The effect of sediment characteristics on the distribution of two subtidal harpacticoid copepod species

.I. e.~p. fu(ir. Biof. Gof.. 1981. Vol. 50, pp. 289- 301 0 ElsevieriNorth-Holland Biomedical Press

THE EFFECT OF SEDIMENT ON THE DISTRIBUTION

OF TWO SUBTIDAL

COPEPOD

WILLIAM Deppurtmcnt

~HARACTERIS~CS HARPACTICOID

SPECIES

S. RAVENEL and DAVID THISTLE’

qf’Oceanographv,

Fiorida

State University,,

TaNahassee, FL 32306,

U.S.A.

Abstract: Abundances of two harpacticoid copepod species, Enh.vdrosoma littorale Wells and Zausodcs cf. arenicolus Wilson, were significantly higher in one of two adjacent subtidal. soft-bottom habitats in St. George Sound. Florida (29”54’N : 84”37’48”W). For Enhydrosoma littorafr. a laboratory-preference experiment indicated that sediment-related factors caused the observed distribution. in a series of preference experiments, differences between the sediments of the two habitats in granulometry and organic matter were shown not to account for the preference. Rather, the preference results from differences in the microbes attached to the sediment particles in the two areas. In contrast, Zausodes cf. arenicolus did not prefer sediments from its area of high field abundance in laboratory preference experiments. indicating that factors external to the sediment, i.e. hydrographic conditions or biological interactions, were responsible for this species’ distribution.

INTRODUCTION

Meiofauna community studies have progressed from the initial qualitative descriptions of the fauna (Remane, 1933; Mare, 1942) to the recognition and use of these ubiquitous organisms as important tools in the study of basic ecological questions (Coull & Bell, 1979). Little is known, however, about the factors which control meiofaunal species distributions. For most species studied, these distributions have been found to be non-random (Gray & Rieger, 1971; Heip, 1976; Heip & Engels, 1977; Thistle, 1978). Attempts to explain observed meiofaunal distribution patterns have concentrated on physical factors. The importance of sediment type in controlling meiofauna community compositions has been shown for subtidal sediments in studies which correlate fauna1 densities with sediment types (e.g., Wieser, 1960; Coull, 1970; McLachlan et al., 1977; Tietjen, 1977; Moore, 1979). Although these studies identify general sediment characteristics which limit the distributions of some organisms, they do little to explain observed differential distributions in relatively homogeneous sediment. Factors affecting meiofaunal distribution patterns have been studied more intensively in intertidal sediments. Here, a number of physical factors have been shown to control the horizontal and vertical distribution of meiofauna. Using a ’ To whom inquiries should be sent. 289

WILLIAMS.RAVENELANDDAVlDTHlSTLE

290

combination

of field studies

that salinity,

temperature,

of the mystacocarid

and laboratory

experiments,

oxygen availability,

Derocheilocaris

Jansson

and grain size limited

remanei.

Similarly,

Jansson

(1966) showed the distribution

(1967) found

that

the distribution of the harpacticoid copepod Parastenocaris vicesima was controlled by oxygen availability, grain size and salinity. Using this approach Gray (1965, 1966a,b,c,d) demonstrated that the distribution of interstitial archiannelids could be explained by differences in temperature, light, oxygen tension, desiccation of sand, grain size, and presence of favorable species of bacteria on the sand grains. Similarly, Gray (1968) showed that high densities of the harpacticoid Leptastacus constrictus which occurred within an area of relatively uniform sediment appeared to be caused by the presence of particular species of bacteria. The purpose of the present study was to determine if Gray’s results could be extended to subtidal sand-bottom meiofaunal communities where physical factors such as temperature, salinity, and oxygen remain relatively uniform over large areas. Two species of harpacticoid copepod (Enhydrosoma littorale Wells and Zausodes c.f. arenicolus Wilson) were the subjects of preference experiments in our attempt to identify the factors controlling their field distributions.

