differential dispersal rates in an intertidal meiofauna assemblage

Differential dispersal rates in an intertidal

meiofauna assemblage

John A. Commito a,b,*, Guglielmo Tita c,1

aEnvironmental Studies Department and Biology Department, Gettysburg College, Gettysburg, PA 17325, USAbDipartimento di Scienze dell’Uomo e dell’Ambiente, Universita di Pisa, Via Volta 6, I-56126 Pisa, ItalycUniversite du Quebec a Rimouski, Institut des Sciences de la Mer, 310 allee des Ursulines, Rimouski,

Quebec, Canada G5L 3A1

Received 25 May 2001; received in revised form 20 November 2001; accepted 27 November 2001

Abstract

Meiofaunal nematodes and copepods disperse passively with sediment bedload, and copepods

also display active emergence and reentry behavior. Epigrowth-feeders may be the nematode feeding

group most susceptible to passive transport because they live closest to the sediment surface. We

used bottom traps at a nematode-dominated intertidal mudflat in Maine, USA, to test the hypotheses

that (1) meiofauna taxa disperse in relative proportions different from those of the ambient

community; (2) copepods have the highest relative dispersal rate (number of individuals trap� 1

day� 1 ambient individual � 1) and are not as tightly linked as other taxa to sediment flux; and (3)

epigrowth-feeders have the highest nematode relative dispersal rate. Results supported all three

hypotheses. Nematodes accounted for 95.8% of the individuals in cores, but only 38.9% of the

individuals in traps. Copepods accounted for 1.5% of the individuals in cores, but 56.7% of the

individuals in traps. Less common taxa also had different relative proportions in cores and traps, as

did nematode feeding groups and individual species. The relative dispersal rate was far higher for

copepods than for any other taxonomic group, and the absolute (number of individuals trap� 1

day� 1) and bulk (number of individuals g sediment � 1 trap� 1 day� 1) dispersal rates for copepods

were equal to those of the 65-fold more abundant nematodes and higher than those for all other taxa.

The non-selective deposit-feeders were the most abundant nematode feeding group in the ambient

community, but the epigrowth-feeders as a group and as individual species had the highest absolute,

relative, and bulk dispersal rates. Non-metric multidimensional scaling (MDS) using analysis of

similarity (ANOSIM) and species similarity percentages (SIMPER) reflected these differences

between ambient and dispersing nematode assemblages. Significant positive regression relationships

between sediment weight and the number of individuals captured in traps for nematodes and some

PII: S0022-0981 (01 )00386 -0

* Corresponding author. Environmental Studies Program and Biology Department, Gettysburg College,

Gettysburg, PA 17325, USA. Tel.: +1-717-337-6030; fax: +1-717-337-6666.

E-mail address: jcommito@gettysburg.edu (J.A. Commito).1 Present address: Department of Biology, Louisiana State University, Baton Rouge, LA 70803, USA.

www.elsevier.com/locate/jembe

Journal of Experimental Marine Biology and Ecology

268 (2002) 237–256

other taxa indicated that they moved passively in the bedload. Lack of a significant regression

relationship for copepods suggested an active behavioral component to dispersal. Meiofauna

populations in this soft-bottom community were highly dynamic, demonstrating that the role of

dispersal must be included in any consideration of the ecology of soft-bottom systems at local and

Keywords: Bedload transport; Benthic copepods; Maine; Meiofauna dispersal; Nematode feeding groups;

Sediment flux

1. Introduction

Marine benthic populations are notoriously variable in space and time. Recent advances

in spatial ecology have revealed the importance of local dispersal in regulating population

dynamics and persistence (Levin and Pacala, 1998; Hanski, 1999; Hiebeler, 2000). Yet

much of our attention in benthic ecology has been focused on dispersal at large spatial

scales, often of planktonic larval stages in hard-bottom systems where adults are per-

manently attached to the substrate (Roughgarden et al., 1987; Underwood and Fairweather,

1989; Reed et al., 2000). For these organisms, postlarval dislodgement and transport often

means death. Soft-bottom systems differ from hard-bottoms in a crucial way: organisms are

not permanently attached to the substrate, so postlarval transport can provide infauna with

opportunities for repeated, local dispersal events.

Comparatively few soft-bottom studies have been conducted of juvenile and adult dis-

persal at local scales such as (in decreasing order) the individual sandflat, seagrass bed,

nekton fall, foraging pit, crab burrow, fecal mound, or sediment bite (see references in

Palmer, 1988a; Palmer et al., 1996; Armonies, 1994; Commito et al., 1995a,b; Fleeger et al.,

1995; Turner et al., 1997; Thrush et al., 2000). Because planktonic larvae are absent in

meiofauna, the active and passive transport of juveniles and adults might be crucially im-

portant to their population performance. Harpacticoid copepods tend to reside near the se-

diment surface, and many species are good swimmers (Palmer, 1988a; Fleeger et al., 1995).

