The effect of grazing by a gastropod, Nassarius obsoletus, on the benthic microbial community of a salt marsh mudflat

Estuarine

and Co+al

Marine

Science

9,

(1979)

121-134

The Effect of Grazing by a Gastropod, Nassarius obsoletus, on the Benthic Microbial Community of a Salt Marsh Mudflat”

Michael L. Pace Department

of Zoology,

University

of Georgia,

Athens,

Georgia

30602,

U.S.A.

30602,

U.S.A.

Stephen Shimmel and W. Marshall Darley Department Received

of Botany,

University

of Georgia,

Athens,

Georgia

17 August 1978 and in revised form 23 October 1978

Keywords: salt marshes;

grazing; microbial community; macrobenthos; mud flats; gastropoda; Sapelo Island

microflora;

Removal of Nassarius obsoletus (Nassaridae: Gastropoda) from a salt marsh mudflat dominated by epibenthic microalgae resulted in highly significant increases in algal and total microbial standing stocks as measured by chlorophyll a and ATP respectively. This effect occurred within three days after the removal of the grazer and was maintained for the duration of each experiment (IO and 12 days). Concomitant with an increase in standing stock was an increase in the productivity (mg C fixed m --2 h -I). The assimilation number [mg C (mg Chl a) -i h -iI, however, was not affected by grazer removal. Energy charge ratios, which assessed the growth state of the microbial community, increased in ungrazed areas during the first few days after consumer removal. Disruption of the sediment surface to stimulate bioturbation by the N. obsoletus population had no effect on algal productivity or on the standing stocks of algae and microbes suggesting that the grazer’s influence is not mediated through physical disturbance. Simulated grazing using a lens paper removal technique resulted in standing stocks and productivity which were higher than N. obsoletus grazed areas and comparable to ungrazed areas. N. obsoletus grazing rates must have exceeded the 5-1076 removal of surface chlorophyll per day measured by the lens paper technique. Natural densities of N. obsoletus reduced both algal biomass and productivity. Although previous studies have documented mechanisms of positive feedback from consumers to producers, in this intertidal mudflat community, grazing appears to act strictly as a negative feedback regulator of benthic algal activity.

Introduction Although estuarine Consumer

macroconsumers are of minor importance as processors of energy in salt marshsystems, they may, in part, regulate microbial processes (POmerOy et al., 1977). maceration and bioturbation often activities of ingestion, egestion, excretion, “Contribution No. 373 from the University Island, Georgia.

of Georgia

Marine

Institute,

Sap&

I21 OJOZ-~~~~/~~/O~OIZI

+ 14 $OZ.OO/O

c

1979 Academic

Press Inc. (London)

Ltd.

122

M. L. Pace, S. Shimmel & W. M. DarleJl

decrease the standing stock but increase the metabolic activity of microbial communities (Barsdate et al., 1974; Brenner et al., 1976; Brock, 1967; Flint & Goldman, 197-j; Dargrave, 1970; Lopez et al., 1977). The present study investigated the effect of grazing by an intertidal gastropod, Nassarius obsoletus (Say), on the benthic microorganisms of a salt mash mudflat. The objective was to determine if N. obsoletm regulates the standing crop of microalgae and the total microbial community and whether these consumers enhance algal productivity, turnover rates or the community energy charge ratio. The Eastern mud snail, A7assurius obsoletus, is principally a deposit feeder capable or subsisting on a diet of algae (Scheltema, 1964; Brown, 1969). Radiotracer feeding studies on N. obsoZetususing intact labeled cores as a feeding surface indicate that the amount of al:gal carbon assimilated by the snails is equivalent to 75 “/o of their daily body carbon losses (Wetzel, 1977). Th e snails also utilize bacterial carbon but are incapable of assimilating carbon directly from Spurtina uZtem.i$Ioru (Loisel) detritus. N. obsoletus may also consume meiofauna, but this aspect was not investigated in the present study or by Wetzel (1977).

