Size partitioning of microbial and meiobenthic biomass and respiration on Brown's Bank, south-west...

Estuarine, Coastal and Shelf Science (1987) 25,647-661

Size Partitioning of Microbial and Meiobenthic Biomass and Respiration on Brown’s Bank, South-west Nova Scotia

Jonathan Grant’ and Peter Schwinghame@ “Department of Oceanography, Dalhousie University, Halifax, Nova Scotia,

Canada B3H 4J1, and bDepartment of Fisheries and Oceans, Marine Ecology Laboratory, Bedford Institute of Oceanography, P.O. Box 1006, Dartmouth,

Nova Scotia, Canada B2 Y 4A2

Received 18 March 1986 and in revisedform 11 May 1987

Keywords: benthic fauna; size composition; oxygen demand; microfauna; bacteria; meiobenthos; continental shelf; Nova Scotia Banks

Biomass and respiration (oxygen consumption) of bacteria, microfauna, and meiofauna were measured in coarse sand sediment from Brown’s Bank (172 m) off Nova Scotia, Canada. Community biomass, excluding macrofauna, had a median value of 35 mg C rn-’ , dominated by bacteria (51 qb), microfauna (25” ,), and a minor meiofauna component (2.5”;)). Protozoan microfauna were mostly microflagellates (colourless cryptomonads). The experimental design allowed partitioning of benthic metabolism without using subtraction from whole community rates. Addition-removal experiments with fauna separated into size categories were used to construct a respiration-biomass regression for all taxa. Respiration rates for fauna1 groups were then calculated from their biomass in the natural sediment. Total microbial and meiofaunal community respiration had a median rate of 055 ml 0, me2 h- ’ which was partitioned into median proportions of bacteria (505,,) microflagellates (27O;), and metazoan meiofauna (4”,). Correlations among fauna1 biomass values from incubated vials of sediment suggested that bacteria were important prey for protozoans. With added biomass of meiofauna, protozoans also became a potentially important source of prey. The results demonstrated the significance of microflagellate protozoans in these sediments and their metabolic and trophic importance relative to meiofauna and even bacteria.

Introduction

Sediment oxygen consumption is often used as a measure of energy flow through benthic communities (Pamatmat, 1977; Zeitzschel, 1980; Patching 81 Raine, 1983). Although this consumption integrates the aerobic respiration of a variety of organisms into a single measure of community metabolism, the proportional contributions of the various trophic or functional groups cannot be assessed. A means of partitioning respiration among these groups would offer more insight into the pathways of energy flow through the benthos. This information is required if the effects of disturbances on benthic ecosystems are to be understood.

Although attempts have been made to estimate the partitioning of benthic respiration among fauna1 groups, most of the methods have been either indirect or limited in resolving

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0272-7714/87/060647+ 15 $03.00/O @ 1987 Academic Press Limited

648 J. Grant &3 P. Schwinghamer

taxonomic or size categories. Many studies have partitioned community respiration by applying literature values of weight-specific respiration to estimates of species biomass (e.g. Davies, 1975). Others have combined laboratory estimates of respiration for various macrofaunal species (Pamatmat, 1968; Smith, 1973; Dye, 1981; Asmus, 1982), or, less frequently, for meiofauna (Dye et al., 1978), with whole community oxygen uptake. Unmeasured components are estimated from literature values based on species biomass and specific respiration values. Respiration by microbiota is usually estimated by subtraction of the metazoan respiration (measured or estimated) from the total biological oxygen consumption.

Estimates of respiration by microbiota usually include bacteria, microalgae, and protozoans, so that energy flow among unicellular organisms is not resolved. Schwinghamer et al. (1986) made detailed biomass measurements of bacteria, microalgae, meiofauna, and macrofauna from an intertidal mudflat community. They then calculated weight-specific respiration using published regressions to estimate the contributions of these groups to benthic respiration and production. Comparing the summed respiration of biomass categories to measurements of sediment oxygen consumption produced good correspondence between methods in estimates of microalgal, meiofaunal, and macrofaunal respiration, but very poor agreement for bacterial estimates. A few investigators have specifically addressed the role of protozoans in community metabolism (Fenchel, 1969; Vernberg & Coull, 1974; Dye, 1981), but in no case has the contribution of protozoans actually been measured.

Bacterial respiration has been estimated by inhibiting it with antibiotics and calculating the bacterial contribution as the difference between antibiotic treatments and total biological uptake (Hargrave, 1970; Smith et aE., 1972; Smith, 1973; Hartwig, 1978; Kemp & Boynton, 1981). After subtraction, partitioning of the remaining respiration has usually been estimated from literature values based on measured biomass. This approach has several problems, mainly that antibiotics do not completely inhibit bacterial respiration (Yetka & Wiebe, 1974; Van Es, 1982; Padgett et al., 1985) and that their effects on other organisms are not well known.

