Muddy Sand Site Research Articles

Bottom mixed layer oxygen dynamics in the Celtic Sea

The seasonally stratified continental shelf seas are highly productive, economically important environments which are under considerable pressure from human activity. Global dissolved oxygen concentrations have shown rapid reductions in response to anthropogenic forcing since at least the middle of the twentieth century. Oxygen consumption is at the same time linked to the cycling of atmospheric carbon, with oxygen being a proxy for carbon remineralisation and the release of CO2. In the seasonally stratified seas the bottom mixed layer (BML) is partially isolated from the atmosphere and is thus controlled by interplay between oxygen consumption processes, vertical and horizontal advection. Oxygen consumption rates can be both spatially and temporally dynamic, but these dynamics are often missed with incubation based techniques. Here we adopt a Bayesian approach to determining total BML oxygen consumption rates from a high resolution oxygen time-series. This incorporates both our knowledge and our uncertainty of the various processes which control the oxygen inventory. Total BML rates integrate both processes in the water column and at the sediment interface. These observations span the stratified period of the Celtic Sea and across both sandy and muddy sediment types. We show how horizontal advection, tidal forcing and vertical mixing together control the bottom mixed layer oxygen concentrations at various times over the stratified period. Our muddy-sand site shows cyclic spring-neap mediated changes in oxygen consumption driven by the frequent resuspension or ventilation of the seabed. We see evidence for prolonged periods of increased vertical mixing which provide the ventilation necessary to support the high rates of consumption observed.

Site-dependent effects of bioturbator-detritus interactions alter soft-sediment ecosystem function

MEPS Marine Ecology Progress Series Contact the journal Facebook Twitter RSS Mailing List Subscribe to our mailing list via Mailchimp HomeLatest VolumeAbout the JournalEditorsTheme Sections MEPS 569:145-161 (2017) - DOI: https://doi.org/10.3354/meps12086 Site-dependent effects of bioturbator-detritus interactions alter soft-sediment ecosystem function Rebecca V. Gladstone-Gallagher1,*, Hazel R. Needham1, Andrew M. Lohrer2, Carolyn J. Lundquist2,3, Conrad A. Pilditch1 1School of Science, University of Waikato, Hamilton 3240, New Zealand 2National Institute of Water and Atmospheric Research Ltd. (NIWA), Hamilton 3216, New Zealand 3Institute of Marine Science, University of Auckland, Auckland 1142, New Zealand *Corresponding author: rgladstonegallagher@gmail.com ABSTRACT: In coastal sediments, macrofauna can trap macrophyte detritus in their burrows, modifying organic matter retention/export. However, the resulting effects on ecosystem function are unclear. We conducted a field experiment to assess how detrital processing by a common bioturbating crab Austrohelice crassa influences benthic metabolism, primary production, and ammonium regeneration at a sand and muddy-sand site. Since the functional role of crabs and detrital decay rates vary with mud content, we hypothesised their interactive effects on ecosystem function would also vary. Sixteen cages (0.36 m2) were established at each intertidal site, and cages were allocated a crab (0 or 35 ind. cage-1) and seagrass detritus (0 or 120 g dry weight [DW] cage-1) treatment. Ten days later, sediment-water solute fluxes were measured. Treatment effects on ecosystem function were site-specific: detritus stimulated benthic metabolism in muddy-sand (by 12 to 29%, p = 0.01), but in sand, effects were dominated by crabs. Metabolism increased by 12 to 21% (p = 0.04), of which 24 to 52% could be attributed to crab respiration. Crabs enhanced ammonium regeneration in sand (p < 0.003), of which <26% could be attributed to crab excretion, but in muddy-sand, the presence of detritus masked this positive effect (dark ammonium flux, crab × detritus interaction: p = 0.03). Crabs and detritus had negative effects on primary production in sand, where crabs reduced primary production by 57% and detritus by 73%. In muddy-sand, variable light conditions made primary production estimates unreliable. Our results emphasise that context is paramount when understanding the effects of reductions in the densities of key species or changes in detrital inputs on soft-sediment ecosystem function. KEY WORDS: Bioturbation · Austrohelice crassa · Detrital decay · Primary production · Nutrient flux · Grapsid · Seagrass · Benthic community metabolism Full text in pdf format PreviousNextCite this article as: Gladstone-Gallagher RV, Needham HR, Lohrer AM, Lundquist CJ, Pilditch CA (2017) Site-dependent effects of bioturbator-detritus interactions alter soft-sediment ecosystem function. Mar Ecol Prog Ser 569:145-161. https://doi.org/10.3354/meps12086 Export citation RSS - Facebook - Tweet - linkedIn Cited by Published in MEPS Vol. 569. Online publication date: April 07, 2017 Print ISSN: 0171-8630; Online ISSN: 1616-1599 Copyright © 2017 Inter-Research.