Methods Study site All experiments were carried out on an intertidal mudflat located on South End Creek Sapelo Island, Georgia. The mudflat borders a Spartinu uZtevni$oru marsh and is exposed only during low tide for a period of about five hours. The biomass of hT. obsoletm on the mudflat ranges from z to II g C me2 (excluding shell material) which corresponds to densities of 480 and 1580 animals m- 2 (Wetzel, 1977). The algal community consists principally of a diverse assemblage of pennate diatoms with a lesser abundance of euglenoids and filamentous blue green algae (Williams, 1962). Seasonal algal fixation rates measured during low tide range from IOO to 400 mg C me2 h-l (D. Wh itney & M. Darley, unpublished data). Ciliate protozoans and nematodes are abundant in these sediments. There arc also high standing stocks of bacteria in the mudflat sediments which rapidly metabolize exogenously supplied organic substrates (Christian, 1976). Experimental design Two replicate field experiments (GREI and GRE2) were carried out to assess the effect of h’. obsoletus removal on the production and biomass of the algal community and the bioniass of the total microbial community. In each experiment a complete randomized block design was used consisting of two blocks with two treatments applied randomly within each block. Treatment I consisted of enclosing an experimental plot with a circular topless cage (45 cm diameter, 6 cm height) made of hardward cloth (6.35 mm mesh) and removing all snails from within. These cages were effective in keeping snails out of the plots while minimizing shading of the algae. Treatment 2 was a control. The plot was marked and the snails were allowed to move in and out of the area freely. No attempt was made in these experiments to control grazing rates. Snails present on the open plots were counted on each sampling day; numbers ranged from 29 to 225 (182-1415 animals mw2). In several initial experiments WC attempted to regulate grazer densities by enclosing snails within cages, but the animals congregated near the cage sides and did not graze normally. In each experiment five or six replicate cores for each parameter (chlorophyll u, ATP and 14C fixation) were taken on the first day (hereafter Day o) of treatment and every one to three days thereafter. Plots were sampled for IO days in GREI and 12 days in GREz.The replicate cores were taken to 5 mm depth at randomly chosen points within each plot using

-

Effect of grazing on benthic microbes

123 -

a IO cm3 polypropylene syringe barrel with the stem removed to make a coring tube (1.6 cm diameter). To assess the effect of snail removal on the metabolic activity of the total microbial community, energy charge ratios were determined in a third experiment employing a design similar to that described above. In this case four replicate cores were removed from each plot on each sampling day. Based on data from previous experiments showing that the microbial community responded to grazer removal within a few days, plots in the energy charge experiment were sampled for only four days. Two potential impacts of grazing by N. obsoletus were evaluated in a final experiment involving four treatments. Treatments I and z were, as before, ungrazed and grazed plots. Treatment 3 involved disrupting the sediment surface in an enclosed plot from which snails had been removed. Sediments were disrupted by dragging a glass rod over the surface for two minutes on each day of the experiment to simulate the bioturbation of the surface that snails cause as they move. An enclosed plot was artificially grazed for Treatment 4 using lens paper. Epibenthic diatoms will migrate up into lens paper which is placed flat on the surface of the mud (Williams, 1963). In this experiment a single layer of lens paper was placed on the surface of the plot one and a half hours before low tide. The paper was then removed at low tide and saved for chlorophyll extraction. Samples for ATP, chlorophyll u, and 14C fixation were taken as described above. Chlorophyll analysis Quantitative analysis of chlorophyll a was done using a modification of the method of Strickland & Parsons (1968). Cores were frozen intact and thensectioned while frozen using a microtome adjusted to slice in one millimeter increments. Estrada et al. (1974) showed that in a New England salt marsh most of the chlorophyll lies in the top half centimeter of scdimcnt. In fact, the photosynthetically active algae must exist in the top millimeter or less of sediment due to the rapid extinction of light (Williams, 1962; Perkins, 1963). N. obsoletus disrupts the surface of the sediment as it moves and appears to graze not only on the surface algae but also on microorganisms which are beneath the surface. Sediment cores were thcrcfore sliced into a O-I mm section which represented the active surface algae and a 2-5 mm section which included the rest of the sedimentary algae available to the snails. A small amount of MgCO, was added to core sections which were then disrupted in 5 ml
I24