They can move passively with eroded sediments when currents are rapid. They also respond

actively to flow velocity and biogenic structures like burrows that alter flow, often emerging

from the sediment in great numbers when water velocity is slowest (Service and Bell, 1987;

Palmer, 1988a; Fegley, 1988; DePatra and Levin, 1989; Armonies, 1994; Sun and Fleeger,

1994; Fleeger et al., 1995). On the other hand, nematodes, which often dominate the

meiofauna of unvegetated mudflats and sandflats, generally show little evidence of active

dispersal (Jensen, 1981; Palmer, 1988a) and probably move primarily by passive transport

in the bedload and water column (Palmer, 1988a; Fegley, 1988; DePatra and Levin, 1989;

Armonies, 1994; Sun and Fleeger, 1994). The nematodes closest to the sediment–water

interface may be most susceptible to erosion and transport (Warwick and Gee, 1984; Eskin

and Palmer, 1985), especially epigrowth-feeders (feeding group 2A, sensu Wieser, 1953)

like Ptycholaimellus ponticus that live in surficial sediment because they rely primarily on a

photosynthesizing food source. Data on other meiofauna taxa are rare and suggest that their

dispersal mode is also largely passive (Palmer, 1988a, Fegley, 1988; Armonies, 1994).

In this study of a nematode-dominated intertidal mudflat, we used bottom traps to

measure sediment flux and three key rates of dispersal for each major taxonomic group of

J.A. Commito, G. Tita / J. Exp. Mar. Biol. Ecol. 268 (2002) 237–256238

the meiofauna community, each nematode feeding group, and each nematode down to the

level of the genus or species. We tested the following hypotheses:

H1. Meiofauna taxonomic groups disperse in relative proportions different from those of

the ambient community.

H2. Copepods have the highest relative dispersal rate (number of individuals trap� 1

day � 1 ambient individual� 1) and are not as tightly linked as other taxa to sediment flux.

H3. Epigrowth-feeders have the highest nematode relative dispersal rate.

2. Materials and methods

2.1. Study site

The study site was Bob’s Cove, an intertidal mudflat in Jonesboro, Washington County,

ME, USA (44�33VN: 67�35VW). The 500-m wide� 600-m long cove is sheltered from the

wind on the north, east, and west sides by steeply rising, heavily forested land, and on the

south by a narrow (300 m) mouth protected by islands and nearby landmasses. Consistent

with this protected basin configuration, the sediment has a smooth, unrippled surface,

indicative of a low-energy environment with slow wind- and tide-generated water currents.

Sediment characteristics were qualitatively assessed and are similar to those at nearby Flake

Point Bar, where mean FF s= 4.87F 0.02 (Beal et al., 2001). Air temperature ranges from

� 35 to 35 �C, and water temperature from 0 to 10 �C. In the summer, shallow water

moving onto or off the flat can reach 21 �C. Salinity is usually about 30 PSU. See Commito

(1982) and references in Commito and Rusignuolo (2000) for additional information on the

ecology of this site.

2.2. Field procedures

At low tide on 10 July, 1994, a grid with 75 positions was established in the mid-intertidal

zone. The grid was a rectangle with sampling positions every 0.75 m along each row (seven

rows with 10 positions each and one row with five positions), with rows 2 m apart. To

determine the structure of the ambient community, cores were taken to a depth of 15.0 cm at

10 randomly chosen grid positions with a 15.0 cm tall�1.3 cm internal diameter, plastic,

cylindrical test tube (cross-sectional area = 1.33 cm2) from which the bottom had been

removed.

To determine the transport of sediment and meiofauna, bottom traps were installed at 10

other randomly chosen grid positions. Traps similar to those of Fegley (1988) consisted of

seawater-filled test tubes of the same dimensions as the coring device (aspect ratio, height/

diameter = 11.5:1) inserted flush with the sediment surface into holes created by carefully

extracting sediment with the same coring device. Traps were left in place overnight and

removed at low tide 1 day later. Core and trap contents were stained with rose bengal and

preserved in 4% buffered formalin.

Deployment occurred during the July spring tide with the lower amplitude. Winds in

the region were light and variable when water covered the site during the trap deployment

J.A. Commito, G. Tita / J. Exp. Mar. Biol. Ecol. 268 (2002) 237–256 239

period, coming from the 190�–280� sector (the protected western and southwestern side ofthe cove), with hourly means of 1.3–7.3 m s� 1 and peak gusts of 7.5 m s� 1 (National

Oceanic and Atmospheric Administration, Mount Desert Rock C-MAN station data).

These wind speeds were typical for July, the month with the lowest 10-year-average and

gust velocities. Thus, tidal- and wind-generated currents were moderate compared to other

dates with more extreme tidal variation and wind speeds.

2.3. Laboratory procedures

2.3.1. Core samples

In the laboratory, meiofauna were extracted from cores following McIntyre and

Warwick (1984) and Heip et al. (1985). Each core sample was rinsed over 1 mm and

63 mm mesh sieves. The contents on the 63 mm sieve were centrifuged at 1800� g in water

for 10 min. The supernatant was passed through a 63-mm sieve, and Ludoxk (specific

gravity = 1.15) was added to the residue in the centrifugation tube to at least five times the

volume of sediment. Three times for each sample, the sediment was brought to suspension

by gentle mixing and centrifuged at 1800� g for 10 min, and the supernatant was passed

through a 63-mm sieve. The final material was rinsed with tap water to prevent flocculation

of Ludox and preserved in 4% formalin. The meiofauna were transferred into a gridded

Petri dish, and the individuals of each taxonomic group were counted. For nematodes, a

subsample of 100 randomly sorted individuals were transferred into 10% glycerol solution

for species identification. When the water evaporated, the nematodes were transferred to a

glycerol drop in a ring of paraffin on a glass slide, each of which was covered with a glass

cover slip, warmed, and then cooled. Nematode species were identified using the keys of

Platt and Warwick (1983, 1988). Their frequencies were multiplied by the total number of

nematode individuals in the sample to provide an estimate of the number of individuals

of each species in the sample. Individuals were assigned to feeding groups according to

Wieser (1953), where 1A= selective deposit-feeders, 1B = non-selective deposit-feeders,

2A= epigrowth-feeders, and 2B = omnivores-carnivores.

