A comparison of meiofaunal settlement onto the sediment surface and recolonization of defaunated sandy sediment

J. Exp. Mar. Biol. Ecol., 1988, Vol. 123, pp. 97-113

97

Elsevier

JEM 01155

A comparison of meiofaunal settlement onto the sediment surface and recolonization of defaunated sandy sediment Stephen

R. Fegley

Rutgers Shelfuh Research Laboratory. Port Norris, New Jersey, U.S.A.

(Received 30 March 1988; revision received 6 July 1988; accepted 2 August 1988) Abstract: Meiofaunal drift (above-sediment transport) and recolonization of disturbed sediments have been examined separately in numerous studies. No studies have attempted to follow both events simultaneously. This issue is important because many recolonizing meiofauna arrive via above-sediment pathways and recolonization rates may depend largely on the number of available recolonizers settling onto the sediment surface. In this study, estimates of the densities of meiofauna settling onto the sediment surface at different stages of a tide were made using cylindrical, settlement traps buried flush to the sediment surface. At the same location and over the same tide, cores were taken of previously defaunated sediment to quantify meiofaunal recolonization. Significantly greater numbers of meiofauna were found in settlement traps at flood tide, which corresponded to the time by which most individuals had recolonized the defaunated sediment. Relative proportions of the more abundant taxa were more similar between recolonized and ambient sediments than with either of these two versus the settlement traps. Only one taxon, ostracods, approached ambient densities in the recolonized sediment by the end of the sampling period: recolonization densities of all other taxa were significantly lower than ambient densities. Harpacticoid copepods generally displayed patterns of abundance in both settlement traps and recolonized sediment that were different than other taxa. The estimated density ofsettling meiofauna greatly exceeded the observed density ofrecolonizing meiofauna at each of four tidal periods (flood, high, ebb, and low tides). The large surplus of potential over actual recolonizers indicates that, on the sandflat examined, other factors, perhaps meiofaunal behavior or microbial dynamics, are more important in determining recolonization rates than the magnitude of meiofaunal drift. Key words: Meiofauna; Meiofaunal recolonization; Meiofaunal settlement

INTRODUCTION

Over the past decade, above-sediment transport of benthic meiofauna has been demonstrated to be ubiquitous and frequent. In shallow-water habitats meiofauna have been observed to disperse laterally via both bedload and suspended-load pathways over substrata ranging from sandy to muddy composition (Bell & Sherman, 1980; Hagerman & Rieger, 198 1; Sibert, 198 1; Chandler & Fleeger, 1983 ; Palmer & Gust, 1985). Indirect evidence exists to suggest that meiofaunal transport occurs in deep-water habitats as

This paper is New Jersey Agricultural Experiment Station Publication D-32001-2-88. Correspondence address: S. R. Fegley, Rutgers Shellfish Research Laboratory, P.O. Box 687, Port Norris, NJ 08349, U.S.A. 0022-0981/88/$03.50 0 1988 Elsevier Science Publishers B.V. (Biomedical Division)

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well (Coull, 1969). Dispersing meiofaunal species are not restricted to a few specialists. The species composition of meiofauna in the water column is similar to that found in the top cm of the substratum and includes both permanent meiofauna and temporary (juvenile macrofauna) forms (Sibert, 1981; Eskin & Palmer, 1985 ; Palmer & Gust, 1985). Although lateral advection of meiofauna is no longer considered exceptional, there is little understanding of how important this process is to influencing marine, soft-bottom community structure and function. An essential component for determining whether meiofaunal lateral advection signilicantly affects benthic systems is identifying the fate of dispersing individuals. Recolonization of artificially and naturally defaunated sediments by dispersing meiofauna in both sandy and muddy habitats has been examined frequently (Scheibel & Rumohr, 1979; Sherman & Coull, 1980; Thistle, 1980; Reidenauer & Thistle, 1981; Chandler & Fleeger, 1983; Sherman etal., 1983). Observed patterns of recolonization differ. Sherman & Coull(1980), Reidenauer & Thistle (198 I), and Sherman et al. (1983) found recolonization rates to be very rapid (ambient densities reached in l-3 days) with little to no apparent changes in relative abundance of the recolonizing species. The authors concluded that the process of drift and subsequent recolonization played, at most, a minor role in maintaining patterns of relative abundance of species in the community. Thistle (1980) documented rapid recolonization (< 24 h) of enteropneust fecal mounds by harpacticoid copepods but found two of the 16 recolonizing species became disproportionately abundant during recolonization. Several studies have recorded both slow recolonization rates (ambient densities reached in several weeks) or changes in relative abundances of the recolonizing taxa (Scheibel & Rumohr, 1979; Alongi et al., 1983; Chandler & Fleeger, 1983). The observations made in the latter studies indicate that disturbance followed by recolonization could be an important process affecting meiofaunal population dynamics. An attempt to identify the components leading to the differing results of the various recolonization studies is frustrated because the studies were conducted in different habitats, examined different taxa, and often lack specific information on the origin of recruiting individuals. Conceivably, differences in recolonization patterns could result from differences in local dispersal patterns. The major source of recolonizing meiofauna is often presumed to be from above the sediments. Chandler & Fleeger (1983) demonstrated that above-sediment pathways were more or at least as important sources of recolonizing meiofauna as within-sediment movements in a muddy habitat. However, they do not provide estimates of how many potential recruits were available from above-sediment sources. In consequence, we do not know what the relationship is between the abundances of potential above-sediment recolonizers and actual numbers of recolonizers from that source. In the present study I sampled potential meiofaunal recolonizers at the sediment surface while quantifying recolonization into defaunated sediments. Specifically this study provides: (1) estimates of meiofaunal settlement from all above-sediment pathways (active epibenthic migration as well as passive bedload and suspended-load transport) and (2) an opportunity to compare patterns of meiofaunal settlement with