M. L. Pace, S. Shimmel & W. M. Darley

the measurement should have been the same in both plots, unless grazing by the snails produced substantial chlorophyllide. However, even if grazing produced chlorophyllidc~, the experimental design was still valid since any increase in chlorophyllide was detected as chlorophyll a in the grazed plot. Carbon-14 Fixation Intact cores were removed to the laboratory and incubated in a chamber containing 1%0, under controlled temperature (25 “C) and light (1740 FE rnp2 s-l) (Darley et al., 1976). The 15 min incubation was carried out during the period of maximum low tide to minimizt the effects of the vertical migration rhythm of the algae (Darley et al., 1976). Following incubation, the top 5 mm of the core were scraped off, exposed to I-ICI fumes, dried to a constant weight, and then ground with a mortar and pestle. Duplicate IO mg subsamples were dispersed in 4 ml of distilled water by sonicating for one minute. Five milliliters of scintillation cocktail (666 ml toluene, 333 ml Triton-XIoo, 4.0 g PPO, 0.1 g POPOP) were added and mixed to form a gel which suspended the sediment. Quench corrections in liquid scintillation counting used an external standard. Adenosine triphosphate assay The method used for the ATP assay was that of Bancroft et al. (1976). Sediment cores wcr’e maintained at ambient temperatures, returned to the lab within one hour of collection. immediately extracted with 16 ml of boiling 0.1 r+NaHCO, (pH 8.4) and the extract frozen. Measurements of ATP were carried out using an ATP photometer (JRB Inc.). Samples were thawed and diluted to 30 ml with 0.1 nl-Tris buffer (pH 7.8) prior to the assay. Firefly lantern tails (Sigma) were the source of the luciferin-luciferase complex. Concentrations were determined by comparing sample counts to appropriate ATP standards. Intact cores could not be frozen prior to bicarbonate extraction, and it was, therefore, not possible to subdivide the o-5 mm depth interval as in the chlorophyll measurements. Instead the top half centimeter of each core was sliced off in the field, and the samples extracted immcdiatelp after returning to the lab. Energy charge ratio The adenylate energy charge is determined by measuring the concentration of the adenylate pool and using the following equation: E.C.-

of each componcm

[ATP]+I/z[ADP] [ATPI-k[ADP]+[AMP]

The methods used to measure adenylates were those of Chapman et al. (1971) and Ilancroi‘t (1977). Sediment core samples were extracted and prepared for analysis in the same mannet as the ATP samples. Details of the solution mixtures and assay procedure can be found in Bancroft (1977).

Results Effect of pazer removal on standing crop and productivity Ungrazed plots had higher concentrations of chlorophyll a and ATP as well as higher rates of production than grazed plots (Figures I and 2). Differences were apparent within one to three days following removal of the snails, and these differences with the exception of ATP in GREI remained throughout the experiments. The data were analyzed using a three way

Effect of grazing on benthic microbes

(a)

I 0

__---_.

1-j

Grazed Ungmzed

1

ICI24C I- - (b) 2oc ,YE

16C,-

;

12c Iz

8C I 4c I= Cl5oc l-

?,

4oc )-

P :

3oc ) ‘1

K

I

2oc )I oc )C)Days

Figure I. Grazer removal experiment I. Mean (a) ATP concentrations (n-6), (b) chlorophyll a concentrations (n=6) and (c) 14Cfixation rates (n-5) in grazed and ungrazed plots at two sites -2iS.E. For any sampling day, site I is represented by the two bars on the left, site 2 by the two bars on the right. Values are for the top 5 mm of sediment T\rith chlorophyll subdivided into O--I mm (upper portion of har) and Z- 5 mm (lower portion of bar) depth sections. analysis of variance. As a consequence of the experimental design, treatment effects (graze1 removal) can be separated from spatial heterogeneity between blocks (experimental sites on the mudflat) and differences between sampling days. Significance levels for the main effects and interactions are presented in Table I. Grazer removal had a highly significant effect on ATP in both experiments. However, in GREI the plots chosen for grazer removal had higher concentrations of ATP than the plots chosen for grazing on Day o prior to the initiation of the experiment (grazed VS. ungrazcd, F-9,4S7, P
126

M. L. Pace, S. Shimmel & W. M. Darley

Figure 2. Grazer removal experiment 2. Mean (a) A’I‘P concentrat (b) chlorophyll a concentrations (n=5), and (c) “C fix&on rates (n:-=5) 111grazed and ungrazed plots at two sites -2jS.E. For any sampling day, site I is represented by the two bars on the left, site 2 by the two bars on the right. Values are for the top 5 mm of sediment with chlorophyll subdivided into O.-I mm (upper portion of bar) and z-5 mm (lower portion of bar) depth sections.