2.3.2. Trap samples

Trap samples contained less sediment and fewer organisms than core samples, so meio-

fauna extraction was simplified. The samples were rinsed with distilled water over 1 mm and

63 mm sieves, the contents on the 63 mm sieve were transferred to a gridded Petri dish, and

counting and identification were carried out as for the core samples. In addition, sediment

was dried at 60 �C and weighed, except for the sediment lost from one trap due to an accident

in the laboratory. That trap could not be included in calculations involving sediment weight.

2.3.3. Dispersal rates

Dispersal rates were calculated in three ways:

Absolute dispersal rate = number of individuals trap � 1 day � 1 = the number of

individuals per trap captured during 1 day of deployment.

Relative dispersal rate = number of individuals trap � 1 day � 1 ambient individu-

al � 1 = the number of individuals per trap captured during 1 day of deployment, divided

by the mean ambient density for that meiofauna taxon, nematode feeding group, or ne-

matode species. For some rare species at our study site, the denominator was equal to or

close to zero, resulting in a relative dispersal rate tending to infinity. To avoid this problem,

relative dispersal rates were calculated for the 10 species that were most abundant in traps,

all of which had mean values >4 individuals trap � 1 and >40 individuals core� 1.

Bulk dispersal rate = number of individuals g sediment � 1 trap� 1 day � 1 = the number

of individuals per trap captured during 1 day of deployment, divided by the sediment

weight captured in the same trap.

2.3.4. Statistical analysis

To examine differential movement into traps, the relative proportions of meiofauna

taxonomic groups, nematode feeding groups, and the five top-ranked nematode species in

ambient community cores were compared to those in bottom traps using the chi-square test.

For the nematodes, differences in species composition between the traps and cores were

examined further by means of non-metric multidimensional scaling ordination (MDS) with

different degrees of data transformation. Untransformed data analysis is more sensitive to

changes in abundant species, while increasingly severe square root transformations are more

sensitive to changes in abundance of those that are increasingly rare (Clarke and Warwick,

1994). The differences between core and trap assemblages were investigated with a one-way

analysis of similarity using the ANOSIM routine (PRIMER v4; Clarke andWarwick, 1994).

The contribution of each species to the average sample dissimilarity between cores and traps

was determined using the SIMPER routine (PRIMER v4; Clarke and Warwick, 1994).

Densities and dispersal rates were compared across taxonomic groups, nematode feeding

groups, and the five top-ranked nematode species with the non-parametric Kruskal–Wallis

test followed by the Student–Newman–Keuls procedure. To determine how closely animal

transport was related to sediment flux, regressions were run on the number of individuals

per trap versus sediment weight per trap. The regression results were used as a crude mea-

sure of dispersal mode, based on the assumption that correlations are stronger for passive

dispersers than for those with active dispersal behavior (Turner et al., 1997).

3. Results

3.1. Ambient community

Nematodes dominated the ambient community, accounting for 95.8% of the individuals

in cores (Fig. 1). Copepods, primarily harpacticoids, were the second most abundant

taxonomic group, followed by kinorhynchs, acari, oligochaetes, polychaetes, ostracods,

sipunculids, and tardigrades, in decreasing order (Fig. 1).

Non-selective deposit-feeders (feeding group 1B) were the most abundant nematode

feeding group, accounting for 39.4% of the nematode individuals in cores (Fig. 2).

Epigrowth-feeders (2A) were the second most abundant feeding group, followed by many

fewer selective deposit-feeders (1A) and omnivores/carnivores (2B) (Fig. 2). Non-selective

deposit-feeders were also the most species-rich feeding group, comprising 39.1% of the

nematode species in cores, followed by epigrowth-feeders (21.7%), omnivores/carnivores

(21.7%), and selective deposit-feeders (17.4%).

Of the 52 nematode species identified in this study, 46 were found in the ambient

community (Appendix A), with a mean of 20.20F 0.73 (SE) species core � 1. Of these 46

species, 16 were never found in traps (Appendix A). The non-selective deposit-feeder

Daptonema sp. had the highest mean density of the five top-ranked species in the ambient

community (df = 4, H = 13.29, P= 0.01; Student–Newman–Keuls P < 0.05), accounting

for 18.3% of the nematodes in cores (Table 1). The densities of the other four top-ranked

species were not significantly different from each other. They were the epigrowth-feeder

Chromadora macrolaima, followed by the omnivore/carnivore Metachromadora sp. 2, the

selective deposit-feeder Terschellingia sp., and the epigrowth-feeder P. ponticus. Thus,

every feeding group was represented among the five top-ranked species.

3.2. Dispersal rates

3.2.1. Absolute dispersal rates

The composition of the meiofauna assemblage captured in the traps was markedly

different from that of the ambient community (Fig. 1). Relative proportions of the

meiofauna groups at higher taxonomic levels were significantly different between cores

and traps whether all the groups were considered individually (df = 7, v2 = 118.80,P < 0.001) or the uncommon taxa were lumped into one group for comparison with ne-

matodes and copepods (df = 2, v2 = 9832.8, P < 0.001).