The grouping together of fauna1 groups has resulted in insufficient information being provided on the metabolic importance of smaller organisms. A promising solution to this problem is the use of addition-removal experiments. For example, Ellison (1984) sieved the 60-250 urn fauna1 fraction from intertidal mud and incubated it with sterile sediment to measure meiofaunal respiration, but he did not measure the biomass of the organisms incubated.

We sought to overcome some of the above shortcomings of previous attempts to divide biological oxygen uptake among bacterial, protozoan, and micrometazoan components of a benthic community. We used a series of size-fractionated respiration experiments to construct a biomass-respiration curve that includes bacteria, protozoans and metazoan meiofauna at a sandy station on the south-west Nova Scotian continental shelf. This curve was then applied to the biomass of each fraction in the natural sediment in order to partition oxygen consumption in this benthic community. These experiments allowed a detailed look at the distribution of biomass and respiration, as well as trophic relations among procaryotes, protozoans, and small metazoans.

Materials and methods

Sediment and organisms were collected during a cruise of the CSS Dawson in November, 1983, using a 0.5 m2 van Veen type grab modified to take a relatively undisturbed sample.

Size partitioning of biomass 649

The study site was a coarse sand bottom (median grain diameter ca. 2 mm) at 172 m depth on the southern flank of Brown’s Bank (42”24.0O’N, 65’51~5O’W). The temperature of sediment in the grab was 9 “C. The sediment had an organic carbon content of 0.1 ‘I ,, dry weight and a chlorophyll a content of 0.2 ug g- ’ dry weight (Schwinghamer, unpubl. data). No redox discontinuity was visible in the grabs. Any porewater that drained from the bottom of the grab was collected in a plastic bucket for use in respiration experiments. Cores and fauna1 samples were collected immediately after retrieval to minimize leakage of fauna from the grab.

Respiration experiments For extraction of fauna into size categories, the surface 2-4 cm of the grab sample was split into several 250-ml centrifuge bottles each containing about 100ml of sediment and porewater. A buffered isosmotic mixture (sp. gr. = 1.15) of sorbitol, seawater, and the non- toxic silica sol Percoll (Price et al., 1978; Schwinghamer, 1981) was added by cannula to the bottom of centrifuge bottles in a volume about equal to that of sediment. After centrifugation, the supernatant containing the organisms was decanted, the sediment resuspended in the silica-sol mixture and the process repeated. The combined supernatant from all bottles was poured gently with filtered seawater (0.45 urn) onto a 15 urn Nitex sieve. The material was then rinsed through a series of Nitex sieves with mesh sizes of 15, 30,62, 125, and 250 urn. Sieve contents were rinsed into scintillation vials and filled to a known volume with filtered seawater (10” C). Equal aliquots of each size fraction were added to each experimental treatment (see below) using a wide-aperture (1 mm) pipette. The sediment from which the fauna had been extracted (‘ washed ’ sediment) was well rinsed with filtered seawater and saved for use in the addition-removal experiments (below).

Microscopic observations on harpacticoid copepods and nematodes subjected to the above treatment were made prior to the beginning of the incubations. Nematodes appeared to be normally active immediately after sieving. Harpacticoids were inactive for several minutes but activity returned within 15 min. No broken or dead animals were observed.

Respiration experiments were conducted onboard in a flow-through water bath connected to the ship’s seawater system (10 “C). The bath was darkened with opaque black plastic bags. Oxygen concentration before and after incubation of sediments was determined with a Radiometer blood gas analyser. For total sediment oxygen demand, three core tubes (25.4 cm2 internal area, 12 cm length) were placed in the grab to obtain a l-5-2.5 cm sediment column, stoppered at the bottom, wetted with porewater to the sediment surface, and filled with 0.45 urn filtered seawater (FSW). Water samples were taken from each core for initial oxygen concentration, and cores sealed with Vaseline coated lids. Cores were incubated for up to 17 1 h to measure oxygen uptake. Final oxygen concentration was at least 58”’ , 0 of initial concentration. Chemical oxygen demand (COD) of the cores was measured with three similarly prepared cores poisoned with 4O ,, formalin. Cores were not stirred during incubation but received mixing from the ship’s motion. Because the incubations extended beyond the duration of the cruise, the cores were transferred to a laboratory of the Bedford Institute at identical temperature, but with no mixing.