the mcans of grazed and ungrazed plots were similar (Figure I). This may indicate an increase in a heterotrophic component of the total microbial community in the grazed plot since no corresponding increase in chlorophyll was observed at these times. In GREz there was a highly significant difference due to grazer removal which was consistent throughout the experiment. In both GREI and GREB there were significant effects due to sites for the chlorophyll and ATP data. This result was anticipated and was the primary reason for the experimental design. Christian (1976) had previously described the variability of ATP within sites and among samples for the salt marsh system. For chlorophyll all main effects were highly significant with the exception of sampling days in GREz. In addition, an interaction was observed between grazer removal and

Effect of grazing on benthic microbes

I27

TABLE I. Summary of three way ANOVAs for chlorophyll, in grazer removal experiments I and 2. *P
GRE Source Grazer Removal (GR) Site (S) Sampling Days (SD) GRxS GRxSD SxSD GRxSxSD

ATP, and ‘“C fixation ***P
I

GRE

2

Chl a ATP

14C

Chl (I ATP

14C

*** *** *** NS ** NS NS

*** NS *** ** NS NS NS

*** ** NS NS *** *** NS

*** NS *** NS *** NS NS

*** *** *** NS NS NS NS

*** * NS NS NS 2% NS

sampling days. The most plausible explanation of the interaction is an increase in the effect of grazer removal with time [Figures r(b) and 2(b)]. A comparison of chlorophyll a values at the two depth intervals indicates that chlorophyll between 2-5 mm depth responded more slowly to the removal of grazers than chlorophyll in the top millimeter. Mean values in the 2-5 mm depth interval were not higher in ungrazed plots (compared to grazed plots) until Day 4 of GREI and Day 5 of GREz [Figures I(b) and z(b)]. The concentration of pheopigments did not show any significant differences between grazed and ungrazed plots (data not shown). Chlorophyll alpheopigment ratios ranged from I to 3 in the top millimeter and were generally below I for the 2-5 mm interval. The impact of grazer removal on algal productivity was similar to its effect on the two measures of standing crop. Mean rates were higher in ungrazed plots on the first sampling day after Day o [Figures I(C) and 2(c)]. Analysis of variance in GREI and GRE2 indicated that for 14C fixation rates, grazer removal and sampling day effects were highly significant. There were no significant differences between sites although the biomass parameters were distinctly different spatially. Measuring 14C fixation rates in the lab under controlled light and temperature conditions may have had the effect of reducing the variability between sites. In GREI a grazer removal x site interaction was observed. Grazer removal apparently had a greater effect at one of the two sites (Figure I). In GREz a grazer removal x sampling day interaction occurred. Similar to chlorophyll, this result appears to be attributable to increased differences between grazed and ungrazed plots as the experiment proceeded. Assimilation numbers [mg C (mg Chl a)-l h-l] were calculated for each experimental plot on each sampling day using the mean chlorophyll concentration in the top millimeter. Although there was no consistent difference in assimilation numbers between grazed and ungrazed plots, there was a definite trend with time as shown by the linear regressions of assimilation number vs. time of day at which low tide occurred, i.e. the sampling time. This trend results from typically high productivity measures on all plots during morning low tides combined with typically high chlorophyll concentrations during afternoon low tides. The difference in productivity is most obvious in GREz between Days 5 and 7 [Figures 2(c) and 3(b)] when sampling was shifted from the evening low tide (Day 5) to the morning low tide (Day 7). Effect of grazing on the energy charge ratio

The impact of grazing on the community energy charge ratio was examined using the same design as in previous experiments. After the initial sampling day ungrazed plots had higher concentrations of ATP and until the last day higher energy charge ratios (Figure 4).

128

M. L. Pace, S. Shimmel & W. M. Darley

Analyses of variance for ATP and energy charge showed that these differences were due to the treatment (Table 2). The transistory increase in energy charge with grazer removal indicated that the microbial community undergoes a phase of growth to a new steady state and then returns to a nominal or stationary phase. There was a considerable reduction in ATP concentration between Days I and z [Figure 4(a)], and a considerable increase in energy charge between Days 2 and 4 [Figure 4(b)]. The reason for these effects is not readily apparent although they may have been due to changes in time of low tide between sampling days combined with changes in air temperature during the course of the experiment (a general warming trend from Day o to Day 4).

4.0

t

0

3.0 \I l

I

Grozed 0 Ungmzec

l A l

0

2.0

:\

Days

07 L--* 00

1000

1600 Tune of low tide

Figure 3. Linear regression of time of low- tide vs. assimilation number [mg C (mg Chl a)-‘h-l] in (a) GRE I, Y=(-0.335)X16.432, ~~-0.885, (b) GRE 2, Y-(---o.xrg)X L3.162, T- -698.

