuspension can be an important mechanism for the exchange of organic matter and associated biota between sediments and overlying water (Hopkinson, 1985; Wainright, 1990), and has been shown to stimulate microbial production in the water column (Wainright, 1987; Ritzrau and Graf, 1992). These studies raise questions as to whether organic matter processing is increased overall, or whether resuspen-

* Corresponding author.

sion simply shifts the major site of mineralization from the benthic to the pelagic environment, There are often order of magnitude differences in the standing stocks of organic matter in sediments and the water column. In addition, the predominant form of organic matter often differs between the environments, e.g. primarily particulate (PO the sediments and dissolved (DOM) in the water column of the Kiel Bight (Meyer-Reil, 1984). Organic matter may degrade more quickly in thz v:ater column relative to sediments because of the relatively oxic, well-mixed conditions in the water column. Even in surficial sediments, organic matter degradation often occurs under anaerobic conditions, at consequently slower rates (Aller, 1994). Processes such as resuspension, which shift a part of the large

0924-7963/97/$17.00 Copyright 6 1997 Elsevier Science B.V. All rights reserved. PII SO924-7963(96)00130-3

Table 1 Literature values of total organic carbon (TOC) content, respiration rates, and specific degradation rates (the ratio of respiration rate to standing stock) of sedimentary (Part A), and water column POM (Part B). Sedimentary organic C was estimated from organic matter or organic carbon content. Water column POC was estimated, as noted, from chlorophyll concentrations, and thus may approximate labile POC. Specific degradation rates of TOC are much greater in the water column than in the sediments (B) Water column

(A) Sediments 3. Specific degradation rate (col. 2/col. I) (X 1000)

Reference

0.20 0.020 0.054 0.026 0.020 0.02 1 0.132 0.132 1,090 Q.152 0.072 0.374 0.278 0.263 0.632 0,096 0,425 0.300 0,900 I.800 I.200 2,c108 0.932 I.190

1.176 0.003 0.038 0.004 0.003 0.004 1.541 tJ.145 4.844 0.144 0.267 1.385 0.195 0.629 1,559 0.343 0.500 I.@00 0,600 2.400 0.615 32,OOQ 2.845 0.738

1 2

8,55

(original units, Le., + 1000) 0.0024

2. Reported 1. Reported TOC content ’ respiration rate (gC m’ ‘d- ’) (gC m”) (5 cm”‘)

7665 4673 85 910 225 1058 270 270 1425 418 418 281

Aver 1636

4. Reported TOC content (gC m-“)

5. Reported respiration rate (gC m- ’ d- ’)

6. Specific degradation rate (col. 5/col. 4)

Reference

0.06

0.12 0.44

14

0.40 0.80 0.60 0.34 1.10 0.03 1.00 c 2.00 c 4.25 ’ 9.00 c 0.50 c 1.50 c 1.50 c 2.50 c 0.55

0.15 0.58 2.30 0.03 0.10 0.22 0.03 0.42 1.68 2.70 2.04 0.60 0.90 3.12 I .56 0.37

2.000 0.550 0.774 d 0.1152 1.44 2.88 0.06 0.28 0.20 1.15 0.420 0.840 0.635 0.227 1.200 0.600 2.080 0.624 0.660

2.75

0.96

0.36

2.10

0.51

0.22

0.80 1.00 c

3 4 6 7 8 9 10 11 12

16 18 19 20 21

22

II

13

14 is

al0 O,27

0.00062

’ To6 is total organic carbon in the top 5 cm of sediment, except in the case of Yoon and Benner (1942) where 7 cm was used. Organic anic 6 content data (both on a dry weight basis) were converted to an areal basis using the relationships % OM = g OM assuming an average sediment density of 1.5 g cm- ‘. converted from chlorophyll concentrations, assuming g C = g Chlorophyll*50 assumes 11.4 kcal (g C)” (Pace et a’., 1984), 13.6 g C (g Chlorophyll)-’ (Patten, 1961) ’ Griffith et al. (1990) (station 1). References: (I) this study, coastal Georgia* , (3) Yoon and Benner, 1992, Texas estuaries; (3) Rowe et al., 1988, continelital shelf off Long Island: (4 M#cinson and Weta& 1982, coastal Georgia, W,S.A.; (6) Kelly and Nixon, 1984, NarragansettBay; (7) Alkemade et al., 1992, laboratory;(8) Smith, 1978, off Martha’s Vineyard; (9) H ve and Phillips, 1989, Halifax Harbor;(10) Balzer, 1984, Kiel Bight: (11) Joiris et al.. 1982, Scheidt Estuary, Bel ium; (12) Gswfet al., 1984, Kiel Bight; (13) Boynton et al., 1981, Chesapeake Bay; (14) Hopkinson, 1985, coastal r&a: (1%)Smith, 1933, coastal Georgia: (16) Patten, 1961, Raritan Bay, New York; (18) Turner, 1978, coastal Georgia; (19) GfifW et al., 1990, coastal Geor@a: (20) Garside and Malone, 1978, New York Bight; (21) Iriarteet al., 1991, North Sea, English Channel: (22) Carmouze et al., 1991, Brazilian Lagoon.