MATERIALSAND

METHODS

The study site was in St. George Sound off the coast of the Florida Panhandle (29”54’N : 84”37’48”W). Sea-grass meadows with interspersed sand patches covered the bottom in this area. The site was located 300 m offshore on the gradual slope of a shoal. Two parallel 50-m transect lines 3 m apart defined the study site. The 6-mm diameter nylon lines were tagged every 10 cm. A 3-m movable cross line, also tagged at lo-cm intervals, was used to form a Cartesian coordinate system enabling sampling

of pre-selected

random

locations

between

the lines.

Two grass-free patches of sediment crossed by the transect lines were chosen for sampling, and a 3-m2 sampling area was defined within each patch. “Area 1” was defined as the strip of sediment bounded by the transect lines between the 13-m and 14-m tags at a depth of 1.6 m at mean low water. “Area 2” was defined as the strip of sediment bounded by the transect lines between the 49-m and 50-m tags at a depth of 2.3 m at mean low water. SCUBA

divers did the sampling.

Preliminary

samples

were taken

in June and

September, 1978. Five pre-selected random positions in each area were sampled each month. A l-cm2 core for organisms, a IO-cm2 core for total organic matter, and a IO-cm* core for sediment granulometry were taken at each position (avoiding macroscopic features such as shells or polychaete tubes). On the boat, the overlying water was drawn off and preserved together with the top 1 cm of sediment in a 10% formaldehyde solution. The samples for total organic matter were frozen on dry ice and stored frozen until analysed.

SEDIMENT

CHARACTERISTICS

AND COPEPODS

291

Due to encroachment of seagrass into Area 1 during the interval following the preliminary sampling, Area 1 was relocated in an adjacent grass-free region bordered by a third transect line. Area 2 and the relocated Area 1 were sampled as above on 11 June, and 25 September, 1979, except that S-cm’ cores were used for organisms. In addition to the cores, undisturbed 385-cm’ samples of the bottom were taken at haph~ardly selected positions (free of macroscopic features) near the sample areas to provide sediment for preference experiments. These sampies were taken by gently inserting a 19.6 cm x 19.6 cm x 10 cm acrylic frame into the sediment, covering it with a lid, and carefully sliding a stainless steel tray under the frame. The entire 385-cm*, l-2 cm thick layer of sediment was then lifted undisturbed and placed in situ into a glass aquarium made to contain the frame and tray. Two such samples were taken near Area 1 and two near Area 2 in the June sampling; in September three samples were taken from each area. The aquaria containing the sediment and overlying water were frozen on dry ice within 30 min after collection and were stored frozen until used in the preference experiments. The harpacticoid copepods in each organism sample were quantitatively concentrated using a sorting trough (Barnett, 1968) which overflowed into a 0.062-mm sieve. The median extraction efficiency by this method in these sediments was IOOO,,, (Thistle, 1980). The sieve contents were stained in Rose Bengal and the harpacticoids were removed under a dissecting microscope. Adults and copepodite stage V’s were identified to species while earlier copepodite stages were collectively counted as juveniles. Sediment total organic matter was determined from weight loss after combustion at 500 “C (Byers et al., 1978). Sediment granulometry samples were analysed by wet sieving at 0.5-4 intervals from 0.5 4 to 4.0 Cpafter removing any macrofauna and detritus on a 0.0 4 (1.0 mm) sieve. Combined silt and clay content was determined by rinsing the sample on a 4.0-4 f0.062 mm) sieve with de-ionized water while collecting the water and suspended particles passing through the sieve in a pan below. The contents of the pan were rinsed into a 1-l graduated cylinder and brought up to l-1 volume with de-ionized water. After stirring thoroughly, a IOO-ml aliquot was removed and filtered through a pre-weighed Whatmanfilter paper in a suction apparatus (Holme & McIntyre, 1971). After drying at 100°C for at least 24 h, the filters were allowed to equilibrate to room conditions and then were weighed. The net weight was mdtiplied by 10 lo give the weight of silt-clay. Cumulative curves were plotted on probability paper, and the graphic mean and inclusive graphic standard deviation were calculated (Folk, 1968). A supply of organisms to be used in the preference experiments was obtained by collecting sediment near the study site and storing it in aquaria which were supplied with running sea water and allowed to overflow continuously. Animals were concentrated from the sediment by decantation into a 0.062-mm sieve. The sieve was then rinsed into a dish from which individuals of the appropriate species could be removed by pipette.