Sedimentary Environment Influences the Effect of an Infaunal Suspension Feeding Bivalve on Estuarine Ecosystem Function

The suspension feeding bivalve Austrovenus stutchburyi is a key species on intertidal sandflats in New Zealand, affecting the appearance and functioning of these systems, but is susceptible to several environmental stressors including sedimentation. Previous studies into the effect of this species on ecosystem function have been restricted in space and time, limiting our ability to infer the effect of habitat change on functioning. We examined the effect of Austrovenus on benthic primary production and nutrient dynamics at two sites, one sandy, the other composed of muddy-sand to determine whether sedimentary environment alters this key species' role. At each site we established large (16 m2) plots of two types, Austrovenus addition and removal. In winter and summer we deployed light and dark benthic chambers to quantify oxygen and nutrient fluxes and measured sediment denitrification enzyme activity to assess denitrification potential. Rates of gross primary production (GPP) and ammonium uptake were significantly increased when Austrovenus was added, relative to removed, at the sandy site (GPP, 1.5 times greater in winter and summer; ammonium uptake, 8 times greater in summer; 3-factor analysis of variance (ANOVA), p<0.05). Denitrification potential was also elevated in Austrovenus addition plots at the sandy site in summer (by 1.6 times, p<0.1). In contrast, there was no effect of Austrovenus treatment on any of these variables at the muddy-sand site, and overall rates tended to be lower at the muddy-sand site, relative to the sandy site (e.g. GPP was 2.1 to 3.4 times lower in winter and summer, respectively, p<0.001). Our results suggest that the positive effects of Austrovenus on system productivity and denitrification potential is limited at a muddy-sand site compared to a sandy site, and reveal the importance of considering sedimentary environment when examining the effect of key species on ecosystem function.

Estimating areal production of intertidal microphytobenthos based on spatio-temporal community dynamics and laboratory measurements

In situ Microphytobenthic community dynamics were combined with laboratory measurement of predominant species by fluorescence methods to estimate the areal primary production. Field investigation of community dynamics of microphytobenthos (MPB) was conducted from August 2006 to August 2007 in intertidal flats of the Nakdong River estuary, Korea. MPB Biomass varied between 0.47 and 16.58 μg cm−3 in the surface 1 cm sediment, with two dominant diatom species, Amphora coffeaeformis and Navicula sp., occupying average 77.2 ± 14.9% of total number of MPB cells. The biomass was higher in the slightly muddy sand sites than that in the sand site, and showed different pattern of seasonal variation. The profile of vertical distribution of biomass was an exponential decrease trend with depth in sediments. The biomass proportions in the uppermost 3 mm were 57.6% and 37.8% with and without the presence of biofilm, respectively. The two dominant species were cultured in laboratory, and their photosynthetic parameters, rETRmax (relative maximum electron transport rate), α (light utilization coefficient) and Ek (light saturation parameter) were derived from rETR (relative ETR)-irradiance curves by Imaging- PAM (pulse amplitude modulated) fluorometry. The rETR-irradiance curves showed no significant difference of photosynthetic activities between the two species. The areal potential production ranged from 0.74 to 2.22 g C m−2 d−1.

Temporal variation in the vertical distribution of microphytobenthos in intertidal flats of the Nakdong River estuary, Korea