Addition-removal respiration experiments were done in 24-ml glass scintillation vials (abbreviated as V) containing 1 cm3 of sediment from the O-l cm layer. The extracted fauna from the various sieve fractions were added to either natural (unwashed) sediment or washed sediment as follows (n = 3-5 vials for each):

650 J. Grant ~3 P. Schwinghamer

natural sediment, no porewater (V + FSW) Natural sediment (total fauna = T) + 1 ml porewater

+ no added fauna (VT) + 15 urn fauna1 fraction (VT + 15) + 30-urn fraction (VT + 30) + 62-urn fraction (VT + 62) + 125ym fraction (VT + 125) +250-urn fraction (VT + 250) + formalin (VF)

Percoll-extracted sediment (washed, W) + 1 ml porewater + no added fauna (VW) + 15-urn fauna1 fraction (VW+ 15) +30-urn fraction (VW + 30) +62+m fraction (VW + 62) + 125~urn fraction (VW+ 125) + 250ym fraction (VW + 250) + formalin (VFW)

Porewater + no sediment Porewater + no sediment + formalin

Each of the above treatments was filled with 0.45 l.trn filtered seawater and sealed, without air spaces, with parafilm under the cap. Initial oxygen concentration was determined from the mean of subsamples from the vials (coefficient of variation= 1 O~~j). Vials were incubated for 167-175 h without internal stirring after which final oxygen concentration was measured. A potential problem of the incubations is the stimulation of respiration which results from handling or otherwise disturbing organisms (Ellison, 1984). The incubation times used were long enough to ensure that an initial stimulation period would have little effect on the overall rates (see also Discussion). Like the cores, vials were mixed by the ship’s motion for the first half of this period, and then transferred to a shore-based laboratory. Non-linearity of oxygen uptake is considered below.

Biomass determination

For biomass determinations, sediment was preserved in 43,, buffered formalin. Organisms were enumerated from initial grab subsamples (VF, O-l cm sediment depth) to measure biomass in the natural sediment, and then in the some of the addition-removal vials after incubation. Bacteria and eucaryotes up to 15 urn maximum dimension were enumerated at 1000 x magnification on a Zeiss ICM 405 inverted microscope with an ocular micrometer grid. A modified acridine orange (AO)-mithramycin epifluorescence method was used (Hobbie et al., 1977; Coleman, 1980). A l-ml aliquot of sample was filtered on a 25 mm, 0.2 urn pore size Nuclepore filter. Prepared acridine orange (Becton- Dickinson #4940) was added and the sample left to stain for 2 min in the dark. The stain was filtered off and the sample rinsed with filtered distilled water. Mithramycin (1 ml,

i 2-5 ugml- , in distilled water) was added and the sample left to stain for 20 min in the dark. The stain was filtered off and the sample rinsed again. The filter was mounted in Cargille Type A immersion oil. The combination of the two stains resulted in an average 2O”/b increase in counts over samples stained with A0 alone. This presumably occurred because cells that fluoresced orange using A0 only, fluoresced yellow with mithramycin and were thus distinguishable from the red fluorescence of the sediment. A drawing tube


was used to project the bacterial images onto a digitizing table, part of a Kontron MOP-40 image analysis system. Cell lengths and widths were measured and cell volumes calculated from the formula for a prolate ellipsoid:

4 V=- n: ab2

3

where V is volume, and a and b are semi-major and semi-minor axes respectively. A minimum of 400 cells was counted from each sample giving a 950/,, confidence interval

of k lo”,; of the total count for the whole sample. Protozoans and metazoans of > 15 urn maximum dimension were enumerated and

measured on the Zeiss ICM 405 inverted microscope coupled with the Kontron MOP-40 image analysis system using a high-resolution video camera. Organisms were separated from sediment by centrifuging twice with Ludox silica sol (sp. gr. = 1.19). All flora and fauna except foraminiferans were extracted with >90”& efficiency using this method (Schwinghamer, 1981). Foraminiferans were enumerated and measured in both the supernatant and sediment of centrifuge tubes. The supernatant was filtered through a 15 urn mesh Nitex screen. The retained material was stained with rose bengal and transferred to 25-ml settling chambers for microscopic analysis. Smaller, more numerous organisms, such as flagellates, were analysed at 160 x or 400 x in transects across the base of the chambers. Larger and less abundant organisms were enumerated and measured at 25 x or 63 x over the entire chamber area.

