Estuarbte and Coastal ~Iarhte Science (x98o) xo, x65-x8o

The Effect of Tidal Resuspension on the Distribution of Intertidal Epipelic Algae in an Estuary

Prisci l la W. Bail l ie and Barbara L. Welsh 23Iarine Sdences Institute, University of Connecticut, Groton, Connecticut, U.S.A.

l~eceived 6 September z978 and in revised form 3o October r978

Keywords : benthic flora; micro-algae; intertidal environment; mud flats; New England coast

The distribution of microalgal biomass in the sediments and water column of a salt marsh and mudflat ecosystem was studied over a fifteen month period by fluorometrie analysis of chlorophyll a. Chlorophyll entered the water column on the early flood tide due to the resuspension of intertidal mudflat sediments and associated epipelie algal flora. By mid-flood tide, chlorophyll values were surpassed by its degradation products. Unusually large numbers of bivalves and other suspension feeders present in the system may have processed the water and released fecal materials, thus raising the concentration of pheopigment in the water column between early and mid-flood tide. By mid-ebb, the pigment levels had declined, possibly due to the settling of algal cells and feces. Resuspension recom- menced briefly by late ebb as the system emptied. Calculations revealed that if io to x5~o of the mudflat sediments were resuspended to a depth of x nun it could account for the chlorophyll levels inside the estuary, which were high compared to neighboring systems. Pigment gradients in the water column and sediments indicated net transport of epipelie algae from the mudflat to the salt marsh where most of the bivalves were located. Suspended material concentrations as well as counts of epipelie and phyto- plankton frustules suggested that epipelie algae in shallow estuarine systems may be art important food source for the filter feeding community when made available twice daily by tidal resuspension of sediments.

Introduction

The purpose of this study was to examine the effect of tidal resuspension of estuarine sediments on the abundance, diztribution and availability of a benthic floral community, the epipelic algae.

The periodic resuspension of sediments in shallow estuarine systems is caused by several physical factors such as wind action on the water surface, tidal currents and convective currents (Anderson, x972, i973, 1976 ). The effect of these combined forces is greatest in shallow water so that resuspended sediments enter the water column on the early flood tide and again on the late ebb. This phenomenon has been well documented in the geological literature but its effect on communities of benthic microorganisms such as epipelic algae has received little attention from biologists.

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x66 P. IV. Baillle ~- B. L. Welds

Epipelic algae are primarily motile pennate diatoms (with two raphes) which inhabit the surface of submerged sediments (Round, x97x ). It has been known for some time that such epibenthic microalgal communities are important contributors to primary productivity in aquatic ecosystems (Gr~ntved, x96o; Gruendling, x97I ; Leach, t97o; Marshall et al., x97t ). Their production far exceeds that of phytoplankton in many shallow systems, both fresh- water and marine (Hargrave, I969). The epipelic standing crop is also exceptionally stable seasonally. In contrast to phytoplankton, they are available throughout the year (Smyth, x955), and may constitute an important energy source in the system, especially between phytoplankton blooms.

The spatial distribution of epipelie microflora is particularly advantageous for benthic grazing populations (Moss, x968 ) since the majority of the cells are concentrated near the sediment-water interface. They migrate upward through the top 5 mm of sediment (Hopkins, x963, x964) toward the light (Palmer, I96o; Perkins, 1958; Taylor, x964) or downward for protection and possibly for nutrients (Gruendling, x97x; Pomeroy, x959).