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patterns ofmeiofaunal recolonization. I also extend our knowledge of meiofaunal lateral advection into a common near-shore habitat, the low energy sandfiat.

MATERIALS STUDY

AND

METHODS

SITE

Sampling was conducted on an intertidal shoal about 40 m west of Piver’s Island in Beaufort, North Carolina (Fig. 1). The general physical and sedimentological characteristics of this region have been described elsewhere (Brett, 1963; Sutherland & Karlson, 1977; Peterson, 1982). The graphic mean (2 1 SE) sediment size at the study site was 0.141 (0.001) mm with a mean ( + 1 SE) silt-clay component of 3.56% (0.27%). Detailed information on seasonal changes in sediment composition for this shoal is provided in Fegley (1985). Predictable differences in the magnitude and direction of tidal currents occur on this sandflat because of its proximity to Piver’s Island. Flood tide flow comes to the sandflat from a southwesterly direction, urnimpeded by any prominent emergent or submergent l~dforms. During ebb tide much of the tidal flow is diverted south of the sandflat by Piver’s Island, which lies to the east. Consequently the magnitudes of the tidal currents differ greatly between flood and ebb tides. However, the directions of flood and ebb tidal currents change only slightly on the sandflat. SAMPLING Tidal

current

speed was determined

by timing the movement

of dye over a measured

distance at a height 30 cm above the sediment surface (except during very late ebb tide when the sandflat was covered by <2 cm of water). The aim of these measurements was not to fully characterize the flow regime in the sampling area but to use mean current speed (n = 10) to delineate different tidal stages: peak flood, slack high, peak ebb, and slack low. The last period did not correspond with actual low tide, when the flat would have been exposed, but was taken when only standing water was left on the somewhat concave-shaped sandflat. During low tide on 3 September 1984, when the sandflat was completely exposed, I placed 40 replicate settlement traps, at 0.5-m intervals, along a transect extending approximately perpendicular to the expected current direction {a shit in current direction of = 20” occurs at the specific site used so the transect was oriented so that both flood and ebb tidal currents impinged on the transect at an angle of x 800). The transect was located at the shoal interior (Fig. 1). Settlement traps were used to estimate the number of meiofauna arriving at the sediment surface. Because settlement traps can be notoriously biased samplers for measuring particle fluxes in the presence of current flows, some discussion of the design considerations and use of the traps in the present study is pertinent here. A large body

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of information examining these effects exists (see references and discussion in Hager-man & Rieger, 198 1, and Hannan, 1984). Of al1 the identified problems that lead to biases in particle sampling the one of greatest concern for the present study is the production of turbulence inside the trap. Secondary circulation patterns arise within any trap when

Fig. 1. Map of the Cape Lookout region of North Carolina with a detailed depiction of the Piver’s IsIand area. I, North Carolina, the arrow points to the Cape Lookout region; 2 Cape Lookout region, the arrow points to Beaufort Inlet; 3, direction of Beaufort Inlet from the study area; 4, Piver’s Island; 5, intertidal shoal used in the study with the transect location indicated by the straight broken line; 6, Beaufort, North Carolina.

ambient flow passes over the trap opening. The faster the ambient current the deeper this induced turbulence extends into the trap. Deposition of particles into the trap occurs at the interface between turbulent and non-turbulent water in the trap (Hatgrave & Bums, 1979; Gardner, 1980a,b; Hagerman & Rieger, 1981). If the trap is not uniform in diameter along its vertical length (i.e., the trap does not possess axial symmetry) then the area of the p~ic~e-~xch~ge interface changes as turbulence moves up and down the trap in response to changes in ambient flow. Consequently, the quality and quantity of the particles collected will be a function of the interaction between ambient current speed and trap shape (Hargrave & Bums, 1979; Lau, 1979; Hagerman & Rieger, 198 1). In addition, if the trap is not tong enough, turbulence eventually extends all the way to the bottom of the trap, resus~end~g particles pre~ous~y collected (Lau, 1979; Gardner, 1980a,bf. This condition flier reduces the reliability of trap-derived estimates of particle flux in unpredictable ways.