Digitized area and length measurements were converted to volume using the formula:

V= 0.8165 (area)2 (length)- ’ (2)

Equivalent spherical diameter (ESD), a shape-independent linear dimension, was calculated as:

Results

Biomass of the natural community

The total biomass of the microbial- and meiofaunal-size organisms in the surface 1 cm of the natural sediment (based on formalin-preserved samples, VF) was 442 mm3 rne2, or approximately 35 mg C m-’ (assuming 1 mm3 fresh volume 280 ug carbon). The median percentage of total biomass in each size fraction measured (not sieve mesh size) is presented in Figure 1. For the purpose of this analysis, the size fractions were combined into three categories: bacteria, protozoa, and meiofauna. The wide ranges of values associated with the proportions of total biomass contributed by each of these groups in replicate samples indicated that, although bacteria dominated, protozoans and occasionally meiofauna made up significant proportions of the biomass (Figure 1).

Bacterial biomass (rods, cocci, filaments) in the surface 1 cm of sediment had a median value of 214 mm3 m- 2 (17.2 mg C rne2) in the VF samples (Table l), similar to bacterial biomass in coarse sediments of the Bay of Fundy (Schwinghamer, 1983), but approximately an order of magnitude less than the general values given in Meyer-Reil(1983) and Ankar (1979). Bacterial numbers ranged from lo’-lo8 ind. ml- ’ and were dominated by l-2 urn rodshaped cells (70%). At least 9094 of all cells were free-living rather than attached to grains.

652 J. Grant &P. Schwinghamer

CILIATES 8 BACTERIA fiFLABELLATES MEIOFAUNA

ESD SIZE CLASS (flrn)

Figure 1. Biomass distribution of benthic bacteria, micro- and meiofauna in sediments from Brown’s Bank on the south-west Scotian Shelf. Values are expressed as percentages in log, size classes of total non-macrofaunal biomass, based on formalin-treated (VF) vial samples. Taxa represented in the size classes and the medians and ranges of their relative contributions to vial biomass are given above the histograms. Median values need not sum to 1000/O. Note that these are median values of percentage size class calculated for each vial, not median biomass values (Table 1) summed to calculate a percentage.

Protozoa were dominated by microflagellates, mostly non-pigmented cryptomonads in the 4-g urn size range. Microflagellate biomass had a median value of 201 mm3 me2 (16.1 mg C m-‘). Ciliates were larger in size (mostly in the 62 pm size group), but made up less than 10% of the biomass in this size class. Live benthic foraminiferans (rose-bengal stained) were rare. Total protozoan biomass was within the range found by Fenchel(l975) in another system dominated by flagellates. Metazoan meiofauna, usually the smallest portion of measured biomass, occurred at a density ranging from O-54 ind. cme2 in the surface 1 cm of sediment, an order of magnitude less than previously reported values for continental shelves (Coull et al., 1982). The latter values, however, represent biomass integrated to greater depths in the sediment than was done in our study. Meiofaunal biomass in the VF samples had a median value of 26 mm3 m-* (2.1 mg C m-‘), which is also at the lower range of average values reported by Fenchel(l978) and Schwinghamer (1983). Harpacticoid copepods (41%), nematodes (18%) and polychaetes (400,) dominated meiofaunal biomass.

Addition-removal experiments Biomass. Washed (VW) and unwashed (VT) sediments with no fauna1 additions showed

significant increases in the bacterial biomass over their respective initial conditions, i.e. washed and unwashed sediment + formalin (VFW and VF) (Table 1). Fauna1 additions resulted in increased final biomass of bacteria in washed sediments (VW + >VW) and unwashed sediments (VT + > VT). Therefore, bacteria grew in all treatments, but they grew more in fauna1 additions. The growth was significantly greater in washed sediment


TABLE 1. Addition-removal experiments: median biomass values of functional groups (mm3 m-‘) and community oxygen consumption values (ml 0, m-’ h-l) in vials incubated for approximately 170 h. Initial conditions at the beginning of the experiment were measured in formalin-treated vials (VF, whole sediment; VFW, washed sediment); treatments with all sizes of added fauna are pooled (VT+, whole sediment plus added fauna; VW + , washed sediment + added fauna). Significance levels in U-tests for differences between medians are at P< 0.05 or better. Note that median biomass is compared across treatments rather than across taxonomic groups. The values are distinct from median percentage composition values which are calculated across groups and given in Figure 1. Values in parentheses are ranges.