Epibenthic microorganisms are, by virtue of their location, susceptible to erosion. In some freshwater streams epiphytic and epilithie forms were found suspended in the water column (Marker & Gunn, x977). A planktonic assemblage in a freshwater creek was mainly derived from the epipelic algae and was utilized by filter feeders such as lamprey larvae, protozoa, amphipods and hemiptera (Moore, x972 ). Benthic diatoms have also been found in the water column of several subtidal marine systems (Karentz & Mclntyre, x977; Marshall, x956; Roman & Tenore, x978 ). In an intertidal saltmarsh creek, tidal flux caused the re- suspension of bacteria (Erkenbrecher & Stevenson, i975, x977). However, we have not found studies in the literature which deal specifically with the resuspension of epipelic algae from intertidal mudflats and salt marshes, even though these shallow systems are "known to support a large and varied diatom flora (Drum & Webber, x966; Riznyk, i973). Resuspension would greatly increase the importance of epipelic algae to an estuary because it would make the cells available, not only to benthic grazers, but also to filter feeders (Marshall, 197o; Roman & Tenore, x978 ).

Methods

The study t2rca Branford Harbor is an estuary on the north central shore of Long Island Sound (Figure x). This system was especially suitable for studying the effects of tidal resuspemion on a sedi- ment community because the study area was dominated by a large centrally located mudflat (co ha). The tidal fiat was bounded on three sides by a salt marsh (e 4 ha) and on the fourth side by the Branford River channel.

The mean tidal amplitude over the mudflat was x.8 m as determined by a tide gauge mounted near the eastern end of the system. Most of the mudflat lay o.I 5 to o'3o m above mean low water and was therefore exposed for two to five hours during 87% of all low tides. The mudflat sediments were about 6% sand, 33% clay and 6x% silt.

The marsh and mudflat supported unusually large populations of macroheterotrophs (Olmstead et al., x976 ). The ribbed mussel, Modlolus demlssus Dilhvyn, was found in the salt marsh in numbers exceeding 2ooo individuals m -~, especially along the banks of the creeks. A large bed of the oyster Crassostrea virg[nica Gmelin was present on a sand bar at the eastern end of the mudflat (station M6). Zooplankton and the mud snail Nassarius obsoletur Say were also present in large numbers.

Epipelic algae in an estuary x67

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Figure I. Ilranford Harbor study area, latitude 4xoz6 ', longitude 7z~ ', showing 9 diurnal stations (Mz-M9) and 4 seasonal stations (Cz-C4). The general direction of water flow is indicated by arrows (F=flood, E=ebb) . The broken line shows the edge of the channel.

TABLE z. Sampling regime. The three parts of the study are compared in terms of the size and location of the sampling area, the time span of each study and the frequency of sampling. The tidal phase, community and variables being studied are also shown

Variable Diurnal studies Seasonal studies Resuspension studies

Location Nine station matrix across Four stations along Single station channel, mudflat and axis of one creek April and I~Iay: M 4 creeks August : C3

Time Span Five months Nine months Three occasions: June-October z975 December *975- 2x April z976, ~o May I976,

August z976 4 August x976. Sampling Four mid-tide sampling One low-tide sampling Successive sampling every Frequency runs during each 25 h run bimonthly z5 min over 2 to 3 h

monthly diurnal between low and experiment mid-tide

Tidal Phase Flood and ebb Flood or ebb April and l~Iay: flood August: ebb and flood

Community Water column Sediments and water "~Vater column column

Variables Pigments, temperat~re Pigments, temperature Pigments, temperature and salinity and salinity and salinity

April and May: turbidity August: cell counts

168 P. IV. Baillie C.q B. L. Welsh

The sampling reghne The effects of tidal resuspension on the distribution of microalgal biomass in the study area was principally measured by fluorometrie analysis of chlorophyll a (Chlo) and its collective breakdown products, pheopigment a (Pheo). A three part study was conducted over a fifteen month period (Table x).

(r) Five monthly diurnal studies from June through October i975 compared pigment distributions in the water column at nine stations throughout the study area at mid-tide. Water samples were collected from a boat which cruised across the system on each of the four half tides during a diurnal period. Vertical profiles had shown that pigments were quite evenly distributed from surface to bottom; all water samples were therefore collected from the upper o.25 m of the water column.