the water colu

1987 estimated that 8% or less bum, 1992 estimated that 20-30% of freshly sedi-

mented phytodetritus was labile.). If the specific degradation rates in Table 1 were based on labile organic matter rather than total organic matter, they would be an order of magnitude higher, but sedimentary rates would remain much lower than pelagic rates. This conclusion parallels the observations of Meyer-Reil ( 19G4)in the North Sea, where bacterial uptake of labeled organic substrates was 1-2 orders of magnitude faster in overlying water than in aerobic sediments, If resuspension accelerates the overall degradation of organic matter, this may result in less carbon burial and lower sedimentary standing stocks of carbon. Other mechanisms which tend to prolong the exposure of organic matter to oxic conditions, such as sediment stirring @lemming and Trevors, 1990) and bioturbation (Alkemade et al., 1992;Aller, 1994), might have similar effects on mineralization and burial. Denitrification is another important process in coastal sediments (Seitzinger, 1988), which occurs under anoxic or suboxic conditions. Prolonged exposure of sedimentary organic matter to oxic conditions during resuspension may decrease the loss of DIN from the system due to denitrification, thereby making more DIN available to phytoplankton. Resuspension events arc difficult to investigate in situ because of the harsh field conditions that prevail

pools of organic matter i

ments (5 cm) and dee

cm). Two additional pools in the wate

that they contained a combination of living and recently dead organic matter, Two physical processes were modeled explicitly, i.e. resuspension and settlement of p~iculat~ material (Fig. lb). These processes controlled the relative strength, and duration of resuspension events, and the subsequent settlement of particulate material back to the benthos. They were user-defined at each time step (1 time step per day) as follows: Settlement = (settling rate) * (settling time),

(1)

“settling rate” had a default value of 0.2 d-l and ‘‘settling time’’ was equal to either 0 or 1 on each day of the simulation, and 4we . . _**

Resuspension = (resuspension magnitude) * (resuspension time)

9

(2)

Sca~onalltv

n

. . . *. . . . . .*....,** .........

Winter

Summer

Resuspension

d

s

lb Time (d)

l’s

begins

via the ’’seasonality ’’ function (see above and Fig. 1~). The frequency and intensity of resuspension events was chosen to approximate those off coastal New Jersey (a shallow water site “or which a detailed data set is available; S. Glenn, Rutgers University, pers. commun.). During 1994- 1995 off New Jersey, between 6 and 12 resuspension events (significant wave height exceeding 2 m) occurred per year, with more intense events associated with hurricanes in the fall, and with winter storms during November to March. We considered only vertical exchanges between sediments and the water column. We did not address the horizontal advection of materials, nor combined vertical + horizontal transport, both of which can profottndly affect the fluxes of organic materials at the sediment-water interface (Smetacek et al., 1984; Gabrielson and Lukatelich, 1985; Muschenheim, 1987; Graf, 1992). It was assumed that each vertical layer - water column, aerobic sediment and anaero-

petted tz change as a function of resuspe

would expect no effect o

toplankto~~ because of tu pension. Biotic flows were regulated by which was parameterized by a function varied according to the sine function “seasonaIi~y’~ (see above, and Table 2). Initial conditions (Tables 2 and 3) were chosen, where possible, for the Georgia coastal zone (Hopkinson and Wetzel, 1982; Hopkinson, 198% 1987; Wainright, 1987, 1988, 1990). The sediment is sandy, and well-oxygenated, with the redox potential discontinuity (RPD) at 2-5 cm of sediment depth,

Fig. 1. (A) Diagram of standing stocks (boxes) and flows (arrows), as produced by STELLATM software. (B) Detail of the modifiers which influence the flows of labile organic matter between the water column and the benthos via “resuspension” and “settlement”. (c) Time series of the parameter “resuspension magnitude”, a function of bbseasonality*v(top panel); time series of “resuspension time” and “settling time” during a typical resuspension event (middle and lowest panels). (D) Detail of the flows and modifiers which influence standing stocks, using phytoplankton as an example.