Copepods and nematodes had far higher absolute dispersal rates than any of the other

taxa (Fig. 1). Copepods accounted for 56.7% of the individuals in traps, despite rep-

resenting only 1.5% of the ambient community density. Nematodes were the second most

abundant taxonomic group in traps, although not significantly different from the copepods.

In decreasing order, there were small numbers of kinorhynchs, acari, ostracods, poly-

chaetes, oligochaetes, and isopods, with no sipunculids or tardigrades.

The relative proportions of the number of individuals of each nematode feeding group

in traps were significantly different from those in cores (df = 3, v2 = 404.1, P < 0.001; Fig.

2). Epigrowth-feeders had the highest absolute dispersal rate of the nematode feeding

groups, accounting for 53.2% of the nematode individuals in traps, compared to 30.2% in

the ambient community cores (Fig. 2). The rate for epigrowth-feeders was twice that of the

non-selective deposit-feeders, which was twice that of the selective deposit-feeders and

omnivores/carnivores.

There was no change between cores and traps in the relative proportions of the number

of species in each feeding group (df = 3, v2 = 1.015, P= 0.798). Non-selective deposit-

feeders were still the most species-rich feeding group, comprising 38.9% of the nematode

species in traps, followed by epigrowth-feeders (25.0%), selective deposit-feeders

(22.2%), and omnivores/carnivores (13.9%).

Fig. 1. Ambient community densities and dispersal rates (meanF SE) for major meiofauna taxonomic groups at

Bob’s Cove, ME. Note vertical axis breaks in three of the four graphs. For ease of comparison, same sequence of

taxa is presented along the horizontal axis of each graph. Nem=nematodes; Cop = copepods; Kino = kinorhynchs;

Acar = acari; Oligo = oligochaetes; Poly = polychaetes; Ostr = ostracods; Sipu = sipunculids; Tard = tardigrades;

Isop = isopods. 0 = no animals; und = undefined because of 0 value in denominator of rate calculation. Underlined

groups denote no significant difference (Student–Newman–Keuls P< 0.05 following Kruskal–Wallis tests).

Of the 52 nematode species identified in this study, 36 were found in traps (Appendix A),

with a mean of 17.30F 1.46 (SE) species trap� 1. Of these 36 species, six were never found

in cores (Appendix A). There were no significant differences in absolute dispersal rates

among the five top-ranked species (df = 4, H = 3.77, P= 0.44). The first-ranked species was

the epigrowth-feeder C. macrolaima, accounting for 16.1% of the nematodes in traps (Table

1). The others were the epigrowth-feeder Parachromadorita sp., followed by the epigrowth-

feeder P. ponticus, the non-selective deposit-feeder Daptonema sp., and the epigrowth-

feeder Chromadorina sp. Thus, only two feeding groups were represented among the five

top-ranked species, with epigrowth-feeders accounting for 88.6% of those individuals.

The relative proportions of the five most abundant nematode species in cores were signi-

ficantly different from those in traps (df = 4, v2=378.2, P < 0.001). There was a clear pattern

in this species composition shift between the ambient community and the dispersers. Of the

Table 1

Five top-ranking nematode species for ambient community density and three dispersal rates

Rank Species Feeding group MeanF SE

Ambient community density (no. individuals core – 1)

1 Daptonema sp. 1B 300.83F 45.66

2 Chromadora macrolaima 2A 258.69F 69.41

3 Metachromadora sp. 2 2B 161.35F 60.66

4 Terschellingia sp. 1A 98.38F 17.85

5 Ptycholaimellus ponticus 2A 97.58F 28.99

Absolute dispersal rate (no. individuals trap � 1 day� 1)

2 Parachromadorita sp. 2A 24.90F 8.87

5 Chromadorina sp. 2A 12.96F 3.40

Relative dispersal rate (no. individuals trap� 1 day� 1 ambient individuals� 1)

4 Axonolaimus sp. 1B 0.18F 0.05

5 Leptolaimus elegans 1A 0.17F 0.05

Bulk dispersal rate (no. individuals g sediment� 1 trap� 1 day� 1)

Fig. 2. Ambient community densities and dispersal rates (meanF SE) for nematode feeding groups (sensu Wieser,

1953) at Bob’s Cove, ME. For ease of comparison, the same sequence of feeding groups is presented along the

horizontal axis of each graph. 1B= non-selective deposit-feeders; 2A= epigrowth-feeders; 1A= selective deposit-

feeders; 2B = carnivores–omnivores. Underlined groups denote no significant difference (Student–Newman–

Keuls P< 0.05 following Kruskal–Wallis tests).

top five species in the ambient community, the species ranked number one (Daptonema sp.),

three (Metachromadora sp.), and four (Terschellingia sp.) dropped to fourth, ninth, and

twentieth, respectively, in the traps (Appendix A, Table 1). The species ranked number two

(C. macrolaima) and five (P. ponticus) in the ambient community rose to first and third,

respectively, in the traps. Thus, the two epigrowth-feeders rose in the rankings, and the three

species from the other three feeding groups all dropped in the rankings. At the same time, the

two new species to appear among the top five in traps were both epigrowth-feeders (second-

ranked Parachromadorita sp., up from number 11; fifth-ranked Chromadorina sp., up from

number 12). The only species other than an epigrowth-feeder to be found in the top five

dispersers was Daptonema sp., the most abundant nematode species in the ambient

community.