Simulating the ejjects of N. obsoletus grazing Bioturbation by 117.obsoZetusdisrupts the sediment surface of the mudflat, and this may have accounted for part of the observed differences between grazed and ungrazed plots in previous experiments. The other obvious potential effect of the snails was ingestion of microbes and algae. The final experiment was an attempt to resolve the effects of these two impacts of snails by simulating bioturbation and ingestion. It is difficult to assess lrow realistic our stirring of the sediment surface was, but visually, the stirring produced the same result as N. obsoletus bioturbation. The lens paper technique of artificial grazing removed s-11 y. of the surface chlorophyll based on extractions of the lens paper and sediments beneath the paper (data not shown).

Effect of grazing on benthic micrboes

.{ 0.6 aa e g 0.4 if 15 02

Figure 4. Energy charge experiment. Mean (b) energy charge ratios (n=4) in grazed and For any sampling day, site I is represented the two bars on the right. Values are for the

(a) ATP concentrations (n=4), and ungrazed plots at two sites --a&S.& by the two bars on the left site a by top j mm of sediment.

TABLE 2. Summary of three way ANOVAs for ATP and energy charge ratio (ECR)” in the energy charge experiment. *P
Source Grazer Removal (GR) Site (S) Sampling Days (SD) GRxS GRxSD SxSD GRxSxSD “Ratios transformed statistical analysis.

ATP

ECR

*** ** *** NS NS NS NS

*** NS *** NS NS NS NS

using arcsine transformation

prior to

Mean chlorophyll concentrations in the top 5 mm of the disrupted and artificially grazed plots were equal to the ungrazed plot [Figure s(b)]. All three of these plots in turn had a higher biomass than the grazed plot as shown by statistical analysis (Table 3). In the surface millimeter mean chlorophyll values were higher in the artificially grazed plot than in the ungrazed plot. This result suggests that a low rate of grazing may actually stimulate the surface algae [Figure S(b)]. ATP and r4C fixation data were similar to chlorophyll in that disrupted and artificially grazed plots were not significantly different from the ungrazed plot (Figure 5 and Table 3) with one exception. On Day 7 the concentration of ATP in the artificially grazed plot dropped to a level significantly lower than the other plots. This effect was not observed for chlorophyll or r4C fixation and is not readily explainable.

130

144. L. Pace, S. Shinmel

I60

i-

& W. M. Darley

Cc)

:

-J

3

5

7

Days Figure 5. Simulation of grazer effects experiment. Mean (a) ATP concentrations (n=6), (b) chlorophyll a concentrations (n=s), and (c) lpC fixation rates (n=5) in artificially grazed, disrupted, grazed and ungrazed plots --R*S.E. Values are for the top 5 mm of sediment with chlorophyll subdivided into O--I mm (upper portion of bar) and 2-j mm (lower portion of bar) depth sections. 14C fixation samples taken on Day 5 were accidentally lost during processing.

Discussion Grazing decreased both the standing crop and productivity of the algal community in each of these experiments. Previous studies, however, have documented mechanisms of positive feedback from consumers to producers including nutrient excretion, increased turnover rate, and mechanical disruption of the structure of the system by consumer activity (Hargrave, 1970; Cooper, 1973; Chew, 1974; Flint & Goldman, 1975; Mattson & Addy, 1975; McNaughton, 1976; Owen & Wiegert, 1976). None of these positive feedbacks appear to be

Effect of grazing on benthic microbes

--

131

TABLE 3. Summary of ANOVAs and Student-Newman-Kuels (SNK) multiple comparison test for chlorophyll, ATP, and r4C fixation in the simulation of grazer effects experiment. Where significant (Pto.05) F values occurred, the SNK test was performed to distinguish differences between means. A=artificially grazed, **P
day

F

0

I’99

3

I'02

;

4*81* 5.85**

ATP

SNK

F

U>G U=D=A>G

2.38 3.60*” 5.90** 4.71*

SNK

U=D=A>G U=D=G,A

“All means were equal; the SNK test is not sensitive even though there was a significant F value.