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To avoid these biases I used cylindrical traps that possessed axial symmetry and an aspect ratio (trap length to opening diameter) of 10. Lau (1979) demonstrated that cylindrical traps with an aspect ratio of 10 would retain previously trapped particles up to a trap Reynolds number (a dimensionless ratio describing unique flow characteristics, R, = pd/vwhere p is ambient velocity, d is trap diameter, and v is the kinematic viscosity of the water, Lau, 1979) of 20000. For the traps used in my study this indicates that resuspension of previously captured particles would not be a problem up to a current speed of 140 cm * s - 1 at the trap aperture. This is four times greater than the maximal, ambient current flow of 35 cm. s- ’ observed 30 cm above the sediment surface. Other potential biases were minimized in my design. When settlement traps are exposed to flow, secondary circulation patterns occur around the outside of the trap. Gardner (1980b) found the majority of particle-laden water entering a trap comes from this secondary flow near the trap opening. To avoid this, the traps were buried flush to the sediment surface so that secondary flows around the trap would not occur or would be minimized. This introduces the possibility that settling particles (meiofauna and sediment grains) that collect on the sediment surface near the opening of the trap could be swept into the trap en masse (Gardner, 1980b). This biases estimates of particle flux from just the water column because sediment and meiofaunal resuspension and horizontal transport do occur. However, I did not intend to estimate water column particle flux per se but net settlement onto a unit area of surface, which presumably is what an area of ambient or recolonizable sediment experiences. My goal was to estimate the total number of individuals arriving at a unit area of sediment, regardless of the specific above-sediment process that delivered the individuals to that location. Finally, live organisms may respond behaviorally to the presence of the trap or the secondary circulation patterns that occur within the trap. No information on how meiofauna respond to traps is available. Consequently, a short sampling interval was used to reduce the opportunity for any “escape” behavior to be effective by the relatively poor swimming meiofauna. This was probably adequate for all taxonomic groups except for efficient swimming, harpacticoid copepod species (Palmer, 1984; Fegley, 1985). Settlement traps were placed into the substratum in the following manner. A circular 15 cm i.d. x 10 cm high thin metal ring was shoved vertically into the sediments. I excavated the sediment contained within the ring and placed it into a bucket for transport outside the sampling area. Then, a 1.2 cm i.d. x 12.2 cm long vial was placed vertically into the center of the excavation and the remaining space was tilled with defaunated sediment. The presence of the defaunated sediment insured that no “contamination” of the trap samples with fauna would occur during sampling by accidental collapse of ambient sediment around the lip of the trap. In addition, I sampled the defaunated sediment throughout the tide to estimate the number of recolonizing fauna. The defaunated sediment was collected from the same general area of the sandflat 2 months prior to the present study. The sediment was repeatedly washed (five times) with distilled water in between an equal number of dryings at 50 “C in an oven. A total of 27 haphazardly

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selected, three cm3 samples of defaunated sediment were inspected by direct microscopic observation 1 wk prior to use. No meiofauna were found. Both the top of the sediment trap and the defaunated sediment were leveled flush with the ambient sediment surface (Fig. 2). Prior to inserting the traps, I filled each with fresh, 37-pm filtered seawater and stoppered the trap with a cork pushed flush to the tube opening. The cork had a 0.3 mm diameter wire projecting from its center. The wire served as a handle for removal of the cork from the trap. When each trap was in place the wire was the only structure rising above the level of the surrounding ambient sediment surface.

cm

\

I

Fig. 2. Schematic diagram of a single sampling site. All measurements are presented in the figure except those relating to the wire extending upward from the cork. The wire was 0.3 mm in diameter and 4 cm long. The wire is the only structure that extended above the sediment surface. 1, sediment surface; 2, ambient sediment; 3, defaunated sediment; 4, settlement trap with cork inserted.

Sampling periods, which began with peak flood and ended with slack low water, were selected on the basis of ambient current speeds. I took high and low tide settlement samples when no directional currents speeds could be detected. Flood and ebb tides were sampled when tidal current speeds were maximal. Inspection of current measurements recorded earlier over an entire tidal cycle from this location disclosed that near-maximal tidal current speeds occur for s 1 h. Consequently, when three successive sets of dye release measurements, taken at 15min intervals, indicated no large increases in current speed, the flood and ebb tide samples were collected.