The vertical distribution of microphytobenthos (MPB) was investigated in intertidal flats of the Nakdong River estuary over different temporal periods. The MPB biomass were measured as chlorophyll a concentration in the sediment, and monitored at two different depth scales. From August 2006 to August 2007, monthly sampling was carried out at four sites to a depth of 10 cm. The vertical distribution exhibited an exponential decline with depth and the slope was closely related to sediment composition and MPB biomass in the surface sediment. The two slightly muddy sand (5–25% mud, <63 μm) sites presented similar trends of seasonal variation to a sediment depth of 10 cm, and no statistical difference was observed in their initial values and the slope of the exponential curves applied to each data set. The site with higher mud content also had higher surface biomass, and the fitted exponential curve had a larger slope than sites with more sand content. The seasonal variations of phaeo-pigments compared to chlorophyll a revealed higher grazing stress on the MPB in summer at the three slightly muddy sand sites, and low grazing stress all year around at the sand (<5% mud) site. In January, April and October 2007, and January 2008, diurnal sampling was conducted at hourly intervals at one slightly muddy sand site. The investigation in January 2007 which was conducted at 1 cm intervals to a depth of 10 cm sediment showed the migration mainly existed in the upper 1 cm. The other diurnal sampling was undertaken to a 1 cm depth at 0.5 mm intervals. An evident migratory rhythm was present, with migration up to the surface during daytime emersion and down into deeper sediment during night and high tide submersion. Migrating cells could reach sediments as deep as 6 or even 10 mm. When the visible thick biofilms formed, the migratory rhythm changed by keeping higher biomass at the surface even at night and during high tide submersion. Generally, the MPB surface biomass was an important factor in structuring the MPB vertical distribution. The vertical distribution to depths of 10 cm was closely related to the sediment composition over the long term, while the MPB migration influenced the vertical distribution over the short term in the top 1 cm of the sediment.

How does shoreline development impact the recruitment patterns of coral reef fish juveniles (Moorea Island, French Polynesia)?

The study examined the effects of coastal embankment building on fish recruitment in three habitat types (beach-rock, white sand and muddy sand) in the near shore and fringing reef habitats of Moorea lagoon (French Polynesia). The results showed a positive relationship between the presence of embankments and the density and species richness of juvenile fish along the shoreline (whatever the habitat types). However, embankments deteriorated adjacent fringing reefs (decrease of live coral), which led to a decrease of fish density on beach-rock and white sand sites, and a decrease of fish species richness on muddy sand sites.

Life history of a dominant polychaete, Polygordius jouinae, in inner continental shelf sands of the Mid-Atlantic Bight, USA

The life history of the polychaete Polygordius jouinae Ramey, Fiege, and Leander 2006 was studied in the sandy sediments of the LEO-15 research area off the coast of Tuckerton, New Jersey (39°27.69′N, 74°15.81′W). Divers collected sediment cores biweekly, from February 2004 to November 2005 to examine the temporal patterns of abundance and size–frequency distributions of P. jouinae. These parameters were also examined (for the first time) for P. jouinae in samples taken in ambient sediments and in three reciprocal 3 to 5-day sediment transplant experiments that were part of a previous, unrelated study at coarse sand and muddy sand sites (∼3 km apart) at LEO-15 in the summers of 1994 and 1995. Analysis of these previous data along with the current data provided a larger set of values for P. jouinae, as well as further insight into the life history, habitat selection, and dispersal of P. jouinae (not considered in previous studies). Polygordius jouinae is gonochoristic and the population was mostly composed of sexually mature individuals in late May 2004 and 2005. The reproductive period usually occurs from May to August and individuals that spawn generally have a 1-year life span. Non-spawning individuals of P. jouinae could potentially live longer than 1 year and therefore the population may sometimes consist of individuals of two generations. Recruitment begins no later than July as recently settled individuals (≤9 mm body length), which were not present during the first week of June 1995, 2004, and 2005, first appeared in July in all 3 years. The smallest individual was 2.01 mm long and 0.07 mm wide and this provides the first estimate of size at initial recruitment. Although some worms in sediment trays likely settled as larvae, most worms were recently settled juveniles (length categories 6 and 9 mm) transported in from the surrounding sediments, presumably as a consequence of re-suspension or bed load transport. As expected, larger individuals were present later in September. Although it is unknown whether P. jouinae were transported actively or passively these data suggest that recently settled juveniles are capable of habitat selection (prefer sand over mud) and show that post-settlement transport and selection influence community structure.

The Effect of Muddy Bottom Sediment on the Abundance and Life Span of the Barnacle, Hesperibalanus hesperius, Epizoic on Scallop Shells