Table 2 Equations for standing stocks and fluxes in the STELLATh’model of the effects of resuspension on organic matter processing in shallow coastal environments, In flux listings, 7 is the maximal transformation rate, (w is the threshold substrate concentration below which no msfmation SCUM; y is satiation substrate concentration above which there is no increase in transformation rate. Standing stocks are in unitsofgm-2,flowsareingms’ d-‘. “surficial” and “deep” refer to the O-5 cm and 5-45 cm sediment depth intervals, respectively SW~ u~&~bie equationsfor standing stocks: Water column: labile PQC(,, = labile poC~,.d,j + (phytoplankton Mortality + Uptake of labile DOC + Resuspendedf,,tfiFiniI + Resuspended,,,,,, + Bioresusgendedrsuficial)+ Consumptionof refractoryJJOC + Consumption of refractory POC - Respiration - Settlement - Production of refractory u&on of refractory POC - Dissolution) * dt = labile DO(&,, + (Leachate + Resuspendedf,,ficialj + Resuspended(,,,,, + Bioresuspendedt,,,ficiJI, + Dissolution - Uptake) * dt refractory Duq,, = refractory DOCt,,dt, + @duction + Resuspended(curfifiillj+ Resuspended,,,,,, + Biotesuspended~,*,ficial,+ Dissolution - Uptake) * dt (II = refractory POC+d,, + (Re!;us~nded~,,p~Cii\i)+ Resuspendedfd,,p, + Bioresuspended~,urric,ol, - Settlement - Dissolution - Uptake) * dt dr)+ (Regeneration(pelng,c,+ Regeneration(,,,,h,,, - Uptake) * dt n(,_d,,+ (Primary production - Phytoplankton Mortality - Respiration - Leachate - Settlement) * dt Surficial sediments: labi’ PQC(,, = labile POC+d,, -t-(Settlement of phytoplankton and labile POC f Uptake of labile DOC + Uptake of refractory DOC +Consumption of refractory POC - Resuspended - Bioresuspended - Dissolution - Burialt,,,fi,i,,) - Respiration - Production of refractory POC)* dt labile Boc(,) Eli:labile DOCq,.d,,+ (Dissolution - Uptake - Resuspended - Bioresuspended - Burial~,,,~ic,,l))*dt refriitory DOq,, = refractory DOCt,_dr,+ (Dissolution - Uptake - Bioresuspended - Resuspended - Burialc,,,ficinl,)* dt refractary POq,, = refr’nctorypoC&J,, + (Settlement + Pro iuction by labile POC - Resuspended - Bioresuspended - Dissolution - Uptake) * dt ~Urq,,,ficial) -I- Uptake of refractory DOC -I- Uptake of labile DOC -t- Burial(,,,fi,,,,I, 3 + Dissolution - Burlalcdcrp,- Uptake - Resuspended) * dt lution + Burial~,urflc,al,- Uptnkc - Resuspended - @urial,,l,,p,) * dt ,cl,)+ (Burial~,,,cl‘,c,alI - Resuspended = Dissolution - Uptake - Burial~d,,,,,) * dt FltrK$: (7, tiy, y, ’‘sthw owarks ” U)“&&en ia pmntheses! Water column: labile Pot * C/7 *Q tQ ELSE T * labile POC * 6/7 *i I-i y-phytoplankton)/ f seven geometric size classes of labile POC-the = labile POC * 6/7; T = 0.6; a = 0.2; y = 6.8: Pace et al. (1984) used T values which averaged 1.6 for grazers; We de~t%asedthis ~ahe to 0,6 to account for the inclusion of detritus in the labile PW compartment] 3. Uptake of DIN by p Primary production* 16/106* 14/12 d ?I THEN T *phytoplankton *QlO ELSE T *phytoplankton *(l-(y-DlN)/(r_a))*QlO ); (r = 0.0028 b; y = 1.12 b] FOC = labile POC * 0.01 [perhaps a conservative estimate, after fig. 13-9 of Valieia (1995) ays of Spartina detritusdecomposition,] tion of RftrrctoV E’OC= OAIQOl* refractory POC [a much slower process than leaching of labile detritus, and due mostly to microbe-derived enzymes] @inlion by phytoplankton= O.(a2* phytoplankton + 0.07 * primary production [an estimate based on a total of 0.1 d - I of T*