Multidimensional scaling ANOSIM results on untransformed data revealed significant

differences between the ambient and dispersing nematode assemblages (R = 0.813, P <

0.00, stress = 0.04; Fig. 3). Clustering was similar over a range of data transformation levels

(square root: R = 0.789, P < 0.00, stress = 0.07; fourth root: R = 0.703, P < 0.00, stress =

0.11), so only the untransformed data results are presented here. One trap was an outlier

(Fig. 3). This trap was far different from the others, having one-tenth as many individuals

and one-half as many species as the trap means.

The SIMPER results demonstrated average similarities of 46.9% between cores and

43.4% between traps. The Bray–Curtis dissimilarity was 82.5% between cores and traps.

The species contributing most to the dissimilarity term were Daptonema sp. (21.25%), C.

macrolaima (13.34%), Metachromadora sp. 2 (8.04%), Terschellingia sp. (7.74%),

Fig. 3. Multidimensional scaling (MDS) plot of nematode species assemblages at Bob’s Cove, ME, based on

numbers of individuals per sample. Cores = ambient community core samples (N= 10). Traps = bottom trap samples

(N = 10).

Sabatieria sp. 1 (5.47%), and P. ponticus (5.15%), with other species having values less

than 5%. These results from the multivariate analysis are in close agreement with those

from the univariate analyses. They reflect the drop of Daptonema sp. and other non-epi-

growth-feeders from high ranks in the cores to lower ranks in the traps, while epigrowth-

feeding species like C. macrolaima and P. ponticus rose in the trap rankings.

Traps contained a mean of 0.92F 0.11 g (SE) sediment trap�1. Regression analysis de-

monstrated a significant positive relationship between sediment weight trap � 1 and number

of individuals trap � 1 for the meiofauna as a whole (Table 2). For taxonomic groups, this

relationship was significant for nematodes, acari, kinorhynchs, ostracods, and the uncom-

mon taxa lumped into one group, but not for copepods and the other taxa. For the nematode

feeding groups, the relationship was significant for selective deposit-feeders, non-selective

deposit-feeders, and epigrowth-feeders. At the nematode species level, it was significant for

five species, including two epigrowth-feeders (Actinonema sp. and P. ponticus) and three

selective deposit-feeders (Desmoscolex falcatus,Halalaimus sp., and Leptolaimus elegans).

3.2.2. Relative dispersal rates

The relative dispersal rate normalized dispersal to a per capita rate by dividing absolute

dispersal by the ambient density. Copepods had a relative dispersal rate of 10.09F 2.18

Table 2

Regression parameters for number of individuals per trap ( y variable) versus sediment weight collected per trap (x

variable)

Y y-intercept Slope r2 P

Total meiofauna � 229.76 712.75 0.54 0.02

Copepods � 147.20 409.03 0.26 0.16

Nematodes � 64.83 262.72 0.63 0.01

By feeding groups

1A Selective deposit-feeders � 7.60 31.59 0.59 0.02

1B Non-selective deposit-feeders � 14.36 63.47 0.57 0.02

2A Epigrowth feeders � 42.34 150.48 0.61 0.01

2B Predators and omnivores 4.73 9.81 0.23 0.19

By species

Actinonema sp. � 2.99 4.10 0.46 0.04

Desmoscolex falcatus � 2.80 5.30 0.66 0.01

Halalaimus sp. � 4.05 6.16 0.46 0.04

Leptolaimus elegans � 7.43 16.68 0.54 0.03

Ptycholaimellus ponticus � 42.84 75.43 0.76 0.00

Other taxa combined � 17.44 40.81 0.86 0.00

By taxonomic group

Acari � 8.53 16.59 0.74 0.00

Isopods � 0.34 0.49 0.24 0.18

Kinorhynchs � 3.45 12.40 0.57 0.02

Oligochaetes � 0.36 0.76 0.13 0.34

Ostracods � 4.62 9.95 0.83 0.00

Polychaetes � 0.43 0.82 0.31 0.12

Sipunculids NA

Tardigrades NA

To save space, only significant ( PV 0.05) regressions are presented for individual nematode species. NA= none in

traps.

individuals trap � 1 day � 1 ambient individual � 1, meaning that traps held 10 times more

copepods than did ambient community cores with the same cross-sectional area (Fig. 1).

This relative dispersal rate was the highest by far among the taxonomic groups. It was

more than twice as high as the rate for the second highest taxonomic group, the ostracods,

and nearly two orders of magnitude higher than the rate for nematodes.

Epigrowth-feeders had the highest relative dispersal rate of the nematode feeding

groups, more than twice the rate of any other feeding group (Fig. 2).

There were no significant differences in relative dispersal rates among the five top-ranked

nematode species (df = 4, H = 8.46, P= 0.08). The first three were the epigrowth-feeders

Parachromadorita sp., Chromadorina sp., and P. ponticus, followed by the non-selective

deposit-feeder Axonolaimus sp. and the selective deposit-feeder L. elegans (Table 1).

3.2.3. Bulk dispersal rates

The bulk dispersal rate measured dispersal per unit of bedload sediment by dividing the

number of animals in each trap by the weight of the sediment captured in that trap. Cope-

pods and nematodes had much higher bulk dispersal rates than did the other taxonomic

groups (Fig. 1). Although the rate for copepods was higher than that of the nematodes, the

difference was not statistically significant.