Barnacles are an important fouling component of molluscs of commercial value, and they may adversely affect their hosts. The abundance of living and dead individuals, survival of newly-settled young and the maximum life span of the barnacle, Hesperibalanus hesperius, encrusting the shells of the scallop, Patinopecten (Mizuhopecten) yessoensis, were compared for two sites differing in bottom sediment composition. It was found that muddy sediment (mud content > 25.5%) had an adverse effect on the survival of this barnacle. The valves of scallops collected from a sandy substratum had significantly more barnacles than scallops sampled from muddy sand. At the sandy site, the mean number of barnacles on the upper valves of 5-8-year-old scallops was 378.4 ± 9.6 individuals per valve, and on the lower valves, 92.3 ± 1.8 individuals per valve; of these, 38.5-48.3% were alive. At the muddy sand site, the mean number of barnacles on the upper valves of 5-8-year-old scallops was only 154.5 ± 4.5 individuals per valve, and on the lower valves, 31.7 ± 1.4 individuals per valve. However, living barnacles made up only 20.1-22.6% of the total number counted. On muddy sand, the number of barnacles per cm2 of scallop valve increased with scallop age up to 3-5 years, after which it declined. However, on sand, the number of barnacles per cm2 of scallop valve did not change with increase in host age from 5 to 8 years. The young barnacles settled in significantly greater numbers on the shells of scallops living on sand than on muddy sand. The mortality of barnacles during the first month after settlement on the upper scallop valves was about 45 and 80% on sand and muddy sand respectively. Old, large barnacles (with a shell base >10 mm), both living and dead, occurred more often on scallops living on the sandy substratum. The largest barnacles had a base length of 15-20 and 13-15 mm, at the sandy and muddy sand sites respectively. The possible reasons for these differences are discussed.

Smothering of estuarine sandflats by terrigenous clay: the role of wind-wave disturbance and bioturbation in site-dependent macrofaunal recovery

Sediment run-off from land has been recognised as a threat to the biodiversity of shal- low estuarine and coastal areas. Extreme rainfall events can cause flooding and landslides, which may result in rapid deposition of fine terrigenous sediments and have serious impacts on benthic communities. We designed a field experiment to study the response of intertidal benthos to such depositions of terrigenous clay. The experiment was conducted at 2 contrasting intertidal sites: a sheltered muddy sand habitat and an exposed sand habitat influenced by wind waves. Terrigenous clay (50% water content) was deposited in replicated experimental plots (2 m diameter) at each site in layers 0, 3, 6 or 9 cm thick. The initial response of the resident macrofauna and subsequent recolonisation was monitored over a period of 408 d. Physical and chemical properties of the experi- mental plots and wave climate at each site were also measured. The experiment demonstrated highly deleterious effects of catastrophic terrigenous clay deposition on estuarine macrobenthic communi- ties. At both sites following clay deposition, the numbers of individuals were reduced by more than 50% after 3 d and by more than 90% after 10 d, irrespective of clay thickness. Mud crabs Helice crassa were the only animals able to emerge through the clay layer. They also exhibited elevated densities in clay treatments over the course of the experiment. After 28 d, a storm occurred (maximum wave height 0.4 m, period = 6 s) which dispersed the clay deposits at the exposed sandy site. Recolonisation of the surficial sediments was rapid at the exposed site following this wind-wave dis- turbance. However, deeper-dwelling animals such as large bivalves had not recovered to levels observed in the control plots by the end of the experiment. At the more sheltered muddy sand site, the clay deposition resulted in long-lasting habitat change; Although the clay was gradually broken up and re-colonised by small crabs and surface dwelling macrofauna, recovery was still incomplete 408 d after deposition. The results emphasise the role of wind-wave disturbance and transport of sediments and macrofauna with bedload, and the importance of bioturbation by crabs as facilitators of macrobenthic recovery after disturbance.

Community structure of the benthos in Scottish Sea-lochs. IV. Multivariate spatial pattern

Most studies of spatial patterns of invertebrates in soft sediments have concentrated on populations of individual species. Those that examined patterns in communities have tended to employ categorical analytical techniques. Using macrofaunal abundance data from van Veen grab samples collected 20 to 100 m apart in known spatial arrangements from Scottish sea-lochs, the relationships between patterns in macrobenthic species composition and distances between samples were explored using matrix correlations in a non-parametric framework. Using a simple definition of spatial structure, i.e. that intersample distances are monotonically correlated with intersample species-similarity, spatial structure at each of seven stations was assessed using non-parametric Mantel tests based on rank-correlations. Changes in community structure were positively correlated with distance at all sites in Loch Etive, on both current-swept muddy sands and soft deep muds. Different components of the macrobenthos contributed to spatial pattern at each site. Simple spatial structure was also detectable at a muddy-sand site in Loch Creran, but neither on soft mud, nor at the soft mud site in the Firth of Lorne. The concept of rank-correlograms was introduced. These were used to examine the extent and form of spatial structure in different components of the macrobenthos at each site. Relationships between similarity and distance were neither simple nor consistent. Results were compared to previous studies which used the same data, and it was concluded that studies carried out at a particular scale, or on a particular component of the benthos, are unlikely to be successful in predicting spatial relationships at other scales or for other components of the benthos. Correlational rather than categorical analyses are recommended for exploratory studies of spatial relationships in the benthos. Analyses of the spatial structure at these seven sea-loch sites suggests that by ensuring that samples are at least 40 m apart an investigator is unlikely to underestimate variability or otherwise invalidate statistical analyses based on the use of the samples as replicates. Spacing samples up to 100 m apart may increase variability estimates, further reducing the chance of concluding that a difference exists when one does not.