MEIOFAUNAL

The method of sampling from a total of 10 randomly

SETTLEMENT

was the same during each tidal stage. I removed

the corks

chosen traps (five from each half of the transect).

care was exerted while removing the sediment

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AND RECOLONIZATION

the corks to keep the openings

surface. After removing

each cork, I inspected

Great

of the traps flush with

the trap visually,

using a

diving mask, to see whether the trap had been dislodged above or below the sediment surface. All traps disturbed in this way were discarded. After 5 min the corks were replaced. Because I could not open or seal all of the traps simultaneously, the traps were sealed in the same sequential order used to open them so that each trap was exposed for only 5 min. I next removed a single core sample from the (formerly) defaunated sediment at a distance not > 4 cm from the just-exposed trap. The suction corer used had an i.d. of 1.2 cm and a barrel length of 5.8 cm. The corer was inserted vertically into the sediment to a depth of 3 cm. I then removed the corer and extruded the contained sediment into a vial containing 4% buffered formalin in seawater stained with rose bengal. Next, the sealed trap was removed from the sediments. Because each trap was completely filled with seawater I could not add preservative to the sample without flooding the trap and risking the loss of specimens. Consequently, half of each trap’s contents was poured into an empty, prelabelled vial and the remaining volumes of both the trap and the vial were filled with 4% buffered formalin in seawater stained with rose bengal. I worked from the downstream side of the transect throughout the study to avoid increasing the number of suspended meiofauna in the water column by my activity that obviously disturbed the sediments. To estimate ambient fauna1 abundances, 10 replicate core samples were collected once during the sampling period from undisturbed ambient sediment immediately adjacent to the transect using the same suction corer and technique. The haphazardly selected cores were collected along the entire length of the transect immediately after the last sampling period. In the laboratory, I employed a flotation technique using a silica colloid, LUDOX-AM, to separate the meiofauna from the sediments (deJonge & Bouwman, 1977). The fauna were collected on a 34-pm screen. Prior to separation of the fauna the volume of sediment collected in each container was recorded. For all parametric comparisons I used Cochran’s test to examine homogeneity of variance and, except where noted, achieved homogeneity

the data for where it was

lacking using power or log transformations. Comparisons of faunal abundances in the traps at different tidal periods were made using a model I ANOVA. Analyses comparing abundances of recolonizing fauna were made similarly. RESULTS

All samples were collected in full sunlight except for the slack low water and ambient sediment samples, which were collected under twilight conditions. The specific times of collection were: peak flood, 1346; slack high water, 1603; peak ebb, 1836; and slack low water. 2038.

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Of the taxa collected in settlement traps in all four tidal samplings, only harpacticoids did not differ significantly in density with the stage of tide (Table I). Nematodes, turbellaria, and ciliates were all collected in greatest numbers at flood tide. Sediment volumes collected in the traps exhibited the same pattern. Regressions of faunal abundance on sediment volume within each sampling period were significant in only two instances (nematodes at ebb tide and ciliates at flood tide). TABLE I Mean ( & 1 SE) number of individuals collected in settlement traps during a S-min interval in each of four tidal stages. The mean ( + 1 SE) sediment volume (cm3) collected and current speed (cm. s _ ‘) measured at each sampling interval is also presented. Results of ANOVA comparing abundances collected in each interval are given behind the taxon name. Orthogonal contrasts are presented below. Results of regressions of fauna1 abundances onto sediment volume collected appear at the bottom. - , no ANOVA conducted;

Taxon Nematodes *** Turbellarians *** Ciliates *** Harpacticoids NS Juv. Bivalves PolychaetesGastrotrichsOstracodsTardigradesSediment vol. *** Current speed-

Flood (n = 7) 15.98 5.16 9.37 0.97 0.97 0.33 0.16 2.42 0.80 2.59 31.93

High (n = 9)

(3.41) (2.00) (2.52) (0.38) (0.52) (0.32) (0.16) (0.97) (0.54) (0.86) (1.69)

1.63 0.76 1.13 0.25 0.0 0.0 0.0 0.12 0.12 0.33 0.0 Orthogonal

Taxon

(0.33) (0.27) (0.46) (0.16)

(0.13) (0.13) (0.09)

Low (n = 9)

3.27 0.37 0.88 0.25 0.0 0.0 0.0 0.76 0.0 0.63 19.48

3.14 1.25 0.50 0.50 0.25 0.12 0.0 0.0 0.25 0.38 0.0

(0.81) (0.81) (0.25) (0.16)

(0.27) (0.08) (0.55)

(1.27) (0.35) (0.27) (0.38) (0.16) (0.12)

(0.25) (0.10)

contrasts

Flood + ebb vs. high + low

Nematodes Turbellarians Ciliates Harpacticoids Sediment vol.