Epigrowth-feeders had the highest bulk dispersal rate of the nematode feeding groups,

more than twice the rate of the next highest group, the non-selective deposit-feeders,

which had rates twice as high as the other two groups (Fig. 2).

There were no significant differences in bulk dispersal rates among the five top-ranked

species (df = 4, H = 3.71, P= 0.45). Four were the epigrowth-feeders C. macrolaima,

Parachromadorita sp., P. ponticus, and Chromadorina sp. (Table 1). The other was the

non-selective deposit-feeder Daptonema sp., the most abundant species in the ambient

community.

4. Discussion

The results of this field investigation provided strong support for our hypotheses

concerning meiofauna dispersal dynamics in an unvegetated intertidal mudflat community.

First, comparison of traps with cores supported the hypothesis (H1) that meiofauna taxa

disperse in relative proportions different from those of the ambient community. Nematodes

accounted for 95.8% of the individuals in cores, but only 38.9% of the individuals in traps.

Copepods accounted for only 1.5% of the individuals in cores, but 56.7% of the individuals

in traps. The less common taxa also had different relative proportions in cores and traps.

The same was true for nematode feeding groups and individual species, with additional

evidence provided by the multivariate MDS analysis.

Second, the results supported the hypothesis (H2) that copepods have the highest

relative dispersal rates and are not as tightly linked to sediment flux as other taxa. The

relative dispersal rate was far higher for copepods than for any other taxonomic group.

Copepods also had absolute and bulk dispersal rates that were higher than those for all taxa

except nematodes. These two rates were not significantly different from those for

nematodes, despite the fact that nematodes were 65-fold more abundant in the ambient

community. There was a significant regression relationship between sediment weight and

the number of individuals captured in traps for nematodes and some other taxa, indicating

that they moved passively in the bedload. But for copepods, there was no significant

relationship, consistent with the idea that there is an active component to their dispersal.

Third, the results supported the hypothesis (H3) that epigrowth-feeders have the highest

nematode relative dispersal rates. The non-selective deposit-feeders were the most

abundant feeding group in the ambient community. However, the epigrowth-feeders were

the feeding group with the highest absolute, relative, and bulk dispersal rates. The non-

selective deposit-feeder Daptonema sp. was the most common species in the ambient

community. But the first, second, and third positions in the species rankings for each of the

three dispersal rates were held by C. macrolaima, Chromadorina sp., Parachromadorita

sp., or P. ponticus, all epigrowth-feeders.

4.1. Nematode and copepod dispersal in soft-bottom systems

Our regression results linking nematodes to sediment flux indicate that passive

transport is the primary means by which nematodes disperse. Although there are reports

of swimming activity in some nematodes (Jensen, 1981; Palmer, 1988a), our results are

consistent with those from studies that found nematodes in the water column and bottom

traps after being eroded from the sediment (Fegley, 1988; Palmer, 1988a; DePatra and

Levin, 1989; Armonies, 1994; Sun and Fleeger, 1994). The epigrowth-feeder genera

Ptycholaimellus, Metachromadora, and Chromadora are known to live near the sediment

surface and have been collected in the water column and in resuspended sediment directly

above the bottom (Bell and Sherman, 1980; Eskin and Palmer, 1985). Eskin and Palmer

(1985) suggested that species in these genera live close to the sediment surface and are

more susceptible to erosion and transport than deeply dwelling species. Warwick and Gee

(1984) made the same argument specifically for P. ponticus. Our results on these genera

and species clearly support their predictions.

Our results are consistent with those of other workers who found that copepods possess

a suite of active emergence and reentry behaviors (see references in Palmer, 1988a;

Armonies, 1994; Fleeger et al., 1995). Fleeger et al. (1995) showed quite clearly in their

flume study that copepods made active sediment reentry choices under no flow conditions,

but behaved like passive particles under conditions of flow. Our traps were deployed over

the full tidal cycle, during which time they integrated a variety of flow conditions, possibly

capturing copepods moving both actively and passively.

The origins and distances traveled by the meiofauna captured in our traps are unknown.

Phytal-dwelling copepods dispersed 20 m away from natural seagrass beds in 1 day

(Kurdziel and Bell, 1992). Artificial substrate collectors designed to mimic filamentous

microalgae and aufwuchs-bearing hard surfaces can rapidly collect meiofauna, especially

copepods, from nearby sources (Atilla and Fleeger, 2000). The nearest habitats of these

types (short, sparse eelgrass, Zostera marina; patches of Chaetomorpha spp. and Enter-

omorpha spp., and rock ledges) were at least 150 m away from our sampling grid. Recent

measurements of dispersal of postlarval bivalves < 1 mm long found a median dispersal

distance of 49.0 cm after 12 h (Norkko et al., 2001), consistent with the view that meiofauna

in our traps after 1 day of deployment probably came from the local mudflat habitat.