Influence of benthic macrofauna on the geotechnical and geophysical properties of surficial sediment, North Sea

Spatial and temporal variations in the structure of an Amphiura-Echinocardium macrobenthic community were studied in relation to geotechnical and geophysical properties of the seabed at a muddy-sand site in the seasonally stratified region of the southern North Sea. Vertical profiles of geotechnical properties were recorded in sediments collected by box corer. Maxima in water, organic matter, and fine particle contents coincided with the presence of the burrowing brittle star Amphiura filiformis and the mud shrimp Callianassa subterranea in the upper and lower parts, respectively, of the cores. A significant relationship existed between the abundance of A. filiformis and the water content of the upper 0.05 m of the bed. There were important temporal variations in rigidity modulus, derived from acoustic shear wave propagation in freshly recovered cores, of the upper 0.06 m of the sediment. The rigidity modulus was 45% greater in January than in May and this has been related to the burrowing/feeding activity of the macrobenthic community; there was an inverse relationship between A. filiformis abundance and bed rigidity. Thus the bed had a lower bulk density and lower rigidity in summer due to biological modification of the sediment fabric. This implies that the bed was less resistant to erosion in the summer. Such an effect may be important during summer storms in the shallowest parts of the seasonally stratified zone (ie close to shelf fronts).

Adult/juvenile interactions of infaunal bivalves:contrasting outcomes in different habitats

Field experiments were used to assess the influence of density vanatlons of adult bivalves on recruitment of juveniles. The generality of these results was tested by running the experiment concurrently at 2 sltes of different sediment grain size and wave exposure. Adults of 2 bivalve species, the deposit-feedlng tellinid Macomona lillana and the suspension-feeding venerid Austrorienus stutchburj~i, were used in the experiment. The experiment consisted of 14 treatments of vanous combinations of densities of live adults or the empty articulated shells of the 2 species. Three species of juvenile bivalve (M. liliana, A. stutchburyi and Nucula hartvigiana) were sampled from the experiment on 3 occasions over a 9 wk period. Treatment effects were consistent over lime, but reflected only small changes In the mean density of juvenile bivalves. The presence of empty articulated shells of adult M. liliana or A stutchburyi, positioned at localities within the sediment which reflected normal living conditions, had no detectable influence on the density of juvenile bivalves. Treatments ~nvolving adult A stutchburyi ~nfluenced only the density of juvenile conspecifics at the muddy-sand site. Treatments involving adult M. liliana produced significant effects on the density of both M. l~liana and N. hartvlgiana juveniles a t both sites. However, these effects occurred in opposite directions a t the 2 sites. At the muddy-sand site, highest juvenile densities were associated with high adult densities, but, at the sandy site, they were associated with the absence of adult M. liliana. The identification of subtle density changes a t the sandy site was unexpected because sediment transport is frequent at this site and associated passive movement of juveniles has the potential to mask local biotic interactions. The change found in the direction of interactions between adult and juvenile bivalves between sltes has irnplications for the generality of such interactions: in particular it highlights the influence that habitat may have on ecolog~cal interactions.

K-systems analysis for determining the factors influencing benthic microfloral productivity in a Louisiana estuary, USA