into a component associatedwith primary production, and a smaller component analogous to maintenance metabolism] water column respiration * 16/ 106 * 14/ 12 d abile POC a = spec, etwarer)*(phytoplankton Mortality + Consumption of labile DOC by labile POC (i.e., bacteria) + Uptake of’refractory D8c: by labile bactet’icr)-I- consumptionof refractory POC by labile POC (i.e., bacteria))

pension of refractor

RESUS/S *refractory POC~,“rficjal)[see flat, 141 18. Settlement of refractory FOC = 19. Settlement of labile POC = SETL * labile POC [see flow IS] 20. Settlement of phytoplankton = SETL * phytoplankton [s 183 *labile T*labiIe POC*O.l *Q 21. Uptake of labile DQC by bacteria = IF labile DOC > y y = 16 b; bacteria = labil POCa0.1 *(l-(y-labile DOC)/(r-ff))*QlO [7 = 3; @= 0. 1 ELSE 22. Uptake of refractory DOC by bacteria = IF refractory DOC ; y THEN 7 * labile POC 7 * labile POC * 0. 1 * ( I -( y-refractory DOC)/(y-a )I * Q 10 [T = 0.03 or much slower than growth on labile DOC; (x = 0.36 ‘; y = 144 ‘; bacteria = labile POC *O.l] 23. Uptake of refractory POC by zooplankton = IF refractory POC > y THEN T * labile POC * l/7 * QlO ELSE T * labile POC * I /7 *(I -(y-refractory POC)/(*f- Q 1)* Q 10 [T = 0.0 15 or one half that of bacteria growing on refractory DOC, a guess; OL= 0.63 ‘; y = 25.2 ’; zoopianbon are l/7 * labile POC] 24. Production of refractory DOC by labile POC = 0.05 *(Uptake ot iabi!e DOC by labile POC + Uptake of refractory by labile POC + phytoplankton Mortality I- Uptake of refractory POC by labile KX - Respiration of water column labile POC) * 0.05 * QlO [5% of uptake is refractory detrital production and 5% of that is DOC, an estimate] 25. Production of refractory POC by labile POC = 0.05 *(Uptake of labile DOC by labile POC + Uptake of refractory D by labile POC + phytoplankton Mortality + Uptake of refractory POC by labile POC - Respiratian of water column labile POC) * 0.95 15% of uptake is refractory detrital production and 95% of that is PQC; an estimate] Surficial sediments: 26. Bioresuspension of labile POC = benthic Respiration * 0.1 * 0.1 [resuspension of material by benthic fauna represents 10% of their activity; 10% of hnthic respiration is faunal] 27. Bioresuspension of labile DOC = Bioresuspension of labile POC * labile DOC/labiIt? PQC [see flow 26; scaled to the ratio of DOC/PX in surficial sediments] 28, Bioresuspension of refractory DOC = Bioresuspension of labile POC *refractory DOG/labile POC bee flow 271 29. Bioresuspension of refractory Pm = sedimentary Respiration * 0.1 * refractory POC/labile POC [see flow 271 30. Dissolution of labile POC = 0.01 * labile POC [see flow S] 3 1. Dissolution of refractory POC = 0.0001 *refractory POC [see flow 61 32. Regeneration r= (besthic RespiWiontsU,rici,,,+dcep)) * 16/ 106 * 14/ 12 ’ 33. RespitWion(,udicialj= QIO * spec.degr.rate~,,,ficisl) * labile POC * 19.55/32 [I9.55/32 is the respiring proportion of the initial standing stock of sedimentaty labile POC, i.e., the macrofauna, meiofauna, and microflora/fauna, as per Gray (198 1) and Tenore (1985)] 34. Transfer of labile DOC from surficial to deep sediments = labile DOC~,urficiol~/50* sedimentation rate + 0.0315 * labile DOCf,“,n,iai) [the second term is a diffusion from above to satisfy initial bacterial uptake. Average sedimentation rate assumed to be 2 mm yr - I (0.0055 mm d’ ’) aerobic layer is 50 mm]