Initial Conditions No fauna added Fauna added

VF VFW VT VW VT+ VW+ n=3 n=3 n=3 n=3 n=lO n=8

Total bacteria 214 496 1224* (155-851) (455-575) (10551737)

Microflagellates 201 (5G93)

264 (138-239) (228-343)

Meiofauna (&2&O) (4230) (0_05)

Oxygen consumption - - 0.74 (0.56-1.17)

1058* (789-1081)

587*t (571-601)

K& 1.29

(06-1.42)

1577* (720-3040)

394* (143-702)

250*$ (17-630)

1.871 (0.95-254)

7618*tf. (1358-9141)

67t$ (1-152)

330: (25-780)

3.04-b: (0.76-3.37)

*Significantly different from initial conditions (biomass values only) tWashed sediment significantly different from unwashed. $‘ Fauna added ’ significantly different from ‘ no fauna added ‘.

additions (VW +) than in unwashed additions (VT +). Exponential growth rates caicu-

lated from the incubation of natural sediment (N,, = N,, err) in the presence of potential grazers, yielded bacterial doubling times ( Td = ln2/r) of 100-l 10 h.

Washed sediments without additions (VW) showed significant growth of microflagellates from initial conditions (VFW) and ended up with greater biomass than did the

unwashed sediments (Table 1). The latter showed no change from initial biomass (VT = VF). Fauna1 additions to washed sediment (VW +) resulted in significant decreases in microflagellate biomass back to starting levels (VFW). In contrast, fauna1 additions to unwashed (VT+) sediments did not result in significant decreases in microflagellate biomass. Therefore, the addition of fauna1 fractions in the unwashed sediment did not affect the growth of microflagellates; in washed sediments growth occurred only in the absence of fauna1 additions.

The addition of meiofauna to experimental vials resulted in significant increases in biomass compared to VW (washed), VT (unwashed), and VF (initial samples with formalin) (Table 1). As intended, the median sizes of meiofauna in the vials increased with increasing mesh size of meiofauna added (Table 2). The vials with larger size fractions (62-250 pm) had greater biomass than did those with no additions or with 15 pm and 30 pm size fractions added (Table 2). Meiofaunal biomass in washed and unwashed additions (VW + , VT +) did not differ significantly (Table 1).

Oxygen consumption. Because the cores included a larger depth of sediment relative to surface area than did the vials, oxygen consumption was lower in vials than in core incubations (Table 3). Both types of incubation vessels had similar proportions of COD

654 J. Grant &I’. Schwinghamer

TABLE 2. Medians and ranges of meiofaunal biomass (mm3 rn-- ‘, including ciliates) and equivalent spherical diameter (ESD) of meiofaunal animals in vials incubated for 170 h. VF and VFW were formalin-treated initial conditions; VT and VW had no added fauna; VT + and VW + had fauna added that were collected on the given sieve size (see text for vial abbreviations)

Vial n Biomass (mm3 rn-$ LSD (w)

VF, VFW 6 59 (4400) 104 (33-425) VT, VW 6 0 (O-5) 115 (O-152) VT+15,VW+15 4 27 (17-30) 70 (46-150) VT+30,VW+30 3 57 (44-98) 68 (39-116) VT+62, VW+62 4 400 (370-630) 106 (38-265) VT+125,VW+125 4 430 (260-780) 172 (76-292) VT+250, VW+250 3 310 (240-420) 181 (33-368)

TABLE 3. Oxygen demand of natural unmanipulated sediment (ml 0, m -’ h-l) and porewater (ml 0, crne3 h-‘) in sh’ b ip oard incubations of samples from Brown’s Bank, Nova Scotia. Chemical oxygen consumption was determined from formalin-poisoned samples

Mean oxygen demand f 1 SD

Incubation Total Chemical (5, Chemical

Cores (n = 3) 1.68kO.48 0.29 k 0.29 17.3 Vials (VT) (n = 5) 0.77 + 0.23 0,16_fO.17 20.8 Porewater (n = 5) 0.02 * 0.003 0 0

(17-21%). The porewater accounted for very little of the oxygen consumed in vials. Addition-removal experiments demonstrated a significant relationship between respiration and total biomass in VT, VT+, VW, and VW + treatments (n = 27, r = 0.68,

P<O.OOl; Figure 2) independent of size structure, i.e. the biomass in each vial was made up of various mixed size classes. The geometric mean (GM) regression (log-transformed) relating biological oxygen demand (R, ml 0, m -’ h-i) to total microbial and meiofaunal biomass (B, mm3 rnm2) was:

R = 0.0029 B*‘“* (4)

In addition, an ordinary least-squares regression of biomass and respiration was calculated. However, the calculated respiration values based on this regression were biased in their estimate of measured R because the intercept was significantly greater than 0 (P< 0.01). In contrast, a GM regression resulted in unbiased predicted values. Regardless of whether equation (4) was used to calculate respiration with summed biomass per vial or individual biomass categories (bacteria, meiofauna, etc.) whose R was subsequently summed per vial, calculated and measured values agreed well over the entire range of respiration values.