(2) From December x975 through August z976, bimonthly seasonal studies compared pigment levels in the water column to those in the sediments of a creek subsystem. A small inflatable dinghy was used which could be maneuvered up the narrow marsh creek in shallow waters without disturbing the bottom. All water and core samples were collected near low tide when the water was less than o. 5 m in depth. Both flood and ebb tides were'sampled at random.

43) Three separate resuspension studies compared pigment levels in the water column between low and mid-tide to several variables associated with the resuspenslon of sediments. During the April and May studies, periodic water samples were collected at a single mudflat station (M4) as water moved into the system on the flood tide. Chlo levels were compared to turbidity measurements. During the August study, both ebb and flood tides were sampled. A series of ebb tide water samples were first collected from the inflatable dinghy just inside the creek (station C3). The dinghy was then allowed to float out across the mudflat with the ebbing tide where further samples were collected. Flood tide waters were followed into the system in a similar fashion. Chlo concentrations were compared to diatom frustule counts.

Laboratory analysis All water samples were collected in brown polyvinyl bottles containing several drops of x~/o MgCO 3 solution, and were filtered within two hours. They were prepared for pigment analysis aecording to the methods of Striekland & Parsons (x972). Millipore filters (o-45 ~rn) were mechanically ground and extracted in zoo% acetone (Jeffrey, x974). Analysis was done on a Turner III fluorometer using a G 4 T4-I lamp, a Coming Glass CS-5-6o primary filter and two secondary filters, Corning Glass CS-2-64 and Wratten gelatin no. 7 o (Loftus & Carpenter, z97z). Chlo and Pheo concentrations were calculated using the equations of Lorenzen (z966).

For the resuspension studies, replicate filters were prepared for diatom counts by boiling in concentrated nitric acid for 3 o rain. The samples were washed and concentrated by eentrifugation and were mounted in Permount. Frustules were counted as pennate or centrie forms at 45 • These water samples were also analyzed fluorometrically for pigments and spectrophotometrically (72o nm) for turbidity.

Sediment cores were collected by hand with PVC tubes (56 X 35 cm ID). The upper 3 to 5 em were extruded into PVC rings and stored overnight in the laboratory (Round & Palmer, x966 ) leaving the aerobic-anaerobic layered structure of the sediment intact. The following day the algae were harvested by the tissue trap method of Eaton & Moss (x966) and by withdrawal of microcores (z cm 2 • 5 mm) from the sediment surfaee with a truncated syringe. Tissue traps were ground and extracted as above, and microcores were extracted overnight under refrigeration (Tietjen, x968 ) but were not ground.

Epipelic algae in an estuary x69

Tlle upward migration of intertidal epipelic algae is at a maximum on the ebb tide during the morning hours and they continue to migrate with the tide in the laboratory for several days (Round & Palmer, 1966 ). Therefore, sampling dates were selected so that ebb tide occurred before noon to ensure maximum harvest of cells from the sediment surface the following morning.

Potential productivity of the epipelic algae was determined during late August. Tissue traps were incubated for 5 h in filtered estuarine water under full sunlight at a2 to 24" 5 ~ CO 2 uptake was measured periodically, using an Oceanographic International carbon analyzer.

TABLE 2. Statistics for the diurnal and seasonal studies. C=Chlo, P=Pheo. N = Number of observations. Y=sample mean. CV---sample coefficient of variation. CVL=mean coefficient of variation due to location in the system. CVs=mean coefficient of variation due to seasonal effects. P/(C+P)=mean percent Pheo in the total pigment complex

Study Samples Pigment AT Y CV CV. CVs P/(C+P)