Table 2 (continued)

36. Transfer of refractory DOC from surficiai to deep sediments = refractory DQC/SO * sedimentation rate + 0.03 IS * tefractorplmq,*rfic,s,) f= fh 341 37. Uptake of refractory DQC by bacteria = IF refractory DQC > y THEN t * labile POC * 0.03 * QIO ELSE r *labile PQC*0,03*(1-(y-refractory DOCi/(y-ruii *QIO[T = 0.02; OL= 0.054 ‘; y = 2.16 ‘; bacteria = labile PQC * 0.031 38. Transfer of labile POC tram surficiai to deep sediments = labile PDC * ( 12.45/32)/50 * sedimentation rate * 1S, where 1.5 is Qmade\tunmg factx [logic similar to tlow 36; i2.45/32 is the ratio of dead/living organic matter (refer to flow 3313 39. Productionof refractory POC by labile PQC = 0.05 *(uptake of labile DOC by bacteria + uptake of refractory 13Qc by bacteria + uptake of labile PQC by fauna-respiration) * 0.95 [see fiow 251 40. Transfer of refractory PQC from surticiai to deep sediments = refectory POC/SO * sedimentation rate * 1.5. where I,5 is a model tuning factor (see flow 381 41. Uptake of refractory PQC by fauna = IF refractory PQC > y THEN 0.0 I * labile POC * 0.03 * Q IO ELSE 0.0 I* labile PQC pl:0.03 * Q IO* ( I -((y-refractory POC)/(y-a )I) [ 7 = 0.02; cx= 13.4 ’; y = 536 ’; fauna = hAbilePOC * 0.031 Deep sediments: 42. Uptake of labile DOC by bacteria = IF labile DOC > y THEN T * labile PQC * 0.105 * Q 10 ELSL: ~*iabiiePQC*O,iO5*(1-((y-iabiieDOC)/(y-cw)))*Q10[~= 1; a=0.03 ‘;y= 1.2 ‘; bacteria = labile PQC * 0.105; an estimate] 43, Uptake of refractory DOC by bacteria = IF refractory DQC > y THEN 7 * labile POC * 0.105 * Q IO ELSE +r*iabiie PQC*O.lOS*(l-((y-refractory DQC)/(y-u)))*QlO [T = 0.01 or I% of growth on labile DOC: OL~JF 0.5 “; y = 20 ‘; bacteria = labile POC * 0.105; an estimate] 44, Uptake of refractory PQC by bacteria = IF refractory PQC > y THEN T *labile POC * 0.105 * QIO ELSE T * labile PCK QO.lQ3 *(I -((y-refractory PBc)/( y-cx))I * Q 10 [T = 0.005 or one half of that for refractory DOC; QL= i34.4 “; y a 5376 “; bacteria = labile PQC*0.105] 45, Burial of labile LXX i-; labile DOC/450* sedimentation rate [see fiow 341 46 Wuriai of refractory DQC = refractory DQC/450 * sedimentation rate [see flow 361 47. Burial of labile PQC = labile POC *(59, i8/69)/450* sedimentation rate, where 59. IS/69 is the fraction of totai POC which is non-living [sea fiow 381 cl83 Buriui of r@fructWy cfructory PQC/4SO * sedimentation rate [see fiow 401 49. Dissolution of Iabilc PQ $0, 51, &?sp

of Milt in surfMa

0.01 * labile P0C [an estimate) * rcfructory PCK [an estimate of cu. I@% of initial hctcriill U~ItilkC] ‘labile P0C * ~9.~2/~~.7~ [9,~2/6~,7 is the respiring fraction ut meiofauna and macrofauna assumed to be 10% af their concentrartions

sediments]

lwnhpm: s~,d~~~.rut~~~~,~~~ u 0.95 d- ’[9S% of the uptake of materials by labile POC is respired. onscrvutivs; CMfith C1987) suggested that PQC in the water column may turn over in hours to days.]

ate, based on Table I] 1 d” ’[an csthte,

RESUS (see text) scusonriity (see text) sedimentation rate = 0.0055 mm d“ SETL (see text)

M/i06

is the Redfield ratio of NC,

based on an anticipated lower degradability of deeper POC]

(Hopkinson,

and 14/12

1986)

is the atomic ratio of N:C.

ments, we varied the values of these two r’ates to evaluate the effect on simulated results. (3) Effect of resuspension This experiment compared the model results obtained under default conditions with those obtained with no resuspension. Resuspension magnitude was set equal to 0 and settling time set to 1 throughout the simulation.