Partitioned respiration in the natural sediment was calculated (equation 4) from biomass values in the VF vials and expressed as the median respiration of the taxonomic


2 0 6 0.5 -

9 l 0 a I I I I I

5x10’ lXlO3 2x103 5x103 lX104 COMMUNITY BIOMASS (mm3. m-2)

Figure 2. Relationship (log,,-log,,) between sediment respiration and biomass in vials incubated for approximately 170 h. The line is the geometric mean (GM) regression (see equation 4 in text). t, Vial incubations in which final oxygen concentration was < 40”,, of the initial concentration.

0.6

0 .25-4 4-15 15-250

ESD SIZE CLASS (pm)

Figure 3. Medians and ranges of percentage total community respiration (shaded bars) contributed by bacteria, microflagellates and meiofauna on Brown’s Bank. These proportional respiration values are derived from the percentage biomass given in Figure 1 (see text). The medians and ranges of the oxygen consumption rates are given in the unshaded bars. As in Table I, these median rates are calculated across vials and not used in proportional respiration calculations.

categories (Table 1). In addition, the proportional biomass (Figure 1) in each vial was used in equation (4) to derive respiration values which were then summed to calculate percentage total respiration, expressed as medians for the various functional groups (Figure 3). Bacterial respiration was a median 50”, of community metabolism. Median microflagellate respiration was comparable to median bacterial rates, underscoring the

656 J. Grant &I’. Schwinghamer

significance of this protozoan group. In proportional comparisons across categories, the median role of microflagellates in total community metabolism was less (27”,,), but still substantial. Other protozoans (ciliates) and metazoan meiofauna contributed little to the overall rate. Due to the variance in biomass values, the range of values for each fauna1 category in Figure 3 was wide. The variance in biomass, and thus respiration, probably reflects microscale patchiness in marine sediments. However, the central tendencies indicated by the median percentage respiration values demonstrate the importance of bacteria and microflagellates relative to meiofauna.

Summation of the median rates of respiration by the fauna1 categories in Figure 3 produced a median estimate of community respiration of 0.55 ml 0, m- ’ h- ‘. When calculated from the median total biomass of the natural community (i.e. summed biomass, no categories), community respiration was 0.38 ml 0, mm2 h- ‘. Both values are in approximate agreement with the measured vial respiration (VT), corrected for COD (0.61 ml 0, -2 h- ‘, Table 3). This suggests that our calculations of respiration for the individual functional groups were not grossly in error, despite potential successional changes in biomass and respiration during incubations. Moreover, the VT values also agreed with whole-core rates when core rates were corrected for their two-fold greater sediment depth (Table 3).

Over the entire data set, the variable which best correlated with 0, uptake was the biomass of rod-shaped bacteria (n=27, r=0.76, P<O.OOl). The five-fold increase in biomass of bacteria in washed (VW + ) compared to unwashed (VT +) addition treatments was reflected in the significantly increased respiration of VW+ over VT+ (Table 1). Meiofaunal additions produced significantly higher 0, uptake than in non-addition treatments (Table l), but meiofaunal biomass was only weakly correlated with O2 uptake (r=0.34,P<0.05).

During the incubation of vials, oxygen depletion in some cases exceeded 90”, of the initial concentration potentially causing a reduction in respiration and a non-linear response between end-point determinations of oxygen. If a non-linear response occurred, greater oxygen depletion in the vials would be observed as lower biomass-specific respiration. To test for this effect, R/B was regressed against the percentage depletion of initial 0,; no linear relationship was found (r = 0.07). In addition, all points on the R/B curve in which oxygen depletion exceeded 6Oqh (Figure 2) were examined. These points did not deviate from the RIB relationship as low values.

Trophic interactions. Simple correlations among the dominant groups of organisms (Figure 4) allow insight into potential trophic relationships. However, it should be stressed that these correlograms are not energy flow diagrams, but serve to show the strength of relationships between variables. For the entire data set [VW, VW + and VT, VT + ; Figure 4 (a)], cocci and rods were positively correlated, indicating similar responses to various conditions. More importantly, microflagellates showed negative correlations with both bacteria and meiofauna, suggesting that bacteria were grazed by microflagellates which, in turn, were grazed by meiofauna.