WILLIAM

292

S. RAVENEL

AND DAVID

THISTLE

The preference experiments were run in acrylic chambers (Fig. 1). To set up a run. plastic partitions were inserted in the three slots. thus forming two adjacent

Fig.

1. Preference chamber with partitions

in place: scale line equals

1cm.

I-cm? chambers. A square core tube with external measurements of 1 cm was used to insert a relatively undisturbed, 0.5cm deep sample of the two sediment types being offered on either side of the center partition. The chamber was then placed in a dish to which sea water was added until the sediment was covered to a depth of 0.5 cm. The center partition was carefully removed, and a single specimen of the selected species was introduced to the center of the chamber with a pipette. Each dish held two chambers. The chambers were placed side by side; sediments of opposite types were placed next to each other to control for unperceived external factors. After 2 h in the dark, the center partition was replaced, the end partitions were removed, and the sediment on each half was rinsed into dishes for sorting. Preliminary observations of the species studied indicated that 2 h was sufficient time to allow the animal to inspect each sediment type several times. Each experiment consisted of the first 20 successful runs. A run was termed unsuccessful when : (1) the test individual was lost; (2) the recovered individual was a juvenile; or (3) the recovered individual was not of the correct species. Unsuccessful runs comprised 56% of those attempted. All sediment used in the preference experiments was taken from the frozen aquarium samples taken on the two sampling days in June and September. 1979. In addition to whole sediment from each area, a variety of treatments was used to investigate the importance of specific sediment characteristics to the two harpacticoid species. “Trough-washed sediment” refers to sediment which was processed in a sorting trough (Barnett, 1968) using running sea water. This procedure removed metazoans, detrital organic material, as well as the silt, clay, and a small amount

SEDIMENTCHARACTERISTICSANDCOPEPODS

of sand from the finer fractions could be caught To replace

on a 0.062-mm

present

in the sediment.

sieve and returned

the silt and clay, the sediment

293

Most of the organic to the sediment

was decanted

through

material

when desired.

a 0.062-mm

sieve

before trough washing. The sea water passing through the sieve was poured into a graduated cylinder, allowed to settle, and then the overlying water was poured off. The resulting concentration of silt and clay was stirred into a volume of moist. “Sonicated sediment” refers to trough-washed sediment equal to the original. sediment which was first trough-washed, then treated with a Biosonic IV ultrasonicator (Bronwill Scientific, Rochester, N.Y.). Two cm3 of sediment at a time were treated in a 30-ml tall-form beaker for 5 min. This was the maximum quantity of sediment which would be completely suspended by the process. After sonication, the sediment was rinsed with autoclaved sea water and frozen until used. The purpose of the sonication was to strip the sand grains of their surface flora. The efficacy of the procedure was tested on Area 2 sediments. Ten 20-g (wet wt) samples were randomly assigned either to be sonicated or to serve as controls. After treatment, the microbial mass of each sample was measured by lipid phosphate analysis (White rt al., 1979a). The 95’1; level of significance was used throughout.