3.1. Default conditions Time courses of all standing stocks from the default simulation are presented in Fi

Water Column OAc9

4

0.29

3

0

2 1 0

20 lO$ O&

‘3o

‘J ‘A’s

‘0-N

‘0 ‘J ‘f ‘M’A’M’

J’J’

A’S’O’ N’

Fig. 2. Time series plots of standing stocks under default conditions, Water column compartments, surficial sediment compartments, and deep sediment compartments are depicted. Units are gC m-* or gN m-*.

Table 3 Initial conditions for standing stocks. Units are g m” Stock s bJ~ =(water, re tory D=(wSw) labile m(water) refractory IQ&c,)

Initial condition

Reference

thic respiration decreased following sion events (Fig. 3b). This result conforms to

4

view that the most active layer of sediments is a layer of floe at the surface (Kemp et al., 1982); once resuspended, this material would contribute to pelagic respiration. Cumulative respiration by surficial sediments over the course of the year represented about 54% of the total, while the water column accounted for 41%, and anaerobic respiration was relatively unimportant (5%; Table 4). Highest biotic rates occurred in summer, at same time that there was little resuspension. Thus, a greater proportion of total mineralization occurred in the sediments at that time. Material accumulated in the sediments during the winter-summer, the timing depending on the particular pool.

refractorY m(surficial)

134

labile ROCfdecp~

0.3

*frrtctory Doqdeep) labile POC~drcpI refmtory poc(deep) -

5-O 68.7

(best estimate “) (best estimate “) (Wainright, 1988) (Oertel and Dunstan, 198 1; Wainright, 1988 b, (Oertel and Dunstan, 198 1) Hopkinson (unpublished) (Whelan et al., 1976 “1 (Whelan et al., 1976 c 1 (Tenore, 1985; Wainright, 1988 d, (Hopkinson, 1985) (Whelan et al., 1976 ) (Whelan et al., 1976 ’1 (Hopkinson, 198s d,

1344

(Hopkinson,1985d,

phytoplankton DIN labile DBC~surtlcial) refractory DOC(su&inl) . labk P861(sudWal)

System respirationrespondedstrongly to resuspension;watercolumnrespirationinc

36 0.7 6.3 1.7 0.28 0.06 OS4 32

a based on total DOC of 2 mg 1”‘, 20 m water depth, 10% labile. POC of 350 kg I”‘, 20 m water depth, 10%

@I1.6 1.2 PO.8

labile in surficial sediments,

0.4 0 I.2 0.8 R

+ biomass of maerofauna,meiafauna, and microfauna (Table 2, flows 33 and 51).

0.4 0

AMJJASONDJFMAMJJASON

d-8UtftiC6

lementarydecreasesin sedimentarystandstocks, Dissolved stocks in the water column ibited decreasesfollowing resuspension,the net esult of the resuspensionof DOM and its consumpndedheterotrophs,Therewas a gradual increase in DOM after resuspensionevents. Despite peaks in primaryproductionin fall and spring (Fig. 3a), the model predicts that phvtoplankton biomassdoes not accumulateuntil winte&ring, in response to resuspensionevents. Lowest water column DIN concentrationsoccurred during the relant summerperiod. Standingstocks of r and nutrientsin anaerobicsediments changed little over the course of the year (Fig. 2c) md were seemingly aff&ctedonly by the strongest resuspensionevents (duringwinter months).