Fegley (1988) sampled meiofauna dispersal over an intertidal flat of well-sorted fine

sand (graphic meanF SE= 0.141F 0.001 mm) for 5-min periods at different tidal cycle

stages with traps similar to those in our study. He found that nematodes, but not copepods,

were sometimes correlated with sediment volume in traps. Moreover, he found that

nematodes were dominant in the ambient sandflat community, but proportionally under-

represented in traps compared to other taxa. Our mudflat results agreed with these from his

North Carolina sandflat. On the other hand, there was a major difference between Fegley’s

findings and ours. Copepods were never commonly captured in his traps, ranking far behind

nematodes, turbellarians, ciliates, ostracods, and tardigrades. What caused these low ab-

solute dispersal rates? Sandy sediment has a greater amount of interstitial space than mud

does, allowing meiofauna to live deeply, where they may be less susceptible to erosion.

Perhaps copepods were better able to resist erosion and hydrodynamic transport than the

other meiofauna taxa were. However, copepods also had low density in Fegley’s ambient

community, only 3% that of the nematodes. Were they uncommon in traps because each

copepod had a low probability of dispersing? Or, alternatively, were they uncommon in

traps simply because they were uncommon in the ambient community?

4.2. Meiofauna dispersal rates

To answer this question, we used Fegley’s data to calculate relative dispersal rates (Table

3). On a per individual basis, the copepods dispersed much more readily than the nematodes,

which is also what we discovered at our study site. Thus, their rarity in the North Carolina

traps was probably due to their low density in the ambient community. The nematodes had a

relative dispersal rate that was 10-fold greater in fast flow (flood tide) than in no flow (high

tide), consistent with the idea that they were dispersing passively. Copepods had a relative

dispersal rate that was only 4-fold greater in fast flow than in no flow conditions. Using

nematodes as a benchmark, copepods had relative dispersal rates that were only 2-fold

higher than the rate for nematodes in fast flow but 5-fold higher during no flow, consistent

with the idea that they actively avoided fast flow conditions and emerged when flow was

slow or nonexistent. Our bulk dispersal calculations from Fegley’s data showed that far

fewer copepods than nematodes were captured per unit of sediment at his site. This result

might explain why we found so many copepods in our traps and Fegley found so few.

Few studies provide the data that Fegley’s (1988) does on the full range of meiofauna

taxa, ambient community densities, and sediment captured in traps necessary to calculate all

three meiofauna dispersal rates. It is too simplistic to use absolute dispersal rate as the sole

estimator of meiofauna dispersal. Depending upon which dispersal rate is being compared,

different conclusions can be drawn when comparing taxa or sites. Another study with data

that can be used to demonstrate this point is Sun and Fleeger’s (1994) analysis of meiofauna

movement in a Louisiana mudflat. They found that copepods as a whole were twice as

abundant as nematodes in traps, so their absolute dispersal rate was 2-fold higher (Table 3;

data presented as they reported it, overall means for the three deployment periods of 24, 48,

and 72 h). However, the copepod relative dispersal rate was 14-fold higher because they

were so much less abundant than nematodes in the ambient community.

These relative dispersal rate calculations from Fegley (1988), Sun Fleeger (1994), and

our study provide evidence that a copepod is much more likely to disperse than is a

nematode. It would be incorrect to assume, however, that this higher dispersal rate is due

only to active behavior in copepods. Sun and Fleeger (1994) assigned life-style desig-

nations to the most common copepods at their site. Despite the higher overall absolute

dispersal rates for copepods than for nematodes, four of the five dominant copepod species

had absolute dispersal rates that were actually far lower than the rate for nematodes, and

the fifth had a rate approximately equal to that of the nematodes.

We calculated relative dispersal rates for these five species and discovered that the three

epibenthic copepods had rates that were 10-fold, 45-fold, and 275-fold higher than the rate

for nematodes. On the other hand, the rates for the tube-building and burrowing copepods

were only 1.6-fold and 3-fold higher, respectively, than the rate for the nematodes. Thus,

the surface-dwelling epibenthic forms all had much higher rates of relative dispersal than

the deeper-dwelling tube-builder and burrower. These species-specific results for copepods

could be due to active emergence, but they also parallel our species-specific conclusions

about nematodes. Despite the obvious differences in morphology, swimming ability, and

behavior between copepods and nematodes, the epibenthic forms within each taxon are

more susceptible to erosion and more likely to disperse than deep forms.

4.3. Dispersal rates and spatial pattern

Armonies (1994) has reviewed the complex landward and seaward movements of

meiofauna resulting from the interaction of wind-generated and tidal currents at his well-

studied field site. Generally speaking, however, the hydrodynamic processes leading to the

Table 3

Comparison of mean meiofauna dispersal rates among studies

Investigation Habitat Ambient Dispersal rate

community

densityAbsolute Relative Bulk

Fegley (1988) Intertidal sandflat, North Carolina, USA

High tide

Copepods 3.35 0.22 0.066 0.76

Nematodes 112.30 1.44 0.013 4.94

Cop/Nem 0.03 0.15 5.14 0.15

Flood tide

Copepods 3.35 0.86 0.256 0.38

Nematodes 112.30 14.12 0.126 6.17

Cop/Nem 0.03 0.06 2.03 0.06

Sun and Fleeger (1994) Intertidal mudflat, Louisiana, USA

Copepods 29.3 28.5 0.97 NA

Nematodes 213.4 14.8 0.07 NA

Cop/Nem 0.14 1.97 14.06 NA

This study Intertidal mudflat, Maine, USA

Copepods 19.21 193.82 10.09 232.07

Nematodes 1240.61 133.89 0.11 180.71

Cop/Nem 0.016 1.45 91.73 1.28

All data normalized to 1 cm2 basis to make comparisons easier, but traps were deployed for different time periods:

Fegley (1988), 5 min; Sun and Fleeger (1994), overall mean for 24, 48, and 72 h; this study, 1 day. Otherwise,

units as in Table 1. NA=No sediment data collected.

establishment and persistence of meiofauna spatial structure remain largely unknown. As

water flows over complex bottom topography (sensu Commito and Rusignuolo, 2000), it

causes spatial and temporal variability in current velocities. As a result, flow-mediated

passive dispersal can create and destroy spatially ordered meiofauna density differences,

even during a single tidal cycle (Sherman and Coull, 1980; Grant, 1981, 1983; Grant et al.,

1997; Hogue, 1982; Warwick and Gee, 1984; Kern and Taghon, 1986; Decho and Fleeger,

1988; Fegley, 1988; Savidge and Taghon, 1988; DePatra and Levin, 1989; Sun and

Fleeger, 1994; Fleeger et al., 1995; Thistle, 1998; Thistle and Levin, 1998).

When hydrodynamic forces are less important, i.e., where tidal- and wind-induced cur-

rents are weak, at slack high and low tides, or in no-flow laboratory experiments, then active

choice may play a more dominant role in determining meiofauna spatial structure (Service

and Bell, 1987; Armonies, 1994; Sun and Fleeger, 1994; Fleeger et al., 1995). Our results

suggest that caution must be used when interpreting the results of static microcosm and

mesocosm experiments (Austen et al., 1998; Schratzberger andWarwick 1998, 1999a,b; Tita

et al., 2000). Compared to typical flow regimes in nature, no-flow laboratory conditions

might cause an increase in residence times within the sediment for nematodes (especially

epigrowth-feeders) and a decrease for copepods. Palmer (1988b) has demonstrated the value

of incorporating realistic flow regimes into microcosm experiments designed to test for

predation and bioturbation effects on meiofauna.

Understanding dispersal rates can shed light on the control of distribution patterns of

meiofauna in the field. The interplay between the regional pool of dispersers and local eco-

logical processes remains poorly understood (Palmer et al., 1996; Thrush et al., 2000), but

the continual movement of meiofauna caused by transport events that are highly variable in

space, time, and intensity could have a dramatic impact on local population and community

dynamics.

5. Conclusion

This investigation demonstrated for the first time, that three measures of meiofauna

dispersal varied in predictable ways on an unvegetated mudflat. Absolute, relative, and

bulk dispersal rates provided different types of information on the movement of

meiofauna. Absolute dispersal is the most commonly reported measure of dispersal in

the literature, yet it provides little insight into the movement of animals in relation to

ambient densities and bedload processes. Comparison of our results with two other studies

showed that absolute dispersal rates of nematodes and copepods varied widely. However,

relative dispersal rates were always higher for copepods than for nematodes, even when

the reverse was true for absolute dispersal rates.

The results presented here quantified the highly dynamic behavior of meiofauna

populations in soft-bottom communities. At our site, meiofauna taxa dispersed in relative

proportions far different from those of the ambient community. Turnover times estimated

from relative dispersal rates (i.e., the inverse of relative dispersal rate) at our site ranged

from much less than 1 day for the mobile copepods and ostracods to 9 days for the

nematodes. These short turnover times indicate that the role of dispersal must be included in

any consideration of the ecology of soft-bottom systems at local and regional spatial scales.

Acknowledgements

We thank Gettysburg College Environmental Studies Department students J.

Abrahamson and D. Risso for assistance in the field. J. Fleeger, C. Lardicci, and F. Mal-

tagliati, and two anonymous reviewers made valuable comments on an earlier version of the

manuscript. This research was made possible by support for J.A.C. provided by the Get-

tysburg College Grants Advisory Commission and the Dipartimento di Scienze dell’Uomo

e dell’Ambiente, Universita di Pisa. [RW]

Appendix A. Nematode species ranks for ambient community cores and bottom

traps. Species ranked in the top 10 for cores or traps are set in bold font

Species Feeding group Rank

Cores Traps

Actinonema sp. 2A 42 21

Amphimonhysterella sp. 1B 26 –

Anoplostoma viviparum 1B 30 –

Anticomopsis sp. 1A 8 8

Antomicron sp. 1A – 34

Aponema sp. 2A – 30

Axonolaimus sp. 1B 9 6

Campylaimus sp. 1B 24 –

Chromadora macrolaima 2A 2 1

Chromadorina sp. 2A 13 5

Comesa sp. 1B – 28

Cytolaimium sp. 1B 40 –

Daptonema sp. 1B 1 4

Desmoscolex falcatus 1A – 14

Diplopeltoides sp. 1A 37 –

Doliolaimus sp. 1B 16 27

Eleutherolaimus sp. 1B 46 32

Enoploides sp. 2B 31 26

Enoplus sp. 2B 31 –

Halalaimus sp. 1A 29 19

Halichoanolaimus sp. 2B 41 –

Hopperia americana 2A 18 28

Hypodontolaimus inaequalis 2A 17 24

Leptolaimus elegans 1A 12 7

Linhystera sp. 1A 15 31

Mesacanthion sp. 2B 33 –

Metachromadora sp. 1 2B 10 12

Metachromadora sp. 2 2B 3 9

Metalinhomoeus sp. 1B 22 23

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differential dispersal rates in an intertidal meiofauna assemblage

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