RESULTS

The analyses of the preliminary sediment samples taken in June and September, 1978, suggested three major differences between the sediments of Area 1 and Area 2. Area 2 had a higher percentage of silt and clay, a lesser degree of sorting, and a higher percentage of total organic matter. On the basis of the 1978 data, a one-tail Wilcoxon T test (Tate & Clelland, 1957) was used to test the significance of these trends in the June and September, 1979, data (Tables I and II). In both months, the predicted differences were found. The harpacticoid copepod abundances obtained from the l-cm’ preliminary cores taken during the summer of 1978 were used to identify apparent species preferences between Area 1 and Area 2. These data allowed a priori predictions of the differential distribution of particular species in 1979 to be tested. In June, 1978, Zausodes c.f. arenicolus abundances were higher in Area 2 than Area 1 (Table III). The prediction that this species’ median abundance in June, 1979, was higher in Area 2 than Area 1 was tested and the difference was significant (P = 0.025, one-tailed Wilcoxon T test, Tate & Clelland, 1957). If this difference in abundance were due to sediment characteristics, it should be duplicated in laboratory preference experiments (e.g. Gray, 1968). An experiment was conducted in which adults of Z. c.f. arenicolus were presented with a choice between Area 1 and Area 2 sediment collected the same day the field data were collected. The results of this experiment and all other preference experi-

294

WILLIAM

S. RAVENEL

AND

DAVID

THISTLE

TABLE I

Area 1 and Area 2 sediment

Inclusive graphic SD

Graphic mean Area

1

2.52 2.31 2.25 2.38 2.22

characteristics

X = 2.33 C#I s=O.12$

X = 0.52 C$ s = 0.05 f#J

x = 0.24 .F= 0.15

.? = 0.63 .r = 0.10

2.32 2.21 2.19 2.22 2.34

0.65 0.67 0.65 0.63 0.66

4 4 4 4 r$

0.85 0.59 0.72 0.51 2.41

1.16 I .09 0.69 I .42 0.85

U = 2.26 4 s = 0.07 4 ._

S = 0.65 4 s= 0.01 Q

.u= 1.02 s = 0.79

.Y= 1.04 .s = 0.28

P < 0.05

P = 0.005

P = 0.005

P < 0.025

TABLE

II

1 and Area 2 sediment characteristics Inclusive graphic

Graphic mean

Area 2

Silt-llay

0.80 0.54 0.62 0.56 0.65

Area

Area 1

‘I,, Total organic matter

0

0.13 0.13 0.50 0.20 0.23

Q Q 4 l#l 4

0.47 0.50 0.59 0.52 0.50

1979.

f#l I$ Q 4 4

Area 2

9 4 4 4 4

in June,

2.36 2.33 2.39 2.20 2.24

0 ‘” Silt-clay

SD

4 I#J r$ ij I#I

1979 “” Total organic matter

4 4 4 4 I#I

0.05 0.14 0.05 0.11 0.17

0.36 0.31 0.41 0.30 0.45

S = 2.30 r$ s = 0.08 Q

?r = 0.56 4 .Y= 0.03 l#J

.u= 0.10 s = 0.05

.I- = 0.36 .\ = 0.06

2.47 2.08 2.24 2.21 2.27

0.58 0.64 0.65 0.64 0.61

4 4 4 4 4

0.46 0.27 0.59 0.45 0.31

0.60 0.65 0.71 0.56 0.52

.v = 2.25 4 s=O.l44

x = 0.62 4 s = 0.03 4

U = 0.42 s= 0.13

\- = 0.61 .Y= 0.07

P < 0.05

P = 0.025

P = 0.005

P = 0.005

C#J q~ 4 rp 4

0.54 0.59 0.54 0.53 0.60

in September,

SEDIMENT

CHARACTERISTICS

AND COPEPODS

295

ments conducted were tested using the upper-tail probabilities for the binomial distribution (n = 20, P = 0.50) (Hollander & Wolfe, 1973). After 20 successful trials, 13 individuals were found in the Area 1 sediment and 7 in the Area 2 sediment. There was no reason to conclude that the species responded differentially to the sediments (P = 0.1316). TABLF