-

sed-deep

I

Fig. 3. The daily primary production (P) and total system respiration rates (R) (a), and the percentages of system respiration occurring in the water column, aerobic sediments and anaerobic sediments (b) under default conditions. The units for primary production and respiration are cumulative gC mB2 during the 600

J J AS

water column was not affected during quiescent conditions, but was higher during resuspension when the specific degradation rate was hig cause there was more sedimentary FGC iu IFSU~== pend. Higher sedimentary specific degradation rates led to increases in both sedimentary and water column

Table 4 Simulated primary production (P) and total system respiration (RI, under experimental conditions. The units are cumulative gC mV2 over the Ml&day simulation period. The last three columnsare the

relative amounts of total system metabolism occurring in the water column, surficialsediments(O-S cm), and deeper sediments (S-50 cm), Default parameters were: rcsuspcnsionfrequency of once per 46 d, resuspensionduration 4 d, resuspensionintensity and biotic rates varied seasonally, Other default conditions as in Tables 2, 3. In Experiment 2, the benthic specific degradation rates were 0.005 and 0.03 d”’ in the “‘low” and “high” simulations, and water column specific degradation rates were 0.475 and 1.43 dmi in the “low” and “high” simulations. Percentages of Respiration did not sum to 100 in Experiment 3 because of a rounding error Experiment

P

R

ON0

J F

(5) Denitrification

125 88 176 107 155 85

43

153 107 215 133 183 107

respiration (58% and 22% higher, respectively) relative to default conditions, but water column respiration decreased as a percen

cific degradation rate was increase conditions, but during resuspension events phyto-

1.6 R

1.2 0.8 0.4 0

1.6 1.2

96 of Respiration

41.3 46.5 36.2 28.2 5 1.4 19.6

54.4 47.4 60.8 66.9 45.1 74.3

83 29.1

63.1

ON

Fig. 4. Comparison of model results using high specific degradation rate (Ct.03 d- ‘, bold lines) versus low specific degradation rate in surficial sediments (0.005 d’ ’, fine lines). A11standing ctnrkc hnw the wits pC’rtV2.

O*B p

water surf.sed deep.sed (I! Default (2) Low sed. degrad. rate High sed. degrad. rate Low water degrad. rate High water degrad. rate (3) No resuspension

J J AS

4.3 6.1 3.0 4.9 3.5

6.2 7.8

0.4 0 AMJJASONDJFMAMJJASON

Fig. 5. Effect of resuspension on primary production (P) and system respiration (R) rates. In all cases, the fine lines depict results using default conditions, the bold lines represent a simulation where resuspension was set to zero. Primary production and respiration are in units of cumulative gC mW2 during the 600 d simulation.

plankton growth increased. This was a secondary effect of increased water column remineralization of resuspended labile POC. Though the absolute amount of benthic respiration was nearly unaffected in this experiment, the relative benthic contribution decreased when water column organic matter 1abiIity (specific degradation rate) increased (Table 4). Total cumulative system respiration increased by 20% (30 gC mW2) over the 600 d simulation compared with the default conditions.

0

1

2

3

3.3. Eflect of resuspension In the absence of resuspension, there were no rapid oscillations of particulate stan water column and surficial sediments. Instead, particulate organic matter stocks remained uniformly low in the water column and perhaps somewhat higher in the sediments. Labile DOC in the water column was higher in the absence of resuspension, presumably because potential consumers, i.e. bacte-

4

FrequencydResuspensim(eventsperseason)

Frequency ofResuspension(eventsperseason)

resuspensioncven!s on system respiration. (a) Cumulative total system respiration(A), (b) cumulahe primary production (P) and (c-e) the percentages of total system respirationoccurring in the watercohmn, surficial sedimentsandin deepsediments. Short, medium and long refer to durations of 1.2 and 4 d, respectively. Default conditions were: 2 events per season,4 d dunltion. Fig. 6. The

r&tionshipsbetweenfrequencyand duration of

e

organisms in an oxic water column was higher than respiration within the sediments. en the duration of resuspension events was held constant at the default value of 4 d, and the frequency was decreased from once every 46 d to once every 92 d, the overall trends in standing stocks were little affected; however, respiration decreased nw?r,?ll (135,&C p+

res

estuarine and coastal waters should be less than 50% in water depths greater than 5 m. Our model can be tuned to conform to that prediction with the following modificaliorrs; (1) i*iA~ci’;o~~ of ;h