About 3000 sediment cores from Baratana Estuary, Louisiana, USA, were incubated between 26 Jul and 25 Aug 1983. For the month, benthic microfloral production measured at a subtidal site consisting of muddy sand was almost twice that measured at a sandy intertidal site (27 g C m-' vs 14 g C m-'). However, maximum productivity rates (230 mg C m-' h') were greatest at the sandy site; these rates occurred under periods of subaerial exposure which were limited to late afternoons on 11 d of the study, yet accounted for 43 % of the total monthly production at the sand site. Other variables monitored were chlorophyll a, light intensity, water temperature, salinity, benthic community respiration, productivity of the water column, biological activity (muddy sand site), wave height (sand site), meteorological tides (wind-induced water levels), tide height, and initial dissolved oxygen. Multichannel information analysis indicated that the information accounted for in productivity by these variables peaked at slightly less than 7 and 3 d periods for the sand site and weak peaks occurred at about 10 and 3 d periods for the muddy sand site. K-systems analysis indicated that the environmental variables acted in combination to influence productivity, and that the combinations changed through time. Physical processes (meteorologic, astronomic, and anthropogenic) largely controlled productivity at the sand site. Productivity at the muddy sand site was influenced by a combination of physical and biological activities.

Siphon nipping: Its importance to small fishes and its impact on growth of the bivalve Protothaca staminea (Conrad)

During four successive experiments spanning 24 months, the bivalve Protothaca staminea (Conrad) grew significantly more at all sizes inside cages that excluded large (> 6 mm) predators and croppers than inside cage controls and uncaged controls in a clean-sand site in Mugu Lagoon, California. For a clam of 2.5 cm initial length, predator and cropper exclusion increased average linear growth to 2.2 times control levels and volumetric growth to 2.5 times control Caging over the same periods had no repeatable effect on Protothaca growth in a muddy-sand site. Laboratory tests of the feeding behavior of Protothaca with and without each of four common consumers suggest that the presence of consumers did not cause sufficient disruption of feeding to produce the growth differences observed in clean sand. However, monthly seining data over 16 months revealed the persistent abundance of three fishes, Leptocottus armatus Girard, Hypsopsetta guttulata (Girard), and Paralichthys californicus (Ayres), containing abundant Protothaca staminea and Macoma spp. siphons in their stomachs. Estimates of habitat-specific intensity of siphon nipping suggest that the rate of cropping per individual Protothaca was 30 to 92% lower in the muddy sand, in part because of a switch to the more abundant deposit-feeding bivalve, Macoma nasuta (Conrad). Thus, habitat-specific cropping differences can explain the between-habitat difference in the response of Protothaca to caging.

Distribuição e ecologia dos decapoda numa área estuarina de Ubatuba (SP)

Benthic macro fauna (Decapoda) was sampled by beam trawl at 6 stations in Bay of Fortaleza and adjacents Escuro e Claro rivers (Sao Paulo, Ubatuba, Brasil). Samples were taken monthly during 1973-1974. Diversity was high in muddy sand sites of Bay of Fortaleza and low at very shallow stations located in the rivers, where the effects of extreme fluctuations of salinity were evident. The changes in species composition appeared to correspond most clearly to differences in salinity. The total number of species of decapods collected in the areas was 27 (14 species of Natantia and 13 of Reptantia). Juveniles of Penaeus brasiiiensis and Penaeus paulensis reach the rivers. Callinectes danae live in the Bay and in the rivers. Xiphopeneus kroyeri is a specie who inhabit the Bay of Fortaleza in high populational density.

Classification and community structure of macrobenthos in the Hampton Roads area, Virginia

Benthic macrofauna was sampled by grab at 16 stations in Hampton Roads and the adjacent Elizabeth River, Virginia, USA. Samples were taken in February, May and August. Sampling sites and species were grouped by a classification strategy which basically consisted of the Canberra metric dissimilarity-measure and flexible and group average clustering. Following reallocations, 8 site groups and 16 species groups instructively classified the 47 sites and 93 species considered in the analysis. The sites were grouped into “associations” on mud, muddy-sand and sand-bottom, and those in the Elizabeth River. Species groupings distinguished a few species most frequent at Elizabeth River or mud and muddy-sand sites, larger numbers of species restricted to muddy-sand and sand or solely to sand sites, ubiquitous species, epifaunal species which were microhabitat-restricted, and seasonal species. An analysis of numerically dominant species in the different associations indicated the relative importance of ubiquitous species and seasonally abundant species. Community-structure statistics (species diversity, species richness and evenness) showed definite spatial and temporal patterns. Diversity was high at sand and muddy-sand sites and low at mud and Elizabeth River sites. This spatial pattern was predominantly one of species richness. At Elizabeth River and mud stations, diversity increased from February to August because of increased evenness, while at sand and muddy-sand stations, diversity peaked in May in response to both high species richness and high evenness. The applicability of “community concepts”, the causes of substrate specificity, seasonality and species diversity, and the effects of pollution on community structure are discussed.