When the washed and unwashed treatments are examined separately, it is apparent that the vial communities developed quite differently during the incubations. Within the unwashed series [VT and VT+; Figure 4 (b)], there was a negative correlation between microflagellates and cocci, but in general no large changes in bacteria or microflagellates occurred as a result of fauna1 additions (Table 1). Meiofauna showed no significant correlation with microflagellates, suggesting that microflagellate biomass was not controlled by meiofaunal grazing in the unwashed sediments [Figure 4(b)]. Microflagellates were


(a) .64** COCCI - RODS

I -.47 ** n=24

MElO

(b) .47* COCCI f----) RODS

- .48* I

pFLAG

J n=l2

ME10

(c) .53* COCCI -RODS

@FLAG

I -.59** n=l2

ME10

Figure 4. Signiiicant correlations among the final biomass of taxonomic groups incubated in vials for 170 h. The numbers for each arrow are Pearson correlation coefficients. *, P~0.05; **, Pi 0.01; n, number of vials. (a) All vials including washed (VW, VW +) and unwashed whole (VT, VT + ) sediments. (b) Unwashed whole sediment (VT, VT + ) vials only. (c) Washed sediment (VW, VW +) vials only.

therefore able to increase by 49-967; in median biomass in the VT + values compared to VF and VT, respectively (Table 1).

In contrast to the VT and VT+ series, the washed sediments [Figure 4(c)] retained more of the negative relationships seen in the pooled data. Apparent meiofaunal predation decreased microflagellate biomass in the VW + vials allowing bacteria to increase seven- fold in biomass over VW values in which microflagellate biomass remained high (Table I). This increase in bacteria was reflected in the higher oxygen consumption of VW + compared to VW (Table 1). In Figure 4(c), rods (ca. 90% of the bacteria in these treatments) thus show a negative correlation with microffagellates. The washing of the VW series removed most of the microflagellates that were initially present in the sediment (compare VF and VFW, Table 1). They appeared unable to recover to original biomass in the presence of increased meiofauna predation in the VW + values; in the absence of meiofauna (VW), microflagellates flourished.

Although the biomass of meiofauna did not differ significantly between VT+ and VW + (Table l), their taxonomic composition was quite different. The VW + treatments had about twice as many small polychaetes as the VT + vials (mean = 3.1 and 1.5 ind. cm 3,

658 3. Grant ~5% P. Schwinghamer

respectively) and fewer harpacticoid copepods (mean= 6.1 and 9.6 ind. crnM3, respectively). Differences in feeding habits of these groups (Montagna, 1984) may have resulted in higher grazing on microflagellates in VW+, and thus the different community development in the two treatments.

Discussion

For smaller organisms, our study is a complete resolution of continental shelf benthic biomass into major taxa. Total benthic biomass on the southern flank of Brown’s Bank was low due to the high-energy nature of the environment; our data for bacterial and meiofaunal biomass were in the range of values reported for coarse sediments in other high-energy locations in the nearby Bay of Fundy (Schwinghamer, 1983). However, when size categories were put into functional/trophic groups (bacteria, microfauna, meiofauna) this benthic community was quite exceptional. Comparing the median proportions across these groups (Figure l), meiofaunal biomass was about 12?& of the bacterial contribution to biomass, in the lower range of other estimates of this proportion ( lO-500;) (Vernberg & Coull, 1975; Fenchel, 1975; Gerlach, 1978; Ankar, 1979). The median percentage of protozoan biomass was 49% of the median percentage bacterial biomass, a high value compared to the maximum of 20% reported in these studies.

We have observed in Brown’s Bank sediments that the biomass of protozoans (mainly colourless cryptomonads) can be greater than that of the metazoan meiofauna, an obser- vation not previously made in shelf environments. While ciliates and foraminiferans are usually considered to be the dominant benthic protozoans, our observations and studies of protozoa from the continental shelf off Washington state (Lighthart, 1969) and from an arctic tundra pond (Fenchel, 1975) clearly demonstrate that flagellates may be an abundant component of benthic protozoa under some circumstances.

In accordance with the total biomass, community metabolism on Brown’s Bank was at the lower range of values found in previous coastal studies (see Smith, 1973; Ellison, 1984). Much of the shelf production is probably deposited in fine-grained basins or slope sediments rather than on the high-energy banks (Grant et al., in press). One of the factors accounting for the low biomass and respiration on Brown’s Bank may be the limited organic input from the water column. Hartwig (1978) reached a similar conclusion for an energetic subtidal sand environment (18-m depth).

The relative contribution of fauna1 groups to community metabolism is often estimated from their biomass (see Introduction). Although we used a similar approach, our community R/B curve was constructed from biomass values measured in the same vials as the respiration. Our curve then allowed partitioning of metabolism from detailed studies of biomass in the natural sediment. We can compare the results of our partitioning experiments to other studies. For the non-macrofauna portion of oxygen consumption on exposed sandy beaches, Dye (1981) estimated that bacteria respired 2-3 times more than either protozoans or meiofauna. Smith et al. (1972) found that in coarse carbonate sediments (Bermuda) meiofaunal respiration accounted for less than 25; of the non- macrofaunal respiration. For Fenchel’s (1969) subtidal sand station, the ciliate metabolism was twice that of the meiofauna (see Vernberg & Coull, 1974). Thus our data agree with previous results from clean sand substrates which show that protozoan respiration can exceed that of meiofauna.