111

Abundances of Zausodes c.f. arenicolus and Enhydrosoma littorale per core: l-cm’ core tubes were used in 1978; 5-cm’ core tubes were used in 1979; P-values refer to results on one-tail Wilcoxon T tests. Zausodes c.f. arenicolus -~-

Area 1

Area 2

Enhydrosoma iittorale

June 1978

June 1979

Sept. 1978

Sept. 1979

0 0 2 1 0

1 5 4 1 12

0

4

0

0

0

0 0

7 1 1

0 2 3 1 2

39 91 10 35 39

2 5 6 2 4

7 4 7 4 8

P < 0.025

P = 0.05

In September 1978, Enhydrosoma littorale was present in Area 1 in sufficiently low abundances that it was not found in our samples. Its abundance in Area 2 was much higher at this time (Table III). This trend was predicted to re-occur in September of the following year. A one-tail test applied to the abundances of E. littorale collected in September, 1979 (Table III) indicated that this species was more abundant in Area 2 than in Area 1 (P = 0.05). A preference experiment similar to that run with Zausodes c.f. arenicolus, but using sediment collected in September, 1979, was run with Enhydrosoma littorale. E. littorale preferred Area 2 sediment (P = 0.0059). In an attempt to identify the specific sediment characteristic(s) that made sediment from Area 2 more attractive to E. littorale than that from Area 1, a series of preference experiments was run (Table IV). The first experiment tested the importance of the silt-clay fraction. Test animals were offered a choice between trough-washed sediment from Area 2 to which the organic material caught on a 0.062-mm sieve during the troughing process was returned, and trough-washed sediment from Area 2 to which the organic material as well as the silt and clay fraction had been returned so that the two sediments

WILLIAM

296

differed

only in the presence

displayed

S. RAVENEL

AND DAVID

THISTLE

or absence

of the silt and clay fraction.

E. littorale

no preference.

In a preliminary

to the second experiment,

had only trace amounts

of lipid phosphate

we determined

that sonicated

and significantly

less than

sediment

the amount

in control sediment (P = 0.008, one-tailed, Wilcoxon T test, Tate & Clelland, 1957) indicating that sonication removed the microbes attached to the sediment particles (Table V). TABLE IV

Summary

of preference

experiments

and results: P-values binomial distribution.

Experiment number

I

refer

to upper-tail

Sediment treatments Trough-washed replaced

Outcome

sonicated

Trough-washed.

sonicated

Trough-washed

Area 2

4rea

10 8

1

0.2517 Area 2

12 6 0.0577

VS

Trough-washed replaced 4

Trough-washed.

Area 2. organics 14 sonicdted

Area 2

11 0.4119

VS

5

Trough-washed. sonicated organics replaced

Area 2,

Trough-washed,

Area 2

son&ted

9

4 0.005’)

VS

Trough-washed

P

0.588 1

vs 3

the

10

Trough-washed Area 2, organics & silt-clay replaced Trough-washed.

for

Area 2. organ& VS

7

probabilities

Area 2

16

Lipid phosphate concentrations (PM PO,/‘g wet wt) in sediments which had been sonicated to remove the microbial flora and in control sediments: one sample was lost during extraction; the values for the sonicated sediment approach the limits of detectability of the technique. Sonicated

sediment

0.0013 0.0017 0.0017 0.0016 -

Control

sediment

0.0123 0.0116 0.0125 0.0139 0.0121

SEDIMENT

The second granulometry trough-washed,

experiment between

CHARACTERISTICS

tested

the importance

the two sediments.

sonicated

sediment

AND COPEPODS

of the differences

Animals

from Area

291

in sand-size

were given a choice

1 and identically-treated

between sediment

from Area 2 so that differences in organic material and microorganisms attached to the sand grains between the areas did not play a role in the experiment. The sediments did not differ in attractiveness (P = 0.2517). The third experiment tested the importance of organic matter. Animals were given a choice between trough-washed, Area 2 sediment and trough-washed, Area 2 sediment to which the organic material had been returned. E. littorale showed no significant preference (P = 0.0577), but given the near significance of the result this experiment was repeated using sonicdted sediments from Area 2 versus sonicated sediment from Area 2 to which the organics had been replaced. No preference was expressed (P = 0.4119). The fifth experiment tested the importance to the harpacticoids of the microbes attached to the sand grains. A choice between trough-washed, sonicated, Area 2 sediment and trough-washed, unsonicated, Area 2 sediment was offered the test animals. E. littorale preferred the unsonicated, microbe-rich sediment (P = 0.0059).