Ebb (n = 9)

Flood vs. ebb

High vs. low

***

***

NS

***

***

***

NS

NS

NS

***

***

NS

NS NS NS

Regressions Taxon Nematodes Turbellarians Cihates

Flood

High

Ebb

Low

NS

NS

**

NS

NS

NS

NS

NS

**

NS

NS

NS

The majority of individual nematodes, turbellaria, ciliates, and ostracods recolonizing the defaunated sediment were present when the first samples were taken (Table II). Comparison of the abundances of these four groups demonstrated no significant

Nematodes NS Turbellarians NS Ciliates NS Harpacticoids *** Juv. BivalvesPolychaetesCastrotrichsOstracods NS Tardigrades NS

Taxon

Mean ( & I SE) densities. taxon are also presented, of subsequent orthogonal next

II

Nematodes Turbellarians Ciliates Harpacticoids Ostracods Tardigadcs

Taxon

14.52 1.86 3.44 0.09 0.0 0.09 0.0 1.08 0.09

(0.56) (0.12)

(0.12)

(3.93) (0.55) (0.89) (0.12)

Flood (n = 1O) 19.23 1.86 3.44 0.69 0.0 0.0 0.19 1.38 0.19

(0.92) (0.16)

(5.50) (1.08) (0.60) (0.75)

NS

* NS

NS

NS

** NS

NS

NS

* NS

NS

NS NS

NS NS

High vs. low

22.56 2.94 3.53 1.08 0.0 0.0 0.0 2.16 0.20

NS NS

(0.16) (0.47) (0.16)

(4.13) (0.81) (0.60) (0.25)

Low (n = 10)

Plood vs. ebb

Ckthogonal contrasts

jO.47) (0.25)

(0.12)

(3.72) (0.48) (1.16) (1.32)

Ebb (n = 9)

Flood + ebb vs. high + low

16.95 1.23 3.35 1.50 0.0 0.09 0.0 0.88 0.35

High (n = 9)

Tidal stage

112.30 9.80 30.28 3.35 0.62 0.09 0.62 2.65 0.97

(0.57)NS

(1.&8)NS

(9.20)** (1.02)** (2.80)** (1.15)” (0.29)(0.12)(0.34)-

Ambient (n - 10)

for each Results of ANOVA comparisons among recolonization densities within each taxon are presented behind the taxon name. Results contrasts are listed below. Results of ANOVA comparisons between low tide recoI~nizat~o~ densities and ambient densities are given to the ambient density values. -, no ANOVA conducted; NS, P > 0.05; *>P < 0.05; **P < 0.01; ***, P < 0.0Ol.

cm? of the taxa collected from defaunated sediment exposed to recolonization. Mean (k 1 SE) ambient densities -cm-’

TABLE

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S. R. FEGLEY

differences over the four tidal stages. The densities of nematodes, turbellaria, and ciliates in the defaunated sediment at the end of the sampling period (one-half tidal cycle) were significantly lower than respective ambient densities. Harpacticoids showed a different pattern. Lower densities of harpacticoids were detected in the defaunated sediments whenever currents were present suggesting that individuals moved into the water column when flow was present. As with nematodes, turbellaria, and ciliates, harpacticoid density in the defaunated sediment was significantly lower than ambient at the end of the sampling period. Two taxa, ostracods and tardigrades, attained densities in the defaunated sediment that were not significantly different from ambient levels by the last set of samples. Juvenile bivalves were sampled only in the traps and ambient cores. I compared separate sections of the transect to determine whether any spatial differences existed in patterns of recolonization at the scale examined. The samples were divided into separate groups based upon which quarter (5 m length) of the transect they were collected. Then separate ANOVAs were used to compare three different tidal combinations, when currents were present (flood and ebb tide samples pooled), when currents were absent (high and low tide samples pooled), and all tidal stages combined. The entire analysis was repeated after grouping the samples based on which half (10 m length) of the transect they were taken. In all cases, the null hypotheses that means were the same regardless of location in the transect or tidal combination could not be rejected. Because variation of abundances between tidal stages could have hidden any pattern, I re-examined the data after ranking the samples by abundance within each transect and tidal stage using a non-parametric Kruskal-Wallis test (Sokal & Rohlf, 1981). Again, I did not detect any significant patterns associated with any combination of location within the transect or tidal stage. Interpreting the sample results from the recolonized sediment is complicated by the temporal sampling schedule. All patches of defaunated sediment were initiated at the same time, prior to tidal inundation of the sandflat. However, sets of separate patches were sampled at different tidal stages. Consequently, each set of patches sampled later in the tide was exposed to settling meiofauna for a longer period of time. Because the time intervals overlap and the patches were randomly sampled, estimation of the mean number of recolonizing individuals arriving between tidal stages can be done by subtracting the mean abundance found in each tidal stage from the respective mean abundance found in the immediately succeeding tidal stage. Comparison of the resulting interval mean abundances assumes that emigration and immigration rates were constant throughout the entire sampling period. No evidence exists to support the assumption so conclusions based on these data must be viewed cautiously. To facilitate comparison of fauna1 abundances in the settlement traps to fauna1 densities in the recolonized sediment I recalculated the means of the former to derive numbers of individuals settling per cm2 and, for the latter, calculated the difference in means between succeeding tidal samplings (Table III). Surprisingly, the density of settling meiofauna, as estimated by the settlement traps, could account for the density of individuals recolonizing the defaunated sediment for virtually every taxon at each