14C

F 2.76 6.89* * 30.01***

-

SNK U=D>G U=D-A>G

enough to detect a difference

operative in the N. obsoletus- benthic algae relationship. Based on previous work and the experimental results of this study, several reasons can be presented as to why they are not. The main limiting factor for the benthic algae of salt marshes and mudflats is light (Pomeroy, 1959; Williams, 1962; Leach, 1970; Sullivan & Daiber, 1975; Van Raalte et al., 1976). Even if N. obsoletus excreted nitrogen or phosphorous at appreciable rates, mudflat algae are in contact with water rich in these nutrients (both flooding tidal and interstitial waters). These animals are unlikely to be a significant source of nutrients for the producer community. In some casesthe mechanical activities of consumers lead to successional events, nutrient regeneration, or increase in habitat which cause increased productivity (Chew, 1974; Gwen & Wiegert, 1976). The plot in which bioturbation of N. obsoletus was simulated had a similar standing crop in terms of ATP and chlorophyll and the same productivity as enclosed ungrazed plots. If bioturbation was a positive stimulus, biomass or productivity would have been higher than the ungrazed plot. Sediment surface disruption, however, had no effect on the microbial community. Studies of the relationship of herbivore biomass to primary productivity indicate that producers are able to compensate for grazing at low grazer biomass by increasing productivity, but above a certain threshold biomass, productivity decreases (Hargrave, 1970; Cooper, 1973; Flint & Goldman, 1975; Fenchel & Kofoed, 1976). When grazing was simulated using lens paper, productivity and chlorophyll values increased relative to the plots grazed by N. obsoletus. Furthermore, mean chlorophyll values for the top millimeter were higher in the artificially grazed plots than in the ungrazed plots implying that a low level of grazing (~--II%,, removal of surface chlorophyll per day) may actually stimulate the surface algae [Figure s(b)]. At its natural densities, N. obsoletus decreases productivity and standing crop of the algae and must, therefore, be above the threshold where grazing stimulates productivity. Another means of evaluating the impact of grazing on a producer community is the assimilation number, which is essentially a turnover rate if one converts chlorophyll a to cell carbon. Cooper (1973) found that a reduction in producer standing crop at low grazer biomass initiated an increase in turnover rate. In studies of blue-green algal mats in which both productivity and biomass (chlorophyll a) were measured (Brock, 1967; Brenner et al., 1976), the absence of grazers resulted in either similar or increased production rates of algal biomass, although the assimilation numbers decreased. In one of these studies the effect of

132

M. L. Pace, S. Shimmel & W. M. Darley

the grazer in maintaining a high assimilation number was attributed to the mechanical activity of the grazer which broke up the mat, facilitating light and nutrient availability (Brock et al., 1969). Th ese differences may be attributable to the physical structure and successional nature of algal mats versus an epibenthic algal community. In a thick algal mat, the community has reached, relatively speaking, a steady state which is limited by selfshading and diffusion gradients of nutrients and gases, e.g. CO, (Wiegert & Fraleigh, 1972). In such a system a consumer population may increase the availability of the limiting factors and thus increase the fixation rate per unit chlorophyll. The epibenthic algae in our study exist at the surface in a layer only a few cells thick (Pomeroy, 1959) in which differential availability of light and nutrients is unlikely. Our data suggest that this community is not in a steady state situation, but fluctuates with the time of day of low tide. In almost all of the experimental plots, a nearly linear increase in chlorophyll was observed when low tide occurred during the middle of the day. These increases correspond to a maximum eqmxre of benthic algae to light. In such a system it is unlikely that a consumer would exert a positive feedback on the algal community. Rather it appears that the impact of the consumer population is restricted simply to removal of algae by ingestion. In our experiments we also found that removing grazers increased productivity and biomass, but that the assimilation number was not affected by grazing. The total microbial community as measured by the ATP assay responded to the removal of grazers in much the same way as the algal community. This was in part expected since benthic algae are a component of the biomass as measured by ATP, although their proportional representation is difficult to estimate. The similarity between the responses of ATP and chlorophyll is also expected because N. obsoletus is a nonselective deposit feeder (Scheltema, 1964). This snail has an unspecialized feeding apparatus with no apparent found that intertidal gastropods ability to sort particles (Brown, 1969). Nicotri (1977) grazing on hard surfaces exhibited selectivity by removing microalgal species which did not adhere strongly to the surface and by differential digestion of certain algal species. Although we do not know if differential digestion occurs, selective ingestion is less likely in a mud situation because organisms are not being scraped from a surface. If the snail is not discriminating among the various microbes it ingests, then one might expect both of these parameters of biomass to be equally affected. have shown that the energy charge ratio may be used to qualitaWiebe & Bancroft (1975) tively assess the growth state of natural microbial communities. The lower energy charge ratio of grazed plots when compared to ungrazed plots is also consistent with the argument that N. obsoletus has a negative feedback on the microbial community. A transitory incrrase in energy charge similar to that found with grazer removal was also observed by Wiebe & Bancroft (1975) with additions of glucose to sediment slurries. The energy charge assay appears to indicate improved growth conditions for microorganisms over relatively short time intervals. Initial work, however, is still being done to determine the importance and meaning of the energy charge ratio at both the population and community level (Wijsman, 1976; Bancroft, 1977). The influence of grazing by N. obsoletus is a significant factor in a matrix of variables which affects the algal community. Of these light and tides seem to be the most important. Several other factors influence the photosynthesis rates of the algal community. These include vertical migration rhythms of the community, possible diurnal rhythms in photosynthetic capacity, and partial inhibition of photosynthesis in full sunlight (D. Whitney & M. Darley, unpublished data). The impact of grazing by N. obsoletus is superimposed on all of these factors.