DISCUSSION The trends observed in the field distributions of Zausodes c.f. arenicolus in June and Enhydrosoma littorale in September are present for two consecutive years, suggesting that they are the result of annually recurring if not permanent differences between Area 1 and Area 2. If these trends were due to differences in sediment characteristics then the preliminary preference experiments presenting a choice between whole sediment from Area 1 and whole sediment from Area 2 should have resulted in a preference for Area 2 sediment. E. littorale showed such a preference justifying further experiments to identify the factor(s) responsible. Although the silt and clay fraction (< 0.062 mm) was a very small percentage of the ambient sediment from both areas, it was significantly higher in Area 2. In Experiment

1 (Table

IV), the presence

or absence

of silt and clay in the otherwise

identical sediments did not result in a preference. These results permitted the use of the trough-washing process without replacement of the fine fraction in subsequent experiments. In Experiment 2, we used sediment from Area 1 and Area 2 which had been trough-washed to remove organic matter and sonicdted to remove microbes to test for a response to the differences in particle size frequencies of the two areas as reflected by the difference in inclusive graphic standard deviations, No significant preference was found. The results of these two experiments eliminated the sediment granulometric differences as factors affecting the distribution of E. littorale. Gray (1968) did the only comparable work on benthic harpacticoids (but see

WILLIAM

298

Hicks

(1977) on phytal

factors influencing on an intertidal

S. RAVENEL

species).

the distribution sand beach.

AND DAVID

He used

preference

of the interstitial

He found

THISTLE

experiments

to determine

species Leptastacus constrictus

no evidence

to suggest that granulometric

differences controlled the distribution of L. constrictus on the beach. Our results indicated a similar indifference to granulometry for two species of subtidal, noninterstitial

harpacticoids.

Experiment

3, comparing

trough-washed

sediment

from

Area

2 with

trough-

washed sediment from Area 2 to which the organic matter had been replaced, did not result in a significant preference. More individuals chose the sediment with organic matter, however, and it was thought that if both organic material and the microbial biomass attached to the sand grains were potential food sources, then a preference for the former may be masked by the presence of the latter in both sediments. We repeated the experiment as Experiment 4 with the difference that the sediment was sonicated to eliminate the microbes attached to the sand grains. This experiment also failed to produce a significant preference. These two experiments eliminated differences in total organic matter between Area 1 and Area 2 as factors contributing to the differential distribution of Enhydrosoma littorafe. This result was unexpected for two reasons. First, although Gray (1968) did not report it, the Leptastacus constrictus abundances in his Table X were significantly positively correlated with the organic content of the sediment (P = 0.02, two-tailed Kendall tau, Tate & Clelland, 1957). Secondly, harpacticoids have been known to feed on microbes (Battaglia, 1970; Lasker et al., 1970; Brown & Sibert, 1977; Rieper. 1978). As a result, we anticipated that the microbial populations on the detritus which constituted the elutriable organic matter of this study would have elicited a response. The results suggested that Enhqldrosoma littorale may not be a detritus-scraping species. Meadows & Anderson (1966, 1968) reported the occurrence of microbial populations on sediment particle surfaces. Gray (1968) showed that the attractiveness of sediment to Leptastacus constrictus could be destroyed by various treatments harmful to microbes (e.g. drying, autoclaving) and then restored soaking in sea water. In follow-up experiments, he showed that laboratory cultures of known bacteria as well as bacteria cultured from his beach restored the attractiveness to varying degrees. Gray concluded that the variation of bacterial types in the field had an important impact on the localization of L. constrictus abundances. Our Experiment 5 addresses this issue. In it sonicated Area 2 sediment, which lacked a microbial flora, was found to be significantly less attractive to Enhydrosoma littorale than unsonicated Area 2 sediment implying that differences in the microbes were the reason that Area 2 sediments were more attractive than Area 1 sediments to E. littorale. These data have extended Gray’s (1968) conclusion to a subtidal, noninterstitial harpacticoid species. The microbial flora of sand grains is complex (Meadows & Anderson, 1966. 1968). The above results suggest that some characteristics of this flora in Area 1 and