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tidal stage despite large differences in time intervals for which the two different treatments were exposed (5 min for settlement traps for each sampling period versus 173, 167, 153, and 122 min for flood, high, ebb, and low tide intervals, respectively, for recolonized sediment). I also compared the proportional representation of the more abundant taxa between the trap, defaunated sediment, and ambient sediment samples. To increase the replica-

TABLE III Estimates oftotal settlement to the sediment surface. cm * and differences between tidal samplings in mean densities of meiofauna in the recolonized sediment. Settlement estimates are based on intervals of exposure of 5 min while the time between samplings of the recolonized sediment are each > 120 min. Taxon

Sample

Nematodes

Settlement Recolonized Settlement Recolonized Settlement Recolonized Settlement Recolonized

Turbellaria Ciliates Harpacticoid

copepods

Total fauna

source

Tidal stage

trap sediment trap sediment trap sediment trap sediment

Settlement trap Recolonized sediment

Flood

High

Ebb

Low

14.14 14.52 4.57 1.86 8.29 3.44 0.86 0.09 28.1 24.0

1.44 2.43 0.67 - 0.63 1.00 - 0.09 0.22 1.41 3.2 3.8

2.89 2.28 0.33 0.63 0.78 0.09 0.22 -0.81 4.3 2.9

2.78 3.33 1.11 1.08 0.44 0.09 0.44 0.39 5.0 6.4

TABLE IV Mean relative proportions ( + 1 SE) of individuals of the more abundant taxa in samples from settlement traps, recolonized sediment, and ambient sediment. Beneath the proportions are the results of comparisons between sample types using an approximate test of equality of means when the variances are heterogeneous (Sokal & Rohlf, 1981). NS - P> 0.05, * - P < 0.05, ** - P < 0.01. Sample

source

Settlement traps Recolonized sediment Ambient sediment Settlement traps vs. recolonized sed. Recolonized sed. vs. ambient sed. Settlement traps vs. ambient sed.

Taxon

(n = 34) (n = 37) (n = 10)

Nematodes

Turbellaria

0.46 (0.05) 0.67 (0.03) 0.69 (0.02)

0.16 (0.03) 0.08 (0.02) 0.06 (0.01)

0.19 (0.04) 0.14 (0.02) 0.19 (0.01)

**

NS

NS

NS

NS

**

**

NS

NS

Ciliates

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tion for this test the mean numbers of individual nematodes, turbellaria, and ciliates were pooled within each sample type across the entire transect and all tidal stages for calculation of the mean proportional abundances. The data were normalized by an arcsine transform (Sokal & Rohlf, 198 1) but homoscedascity of the variances associated with these means could not be achieved by further transformations. Consequently, the means of the proportions were compared using a test that does not assume variances to be similar (Sokal & Rohlf, 1981). The proportion of nematodes was significantly lower in the trap samples than either the defaunated or ambient sediment samples, both of which were similar (Table IV). The proportions of turbellaria did not differ in any comparison. The proportion of ciliates differed in only the comparison between recolonized and ambient sediment samples.

DISCUSSION

The highest abundances of most taxa were found in settlement traps during flood tide sampling when current speeds were greatest. This pattern suggests that, generally, above-sediment faunal abundances are related positively to ambient current speed. Palmer & Gust (1985) demonstrated the same relationship between advecting fauna and current flow (specifically, friction velocities). However, this relationship does not appear to be linear. Inspection of the mean faunal abundances and sediment volumes shows that ebb tide samples are more similar to high and low-tide samples than flood tide samples (Table I). For example, the proportions of mean sediment volume, relative to the mean flood tide value, are 0.24, 0.13, and 0.15, respectively. This is not surprising for the slack tide samples, when no current was present to erode and transport sediment, but the ebb tide mean current speed was 61% of the mean flood tide current speed. This implies that ebb tide current speed was below the critical erosion velocity (Dyer, 1980) for the majority of ambient sediments on the sandflat. Because faunal abundances follow a similar pattern across the tidal samplings, sediment critical erosion velocities may be one important factor determining susceptibility to erosion for most meiofauna. Laboratory studies have shown that the majority of meiofaunal individuals avoid the sediment surface when currents are present (Palmer, 1984; Fegley, 1985). Meiofauna also avoid the sediment surface when currents are present in the field (Fegley, 1987). Consequently, prior to the beginning of sediment erosion, little passive entrainment of meiofauna may be expected. Harpacticoid copepods did not conform to this general pattern. No significant differences were found in harpacticoid copepod abundances among settlement traps over the separate tidal stages. Beyond the possibility that the variances among the replicates were too great to detect significant differences, two, non-exclusive explanations could account for this absence of pattern. First, harpacticoid copepods are relatively strong swimmers (Palmer, 1986) and could have escaped the settlement traps during the .5-min exposure interval. The abundances observed in the settlement traps