Beyond the metabolic importance of the protozoa, our results allow insight into their trophic role in this benthic community. Some cryptomonads are heterotrophic (Nisbet,


1984) and in our incubations they apparently had a major impact as grazers on bacteria. Flagellates and other protozoans are well-known predators of bacteria (Lee, 1980; Fenchel, 1982). Meiofauna also consume bacteria (Tietjen, 1980; Montagna, 1984) and could potentially compete with protozoans for this food source. However, the addition experiments suggested that, if meiofauna are abundant (note range of relative biomass in Figure l), they may be important predators of the microflagellates and thus act to reduce grazing pressure on bacteria. Previous studies have shown that meiofauna consume ciliates (Lee, 1980; Rieper & Flotow, 1981), but the role of protozoans as prey of, or competitors with meiofauna requires more study.

In manipulating the relative abundance of benthic organisms, we may have affected synergistic interactions that influence community respiration (Hargrave, 1970; Kemp & Boynton, 1981; Pamatmat & Findlay, 1983). However, our R/B curve included potential negative (organisms added), positive (organisms substracted), and equilibrium (natural sediments) interactions. Our results emphasize that in studies of community rate processes, mass-scaling must be coupled with ecological scaling (see also Banse, 1982).

Our respiration calculations do not accommodate size-specific rates, i.e. using equation (4), an equivalent biomass of bacteria and meiofauna would have equivalent respiration rates. Because smaller organisms have a higher weight-specific respiration, bacteria should have a higher respiration rate than an equivalent weight of meiofauna. We therefore compared our empirical respiration rates (based on equation 4) for bacteria, microflagellates, and meiofauna with two size-corrected methods of calculating respiration from biomass.

Schwinghamer et al. (1986) derived empirical relationships between body size and specific production (P/B) over a size spectrum of marine benthic organisms ranging from 10-l’ to 10 g wet weight per individual. We used body size of Brown’s Bank taxa to calculate P/B, multiplied by B (biomass in the vials), and used P in estimating respiration according to equations in Schwinghamer et al. (1986). As a second size-corrected method, we used the equations of Hemmingsen (1960) who derived relationships between oxygen consumption and dry body weight for individual organisms. Individual respiration values were then multiplied by the density of individuals to obtain group respiration.

We applied both of these methods to bacterial, microflagellate, and meiofaunal biomass in each vial. In all cases, calculated bacterial respiration greatly exceeded measured respiration, sometimes by three orders ofmagnitude. We suggest that neither of these size- corrected methods is suitably applied to bacteria (see also Banse, 1982; Schwinghamer et al., 1986). Compared to our experimental results, both methods overestimated microflagellate respiration by a factor of 3-10. Hemmingsen’s method consistently gave a two-fold overestimation of our values for meiofauna respiration, probably because his data were based on laboratory respiration of organisms in culture. The method of Schwinghamer et al. underestimated our meiofauna values by a factor of 0.6.

Our approach provides reasonable estimates of benthic bacterial respiration on Brown’s Bank because it is based on communities of organisms rather than isolated cultures. Bacteria as a group contributed more than the other groups to community respiration (29-70”,,), and their biomass accounted for 56”,, of the variance in this respiration. Size-corrections would therefore greatly inflate our estimates of total community respiration based on summation of median rates. As indicated previously, the present summed estimates agree well with rates measured in whole sediment cores.

We conclude that construction of an R/B curve for a benthic community provides a means of partitioning sediment respiration among fauna1 groups. Medians of relative

660

biomass and respiration demonstrate general trends, the ranges indicate possible combinations of taxa and activity, and addition-removal experiments show the variety of trophic interactions resulting from these combinations. Specifically, our experiments demonstrate the metabolic importance of non-ciliate protozoans in a shelf sediment community, and suggest that they are predators of bacteria and prey for meiofauna. Understanding the trophic/functional relations between bacteria, micro- and meiofauna must be preceded by further information on the biomasss, diversity, and feeding ecology of protozoans in marine sediments.

Acknowledgements

This study was part of the benthic ecological research being carried out under the South- West Nova Scotia Fisheries Ecology Program, a joint undertaking of the Marine Fish Division and MarineEcology Laboratory (Bedford Instituteof Oceanography), Dalhousie University, and St Andrews Biological Station. Additional support was provided by the Natural Sciences and Engineering Research Council of Canada. We thank B. T. Hargrave for helpful comments on the manuscript.

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