SEDiMENTCHARACT~RlSTlCSANDCOPE~ODS

299

Area 2 differ. The identification of these differences is a technically formidable problem (Meadows & Campbell, 1972). Gray’s (1968) approach using stock cultures is suggestive but necessarily artificial. Even his use of bacteria isolated from natural sand bears no necessary relationship to the microbial differences actually perceived by the harpacticoids in the field because of the selectiveness of the culturing process (Jannasch & Jones, 1959; Alexander, 1971; White rf al., 1979b). To answer this question, new techniques are required which assess microbial populations non-selectively. Although the En~~~r~‘~~~~ tittorale data support the ~e~t~st~cus constri~fu.~ data, microbes do not explain harpacticoid species’ localizations in general. In June, Zuus0die.s c-f. are&ohs was significantly more abundant in Area 2 than Area 1. It showed, however, no preference in the laboratory for Area 2 sediment. This result suggests that sediment-related factors including microbes do not underlie the field distribution. Rather, the results suggest that the species’ localization is controlled by differences in environmental factors not duplicated in the laboratory. One physical difference between Area 1 and Area 2 which could affect the distribution of Z. c.f. arenicolus is the degree of turbulence. Because Area 1 is located in slightly shallower water, it is affected more by the small waves generated daily in summer by increasing wind strength in the afternoon. Ripple marks were sometimes observed in the sediment of Area 1 but never in Area 2. Z. c.f. ff~e~~c~Z~~ is a large, dorsoventrally flattened species and may be affected by such turbulence. Biological interactions could not be duplicated in the laboratory and may also affect the distribution of 2. c.f. arenicolus. Recent studies have implied that macrofauna1 predation can be important in regulating meiofauna populations (Feller & Kaczinski, 1975; Bell & Coull. 1978; Buzas, 1978; Sibert, 1979). Sibert (1979) reported that juvenile chum salmon preyed heavily on harpacticoid copepods and on one large species in particular. Area 1 is closer to an area of dense seagrass than Area 2. Juvenile pinfish (Lug&on rhomboides (Linnaeus)) are present in the grasses in large numbers from the spring through summer and could be a factor affecting the distribution of Z. c.f. arenicolus. III this study, we have shown that differences between two subtidal sandy sediment areas in their microbial flora affect the distribution of a species of harpacticoid copepod. A second species appears to be influenced by factors external to sediment characteristics, however, suggesting that factors other than microbes can be important in controlling harpacticoid population distributions.

ACKNOWLEDGEMENTS

R. Dennis and J. Reidenauer dived with us. D. C. White and his co-workers did the lipid phosphate measurements. P. A. LaRock lent his ultrasound machine. K. Fauchald, P. A. LaRock, J. Reidenauer, R. C. Staley, K. Sherman, and A. Thistle

300

WILLIAM

S. RAVENEL

AND DAVID THISTLE

read and commented on the manuscript. We thank these people for their kind help. The research was supported by Office of Naval Research Contract N 00014-75-C-0201 and a Grant-in-Aid-of-Research from the Scientific Research Society of North America.

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SEDIMENT

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AND COPEPODS

301

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