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for this group probably result from both passive and active movements. Consequently, the pattern of settlement to the sediment surface for harpactoids could have been obscured by their active behavior. Second, some harpacticoid copepod species swim into the water column when water currents are absent (Walters & Bell, 1986). Sediment erosion does not appear to be a prerequisite for above-sediment movements for many harpacticoid species. Regressions of faunal abundances onto sediment volume collected were generally non-significant. Differences in both erodibility and settlement characteristics between fauna and sediments probably contribute to the low number of positive regressions. Regardless of the specific factors involved, sediment particles are not good models for predicting meiofaunal transport dynamics. Palmer & Gust (1985) reached a similar conclusion in their study comparing above-sediment abundances of meiofauna and suspended sediment. Comparison of mean abundances of taxa in the recolonized sediment at each tidal stage (not differences in abundance) demonstrated no significant differences in abundance for any group except harpacticoid copepods. For the majority of taxa, the amount of recolonization that occurred over the entire tide was completed by the first sampling. This corresponds to the time when the greatest numbers of potential recolonizers were available as indicated by settlement traps. In contrast, the highest mean abundance of harpacticoid copepods in recolonized sediments occurred at the high tide sampling, the lowest mean abundance at flood tide. The settlement traps demonstrated that harpacticoid copepods were present at the flood tide sampling. Once again, the majority of harpacticoid copepods apparently acted differently than most other benthic meiofauna. They recruited into defaunated sediments when current flows were absent; both high and low tide abundances of harpacticoid copepods were significantly higher than flood and ebb tide abundances. The pattern of higher benthic abundances of harpacticoid copepods occurring at slack and, especially, slack low tide has been seen before (Palmer & Brandt, 1981). Except for tardigrades and ostracods, faunal abundances of all taxa were significantly lower in recolonized sediment collected at slack low tide than in ambient sediment collected at the same time. Recovery of defaunated, sandy sediments to ambient densities typically takes several days to weeks (Scheibel & Rumohr, 1979; Alongi et al., 1983). The absence of a significant difference between tardigrade abundances in the recolonized and ambient sediments is surprising and probably results from high withinsource variance obscuring any between-source differences (the mean abundance of tardigrades in the recolonized sediments is 21 y0 of the mean ambient abundance). Slow crawling tardigrades are noted for their ability to adhere to sediment grains (Swedmark, 1964) and seem unlikely candidates for rapid dispersal and recolonization. Alternatively, ostracods move rapidly (Fegley, 1985). Their mean abundance in the recolonized sediment was 82% of the mean ambient sediment abundance. This far exceeded the levels of recolonization observed for any other taxon (nematodes, 20% ; turbellarians, 30%; ciliates, 12%; harpacticoid copepods, 32%). Either a large proportion of the

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ostracod population participates in rapid dispersal over a tide or a few ostracod species specialize in exploiting disturbed patches of sediment. Ostracods are often ignored in meiobenthic studies because their relative abundance is generally low (Hulings & Gray, 1971), but the results of this study imply that they may be very active in soft-substratum benthic dynamics. Determining whether recolonizing individuals arrived via within- or above-sediment pathways is not possible because the defaunated sediments were exposed to both settlement of drifting meiofauna as well as lateral immigration of meiofauna from surrounding ambient sediments. Chandler & Fleeger (1983) demonstrated that both above- and within-sediment pathways are used by nematodes and harpacticoid copepods that recolonize muddy sediments. Both pathways are probably important in sandy habitats as well but the evidence in the present study is conflicting. The relative proportions of the abundant taxa in the recolonized sediment were more similar to the relative proportions of the same taxa in ambient sediments than in settlement traps, suggesting infaunal migration was the primary source of recolonizing individuals (Table IV). However, examination of the differences in faunaI abundances between tidal stages within the recolonized sediment reveals that the pattern of when recolonizing individuals arrived corresponds closely to the pattern of faunal abundances observed in settlement traps (except for harpacticoid copepods): the highest numbers of individuals recolonized the defaunated sediment by the flood tide sampling with little difference among the numbers of individuals arriving among the three subsequent tidal stages (Table III). How does the comparison of abundances of recolonizing individuals compare to the estimated abundances of potential settling meiofauna? Despite a minimum 24-fold greater time of exposure of the defaunated sediment over the settlement traps, the density of settling meiofauna was virtually the same as the incremental increases of meiofaunal densities in the recolonized sediments. The disparity between potential and actual numbers of recolonizing individuals is even greater if any portion of the recolonizing individuals arrived via the within-sediment pathway. Either settlement traps grossly overestimate the number of potential recolonizing individuals by overtrapping or the majority of the meiofauna dispersing above the sediments could not or elected not to recruit into defaunated sediment. Selection and placement of settlement traps were made with great care to avoid overtrapping settling meiofauna. Most investigators using cylindrical settlement traps regard them as fairly accurate estimators of actual fluxes. Only Gardner (1980b) found evidence that cylindrical traps can overtrap in the presence of rapid flows. Closer inspection of the results of the present study reveals several reasons why overtrapping bias is an unlikely explanation for the observed, large differences between potential and actual recruitment densities. First, the over-trapping Gardner (1980b) found occurred primarily with particles < 64 pm in diameter, which is much smaller than the majority of meiofauna I collected. Second, assuming that settlement traps did overtrap to the maximum amount found in Gardner’s study (two to three times greater than actual flux),