Diurnal mid-tide

Seasonal low tide

Water C 148 16 .o 85"4 4o'55 82"2 column P 148 29"4 80-2 32"ox 78"9 64"8

Water C 62 54"2 I37"8 23"7 x27"z column P 62 12"7 xxx'8 7x'2 xx9"3 I9"o

Epipelon C 67 xo .6 96"2 7r'2 83"6 tissues P 67 o . o

Epipelon C 29 698"2 40"7 36"6 33"7 microcores P 29 27x'x 78"4 5t'7 62"4 28"0

Results

The water cohmm at mld-tlde Mean Chlo levels in Branford Harbor at mid-tide were substantially higher than those reported for neighboring systems at the same time of year. For instance, values in the study area during July averaged at" 7 mg m -3 with maxima as high as 48.5 mg m -a. In Long Island Sound and Flax Pond, July levels do not exceed 1o mg m -a (Conover, .t956; Hardy, 1969; ~IoI1, 1974).

Variability of pigment distribution in the system was high at mid-tide, as shown by the coefficients of variation (Table 2). Sample variances were heterogeneous, which precluded a two-way analysis of variance (Sokol & Rohlf, z969). Coefficients of variation (CV) were therefore computed for each row (sampling run) and for each column (station) and then averaged to give mean coefficients of variation due to season C ~ s ) or location C~L) . They showed that the major portion of variability at mid-tide was due to seasonal changes rather than to spatial heterogeneity across the system.

The spatial distribution "of pigments in the three main areas of the system, channel, mudflat and marsh, varied from month to month (Figure 2) and from tide to tide within the same month. In July, for instance, pigment levels were highest near the marsh on the flood and in the channel on the ebb. In August on the flood, highest concentrations were found over the mudflat. In all three areas of the system, however, the levels of Pheo were consis- tently higher than those of Chlo at mld-tide, both on the flood and on the ebb.

Regressions of Chlo against Pheo by tidal phase (Figure 3) showed that the pigments were linearly related and that there was no significant difference between the slopes of flood and ebb (a=o.os) . There was also no significant difference between flood day and night slopes,

x7 o P. IV. Baillle & B. L. Welsh

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Figure 2. Mean diurnal mld-tide pigment concentrations in the three major areas of the system. R=r ive r channel, F=mudflat , C =marsh creeks. Solid l ines=Chlo, dashed lines =Pheo. Open circles = day, closed circles :-night.

but ebb tide regressions showed a highly significant difference in the Pheo]Chlo ratios which was dependent on photoperiod; there was relatively more Pheo in the water column during the day than at night.

The magnitude of net tim,: for both Chlo and Pheo was linearly related to pigment concen- tration (a=o.os) and declined seasonally from June through October (Figure 4). Fluxes in Flax Pond showed similar seasonal effects (Moll, i974). The direction of pigment flux in Branford Harbor showed that there was significantly more pigment on the flood tide than on the ebb (a=o.os) during all months except July. The high ebb tide pigment levels in July appeared to be concentrated in the channel and at the northern end of the mudflat.

The water cohmm at low tide Seasonal Chlo and Pheo concentrations in the water column at low tide proved to be even more variable than those at mid-tide (Table 2). Over a nine month period, both Chlo and Pheo were more variable from week to week than from station to station along the axis of the creek. The four stations were closely situated (Figure z) so that water moving in and out of the creek was relatively homogeneous during one sampling run. Samples taken two weeks apart, on the other hand, could differ by an Order of magnitude. This is not unusual during bloom conditions in local waters. However, the highest Chlo levels found in the creek (I29"o to 419"3 mg m -~) occurred early in May, two months after the usual Spring bloom period in Long Island Sound (Conover, z956).

The spatial distribution of Pheo concentrations in the creek water column followed a positive gradient from the mouth to the upstream areas. Tests, using Kendall's coefficient of rank correlation, revealed that Pheo values upstream were significantly higher than those near the mouth (a----o.oo2). The Chlo gradient was not significant.

Epipel;c algae in an estuary x7r

In these low tide studies, Chlo was clearly the dominant pigment on the early flood (Figure 5). This trend was the reverse of that seen at mid-flood (Figures 2 and 3) when Pheo was the more important pigment. By late ebb ticle, Chlo and Pheo levels were nearly equal.