Effect

of grazing

on benthic microbes

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Feeding studies have indicated that microorganisms are important food resources for the abundant macroconsumers that live in the salt marsh (Teal, 1958; Wetzel, 1977). Our results illustrate a reciprocal and potentially important impact of macroconsumers on the microbial community. This impact may have important consequences for microbially mediated processes of the salt marsh-estuarine ecosystem. Acknowledgements This research is part of two theses submitted by the first two authors to the University of Georgia in partial fulfillment of the requirements for an MS. degree. We would like to thank Dr Keith Bancroft who helped with the energy charge assay and Barbara Wimpee who drafted the figures. Drs R. G. Wiegert, J. W. Porter, and K. G. Porter provided both helpful discussion and critial review of the manuscript. References Bancroft, K., Paul, E. A. & Wiebe, W. J. 1976 The extraction and measurement of adenosine triphosphate from marine sediments. Limnology and Oceanography 21,473-480. Bancroft, K. 1977 The use of the adenylate energy charge to measure growth state in natural ecosystems. Ph.D. dissertation, University of Georgia, Athens, Georgia. 126 pp. Barsdate, R. J., Prentki, R. T. & Fenchel, T. 1974 Phosphorous cycle of model ecosystems: significance for decomposer food chains and effect of bacterial grazers. Oikos 25, z39-z51. Barrett, J. & Jeffrey, S. W. 1964 Chlorophyllase and formation of an atypical chlorophyllide in marine algae. Plant Physiology 39,4447. Hrenner, D., Valiela, I. &Van Raalte, C. D. 1976 Grazing by Talorchestiu longicornis on an algal mat of a New England salt marsh.. Journal of Experimental Marine Biology and Ecology 7, 255-262. Brock, M. L., Wiegert, R. G. & Brock, T. D. 1969 Feeding by Puracoenia and Ephydra (Diptera: Ephydridae) on the micro-organisms of hot springs. Ecology 50, 192-200. Brock, T. D. 1967 Relationship between standing crop and primary production along a thermal gradient. Ecology 48, 566-57 I. Brown, S. C. 1969 The structure and function of the digestive system of the mud snail Nassariur obsoletus (Say). Malacologia 9, 447-500. Chapman, A. G., Fall, L. & Atkinson, D. E. 1971 Adenylate energy charge in Escherichia coli during growth and starvation. journal of Bacteriology 108, 1072-1086. Chew, R. M. 1974 Consumers as regulators of ecosystems: an alternative to energetics. Ohio~ournnl of Science 74, 359-369. Christian, R. R. 1976 Regulation of a salt marsh soil microbial community: a field experimental approach. Ph.D. dissertation, University of Georgia, Athens, Georgia. 132 pp. Cooper, D. C. 1973 Enhancement of net primary productivity by herbivore grazing in aquatic laboratory microcosms. Limnology and Oceanography 18, 31-37. Dailey, W. M., Dunn, E. L., Holmes, K. S. & Larew, III, H. G. 1976 A i4C method for measuring epibenthic microalgal productivity in air. Journal of Experimental Marine Biology and Ecology 25, 207-217.

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