609 d- *) z?r,(j# y,y$erc&s::

mineralization decreased to 35% of the system total (Fig. 6). Sediment mineralization was unaffected, although its relative contribution to ltotalsystem respiration increased. Increasing the frequency ot resuspension had the opposite effects, 3.5, E’ecr of denitrification The loss of regenerated DIN as N, via denitrification caused a major change in system metabolism. Total respiration under the influence of denitrification was about half that of the default simulation. Much of the effect was concentrated in the water column; the contribution of pelagic respiration to the system total decreased. The decrease in regenerated DIN affected the phytoplankton standing stocks, ecpecially after the first autumn (approximately day 2501, when pulses of DIN associated with resuspension began to decrease in amplitude relative to the

to reproduce Comparison of our simulations with and without resuspension (default vs. Simulation 3) d~m~nstr~t~s the importance of resuspension in determini the sites and rates of orgauie matter processing. SW* pension caused a large increase in total system or= ganic matter mineralization, and a shift in the site of mineralization from predominantly benth equal split between the benthic and pel ments. The temperature-dependentseasonality of biolo cal rates had important effects on our results. The Q10 of many organisms is iv the range of 2. Therefore, given a 20°C annual temperature range in many temperate environments, a 4-fold range of specific respiration rates might be expected (Grant and Hargrave, 1987; Hargrave and Phillips, 1989). However,

the Wiittef :Finimum of telnperature (and biotic activ-

ity) coincides with a peak in resuspension activity, because of the prevalence of winter storms. The two effects tend to counteract one another in our simulations.

Although summer resuspension events are generally less frequent, respiration rates are higher, and the potential effect of resuspension on overall mineralization rates should be large. Human activities such as bottom fishing by draggers (Churchill, 1989) or dredging (Brown and Clark, 1968) can cause resuspension of sediments. Commercial shrimp trawling activity creates extensive plumes of resuspendedsediments in the near-shore zone (out to ca. 4 miles from shore) during the period between June to December in coastal Georgia. If the water column is stratified, and resuspended sediments are confined below the pycnocline, then oxygen could become

severely depleted. Decreases in dissolved oxygen have frequently been observed in plumes of sediment resuspended during dredging operations (LaSalle et al., 1991, and references therein), However, the proportions of this mption which are biological vs. chemical often not known and probably are s of our simulations sug s demand may be biologis of specific degradation on in the ratio of benthic to

ity of benthic organic matter led to ic and water column respiration, lagic organic matter in benthic respiration, Hopkinson(1985) noted that frequent resuspension could lead to relatively refractory POC in sediments of the Georgia Bight; this would have the effect of reducing the specific degradation rate. S;mireachedby Hartwig (1976) and (1980). Wainright ( 1987) sugmay increase du because of the rates of small,organically rich particles relative to heavier,inorganic particles. This effect could be simulated by adding separate settling f~~tion~

for labile and refractory POC. A lower settling rate for iabiie PM than rthciiir~ BCX -+ou!d lead to increasing lability of total POC (labile + refractory) with time, but this was not done Denitrification, which takes in anaerobic sediments, and in large suboxic water masses such as upwelling regions, can cause the loss of 50% of DIN from coastal marine systems (Seitzinger, 1988). This loss of DIN may severely affect phytoplankton production. If resuspension serves to re-inject sedimentary organic matter back into a relatively oxic environment, then denitrification could be

available for primary diminished, leaving more with denitrification production. In our simu present, there was a clear decrease in primary production, leading to a low P:R ratio (0.52). We stress that our model was not designed to accurately predict C:N ratios or P:R ratios, however, this low P:R ratio suggests a tendency towards heterotrophy when denitrification comprises a significant part of mineralization. These results suggest that resuspension may contribute to the extraordinarily high pelagic primary production rates in nearshore Georgia waters (Thomas, 1966; Hopkinson, 1985) by suppressing denitrification activity. Our study has focused on the effects of physitally-farced resuspension. Logically, our model could be extended to the effects of biolo resuspension (see e.g. Graf, 1992). tions we included bio-resuspension indirectly, as a on of the sedimentary respiration rate, but we no attempt to isolate the effect of biotically induced resuspension, We have presented results from a model which simulates the effects of sediment resuspension on organic matter processing. Our results confirm the hypotheses generated in previous studies, i.e., that resuspension may determine the sites and rates of organic matter mineralization in shallow environments (Hopkinson, 1985,1987), and that storm events are important in influencing the distributions of particulate organic matter and the timing and magnitude of primary production (Haines and Dunstan, 1975). However, these results require verification in the laboratory and in the field. Future efforts should incorporate advective transport to and from the present model’s domain, and an explicit treatment of stratification in the water column
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