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Figure 3. Linear regression of Chlo and Pheo by tidal phase. Open circles=mid- flood closed circles=mid-ebb. The box encloses statistics by tida ! phase and by photoperiod: regression equations; the level of the significance of linearity (a); the amount of variability explained by the correlation of Pheo and Chlo (Ra); the correlation coefficient (R).

Tide Photo Regr.eq. a R t R

Flood Y=6"673 + x.4o9X o.oooox 0"727 0.852 Day Y=6"829+ x'42oX o.oooox 0"702 0"838 Night Y=7"9o3+ x'z74X o'oooox 0.805 0"897

Ebb Y----4"727+ z'574X 0.00001 0"706 0"84 ~ Day Y--o'647+ 1"968X o'oooox o.74x o.86I Night Y-~7"648+ x'oo7X o.oooox 0"907 0"952

The sediments The distribution of pigments in the sediments of the creek was less variable than expected (Table 2). Chlo from tissue traps, which harvested only live and motile algae, showed a CV that was considerably lower than that for the water column at low tide. Microcore extracts also showed relatively low CVs. Variability in the sediments due to station location was

172 P. IF. Baillie & B. L. Welsh

similar to that due to seasonal changes, which signified that the distribution of epipelie algal biomass was seasonally stable.

Chlo harvested by the tissue trap method was distributed along a spatial gradient (a----- o.ooz) with significantly higher values upstream in the creek sediments and lower values over the mudflat. A similar gradient appeared in the microcores for Pheo (a=o.oos) but not for Chlo.

Comparisons of Chlo from tissue traps and mlcrocores (Figure 6) showed that values obtained by the two methods differed by one to two orders of magnitude. Since the tissue traps collected only migratory cells which had reached the surface of the sediment, our data indicated that only a fraction of the algae were located exactly at the sediment-water interface

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Figure 4. Tidal flux of Chlo and Phco by station and by month. Each po in t= the difference between a flood tide pigment value and one for the following ebb at a single station.

at any one time. Other investigators have suggested that the epipelie assemblage does not migrate en masse in one direction, but that some ceils move upward while others are moving downward (Round & Eaton, r966 ). The Eaton-Moss method proved to be a sensitive indicator of distribution patterns in the system but, used alone, it would have resulted in an underestimation of pigment levels present in the sediments. Total sediment Chlo, on the other hand, may overestimate living algal biomass in the epipelie habitat ~Tarshall et aL,

x97x).

Epipellc algae in an estuary x73

The maximum photosynthetic rate of the algae in the tissue traps was 5'3 mg C fixed-rag Ch lo -Lh -x. This rate was considerably higher than that given by Colijn & van Buurt (i975) for intact sediment samples. Freeing the diatoms from the sediment allowed estimation of potential productivity in the epipelon and simulated tidal resuspension of the cells.

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Figure 5. Mean pigment levels in the water column for four creek stations at low tide�9 Open bars =Chlo, stippled bars =Pheo.

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Figure 6. Chlo in tile creek sediments'.harvested by tissue traps and mic:'ocores.

z74 P. IV. Baillle ~ B. L. Welsh

Sediment-water column interactions Water on the incoming tide in Branford Harbor often appeared turbid at mid-tide, while ebbing water was clear. These observations were substantiated by suspended material con- centrations (Bohlen & Cundy, 1978 ). The suspended load in the water column (Figure 7) was significantly higher near low tide, regardless of whether the tide was flooding or ebbing (paired variates t-test, a=o.oQ. Resuspension of sediments occurs most readily in shallow water (Anderson, z973; Settlemyre & Gardner, 1977). Our qualitative observations of murky water at mid-flood reflected the fact that tidal resuspension had already occurred earlier in the tidal phase and that some of the sediment was still suspended. Clear water at mid-ebb indicated that the sediment had settled and that the second resuspension at the the end of the tidal cycle had not yet begun.