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correction for this bias in the estimated number of potential recolonizing individuals settling from the water column still results in a large disparity between estimated numbers of settling meiofauna and numbers of recolonizing individuals. Third, Gardner (1980b) found that cylindrical traps, regardless of aspect ratio, were near perfect estimators of natural fluxes when used in the absence of current flow. Inspection of the high and low tide samples, which were taken when currents were absent, demonstrated that the amount of actual recolonization is still much lower than settlement trap estimates indicate they would be if just settling onto substrate were the only important factor. Although settlement traps may overestimate the actual number of potential recruits, especially when current flow is present, the large difference between that estimate and the observed amount of recolonization demonstrates that many meiofauna failed to recolonize the defaunated sediments. It is not clear why recolonization into the defaunated sediment was inefficient. I can suggest two possibilities, both related to the condition of the sediment used. Ambient sediments are usually bound by mucous exopolymers secreted from infauna (Fazio et al., 1982) that reduce the erodibility of the sediment (Jumars & Nowell, 1984). The defaunated sediment used in this study had been cleaned and dried, undoubtedly removing much of the binding material. Consequently, surticial sediment in the patches of defaunated sediment may have been more unstable in the presence of flow than ambient sediments making it more difficult for settling meiofauna to maintain position on the sediment surface long enough to burrow more deeply. The defaunated sediment may also have been chemically unattractive to the meiofauna. Meiofauna have been observed to aggregate in areas where potential food, presumably bacteria and diatoms, is concentrated (Gerlach, 1977; Hogue & Miller, 1981). The clean sediment used in this study may not have illicited any burrowing responses by the meiofauna because of an absence of microflora. Recolonization studies conducted in sandy sediments that are naturally disturbed (such as enteropneust fecal mounds and ray pits; Thistle, 1980; Reidenauer & Thistle, 1981; Sherman et al., 1983) demonstrate recovery to ambient densities at rates considerably faster (days vs. weeks) than those observed in studies that use artificially defaunated sediments (Scheibel & Rumohr, 1979; Alongi et al., 1983). Alongi (1985) found that natural disturbances can reduce meiofaunal densities without significantly altering bacterial and diatom densities. Rates of meiofaunal recolonization into artificially defaunated sediments could rely more on microbial recolonization and population dynamics than meiofaunal dispersal rates. To summarize, meiofauna can influence their own dispersal rate and fate. Several studies have shown that many meiofaunal species actively change the probability of their entrainment by flow (Rieger & Ott, 1971; Palmer, 1984; Palmer & Malloy, 1986; Fegley, 1987) and the present study indicates that the return of drifting meiofauna to the sediments may be complicated by active behavior as well. These observations suggest that diurnal, seasonal, and life-history events that alter drift-associated behavior would produce different patterns of meiofaunal dispersal and recolonization. Conclusions about the degree of importance of meiofaunal drift in any habitat should explicitly

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describe the temporal regime under which sampling occurred. In addition, future studies would profit by separating the observed meiofaunal species into functional groupings. Most of the harpacticoid copepods collected in the study, though not identified to species, exhibited body characteristics typical of epibenthic species (Hicks & Coull, 1983). The different patterns of appearance of the harpacticoid copepods in the recolonized sediments and settlement traps were more likely a characteristic of an epibenthic mode of life than taxonomic affinity.

ACKNOWLEDGEMENTS

I gratefully thank the staff and faculty of the UNC Institute of Marine Sciences for their assistance and use of facilities. Equipment was purchased using Lerner-Gray and Sigma Xi grants-in-aid. Financial support during the study was supplied by N.C. Sea Grant 04-7-158-44121 awarded to C. H. Peterson and during manuscript preparation by the N.J. Fish. and Aquac. Tech. Ext. Center. I thank B. J. Barber, D. R. Colby, M. E. Hay, C. E. Jenner, J. Kohlmeyer, B.A. MacDonald, C. H. Peterson, R. M. Rieger, S. Vogel, T. G. Wolcott and two anonymous reviewers for their comments on the manuscript and H. and V. Page for figure preparation. This study was part of a dissertation submitted to the Biology Department, University of North Carolina at Chapel Hill.

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