The tidal resuspension studies examined these phenomena in detail during the critical two hours before the system was drained and after it began to refdl. As water from the Sound began to move into the system during early flood (Figure 8), salinity increased over the mudflat, possibly due to the release of highly saline pore water from the sediments when resuspension occurred (Welsh, 198o ). After 2 to 3 hours, salinity values over the mudflat approached those of the channel as.equilibrium was achieved across the system at mid-tide. Turbidity followed a similar pattern. Values over the flats eventually approached those of the channel due to settling and dilution effects. During the KIay experiment,

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Epipelic algae in an estuary z75

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Figure 8. Early flood tide dynamics. Tidal resuspension effects on salinity, turbidity and Chlo are shown. Each point represents a percent of the maximum value attained by the variable during the experiment. Solid lines ---- mudflat'(station ~]4) and closed lines = channel (station M3). The time of day appears on the abscissa.

sediment temperatures were cooler by 6 ~ than the overlying water on the early flood but both water and sediment temperatures converged toward those of the channel as the tide progressed. Chlo, during this period, tracked well with turbidity and showed a precipitous decline which indicated that the initial surge of resuspended sediments was rather short-lived, as reported by Anderson (x972, x976). The instability of all four variables, salinity, tempera- ture, turbidity and Chlo indioated that the early flood tide was a period of dynamic change over the mudflat.

The August experiment compared flood tide to ebb (Figure 9). Temperature and salinity were lower and less variable on the ebb though a slight rise occurred in both variables over the mudflat as the water drained from the system. On the flood tide, water temperatures rose 6. 7 ~ in 53 rain as cool channel water spread across the flats which had been exposed to the sun for three hours. Chlo and the number of diatom frustules in the water column increased on the ebb tide as depth decreased. On the flood, both variables reached their maximum values immediately and then declined as microalgae settled or were filtered from

r76 P . W . Baillle ~ B. L. Welsh

the water column. The total number of frustules was linearly related to Chlo concentration on both tides (a=o.os) . Of the frustules counted, 73 to 76% were pennatcs, which further links Chlo levels in the water column to the tidal resuspension of sediments. A phytoplankton community would normally contain mostly centrie forms (Conover, x956 ) whereas the epipelie algae are dominated by pennate diatoms (Round, I97i ).

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Figure 9. Comparison of late ebb to early flood tide. Each point represents a percent of the maximum value attained by the variable during both tidal phases. Broken l ine=creek (S ta . 133) , open line---mudflat (S ta . M 7 ) , solid l ine=channel edge (S ta . M 6 ) . Open circles=centrate diatoms, open triangle3=pennate diatoms. T i m e of day appears on the abcissa.

Conclusions

The coulse of tidal resttspetlsioll At low tide the mudflats are exposed to the temperature of the atmosphere and, during the day, to the heat of the sun. As the tide begins to rise, water spills over the edge of the channel and spreads into a thin layer over the flats. ~fany rapid changes begin to occur. The difference in temperature between the water and sediment causes convection currents which, in combination with tidal currents and wind action on the shallow water, results in the resuspen- sion of sediments (Anderson, x973, x976 ), and their associated microflora. A pulse of Chlo is releascd into the water column together with the pheopigmcnts also present in the sediment (Figure 5).

Epipelic algae in an estuary x77

Figure I o. The course of tidal re-suspension. Hypothetical levels of Chlo (open bars), Pheo (hatched bars) and suspended materials (solid lines) are shown at various times in the tidal cycle and possible events are listed. x. Resuspension of epipelio algae increases chlorophyll level in water column. 2. Suspended material increases due to tidal resuspension of mudflat sediment. 3. Chlorophyll decreases due to selective filter feeding and settling of eells~ fecal materials released, pheopigments increase. 4. Suspended material settles out. 5. Suspended material has equilibrated. 6. Chlorophyll and pheopigments have settled out. 7- Late ebb resuspension begins. Suspended material increases. 8. Epipelic algae are again resuspended.

Once suspended, the cells are potentially available to filter feeders such as bivalve molluscs (iX{arshall, 1967) , zooplankton (Sbuman & Lorenzen, 1975) and other arthropods. The consumption of the microalgae and the release of waste materials raise the relative level of Pheo by mid-flood (Figures 2 and 3). By late flood the heaviest fraction of the sediment has settled out of suspension. Fine materials (such as microalgal cells) are probably the last to descend (Anderson, x973). As the tide turns and ebb begins, the amount of suspended material in the water column has stabilized (Figure 7). By mid-ebb tide the Pheo fraction has declined (Figure 3) due to settling of feces and pseudofeces (Zabawa, i978 ). As the depth of the water decreases, the effects of wind agitation become more pronounced. Resuspension reoccurs briefly as the system empties (Figures 7 and 9).

The significance of tidal resuspension Tidal resuspension breaks down the biphasic nature of the sediment-water interface, causing a temporary, but important intermingling of resources. Suspended epipelic algae in Branford Harbor become a valuable component of the plankton because of their abundance, seasonal stability and spatial distribution within the system. Suspended food reserves from estuarine sediments are "known to be utilized by the filter feeding community (Marshall, i967, x97o ). Bivalves in particular have elaborate sorting mechanisms to cope with the melange of suspended particles and can feed under extremely turbid conditions (Purchon, 1968 ). Benthic diatoms form an important component of their diet (Marshall, x97o ).

The epipelic algae are abundant in Branford Harbor and therefore constitute a major energy source in the system in terms of primary production and standing crop. ~laximum potential primary productivity of i28 g C m-2year -1 compares favourably with x29 g C m -2 year - t for Spartina alterniflora in the salt marsh Warren & Niering, x976) and 39 g C m--" year-* for Ulva lactuca on the mudflat Welsh, 198o ). Sediments in the study area contain one to two orders of magnitude more microalgal biomass per square meter

178 P. IV. Baillie ~q B. L. Welsh

than does a cubic meter of Sound water. Calculations reveal that if only xo to I5% of the mudflat sediments were resuspended to a depth of x mm, it could account for the difference between pigment levels in the water column inside and outside the system.

The epipelic standing crop in the study area is more stable seasonally than are the phyto- plankton of Long Island Sound. Epipelie biomass fluctuates periodically but does not show the population crashes so typical of phytoplankton. Sediment flora are slow to respond to environmental changes because of the stability of the benthic habitat (Gruendling, x97x; Round, x96z ).

Tidal resuspension is a significant factor in determining the spatial distribution of inter- tidal epipelic algae. Shelter is important in the distribution of benthic microalgae (Grontved, x96o ) and erosion can lower the standing crop (Moss, x968; Round, x96z ) over the mudflat. Anderson (x973, x976 ) demonstrated that the outer reaches of a tidal flat show maximum scour whereas the inner areas are primarily depositional. Our studies showed that the diatoms were distributed along a gradient in the creek sediments with highest levels upstream in the more protected region of the subsystem. Pheo gradients indicated that heterotrophie activity was greatest in the salt marsh.

In summary, tidal resuspension affects the spatial distribution of the epipelie algae in two ways. The cells are suspended vertically into the water column, thus becoming available to the filter feeding community. This is important because of the abundance and seasonal stability of the epipelle standing crop compared to phytoplankton. Tidal resuspension also affects the horizontal distribution of the algae by causing net shoreward transport of the cells to the marsh where extensive beds of bivalves are situated.

Acknowledgements

This project was supported in part by Waterways Experiment Station, U.S. Army Corps of Engineers Contract No. DACW 33-75-C-oo59. Our thanks go to W. F. Bohlen and D. Cundy for providing suspended material data and to F. R. Trainor and P. H. Rich for their generous assistance and advice throughout the project.

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