Introduction to the study of meiofauna

The science of sediment toxicology essentially began in the late 1970s. It was largely a product of dredging concerns and recognition of widespread contamination of sediments. During the past few years, sediment toxicity research activity has increased dramatically. Currently, most tests are of an acute nature with fewer available for determining sublethal endpoints of chronic toxicity. Test systems of single and multiple species have included most levels of biological organization in aquatic ecosystems and have been conducted in the laboratory on whole sediments, interstitial waters, elutriates, or other extractable fractions under a wide variety of conditions. Evaluations of methodological effects and comparisons with in situ toxicity using surrogate test species and indigenous communities have, on occasion, shown significant differences in test responses. These differences may be attributed to laboratory-controlled parameters (e.g., light, species, life stage, exposure conditions, test phase, spiking method); sampling and laboratory-induced disruption of sediment integrity; alteration of toxicant partitioning due to manipulations and temporal effects; and failure to recognize other influencing ecosystem variables (e.g., organism niche and life cycle, sediment partitioning and gradient dynamics, physicochemical and biological process integration, biotic and abiotic disturbances, micro- and macrobiota patches, food-web interactions). Optimizing and standardizing test methods will require further studies of these variables to improve inter-laboratory comparisons and ecosystem validity. Despite the many unknowns that exist, a variety of sediment toxicity tests have been effectively used in assessing toxicant contamination by measuring the bioavailable fraction of the in-place pollutants. The optimal assays vary with the study and its objectives. Intergrative studies using several chemical, community, and toxicity measures are currently the most effective at defining ecosystem perturbations.

https://setac.onlinelibrary.wiley.com/doi/pdf/10.1002/etc.5620101204

Assessing the toxicity of freshwater sediments

Subtidal seagrass habitats are prime candidates for the application of principles derived from landscape ecology. Although seagrass systems are relatively simple compared to their terrestrial counterparts in terms of species diversity and structural complexity, seagrasses do display variation in spatial patterns over a variety of scales. The presence of a moving water layer and its influence on faunal dispersal may be a distinguishing feature impacting ecological processes in the subtidal zone. Studying seagrass-dominated landscapes may provide a novel approach to investigating questions regarding self-similarity of spatial patterns, and offers a new perspective for analysing habitat change in a variety of marine environments.

https://www.cell.com/trends/ecology-evolution/pdf/0169-5347(94)90041-8.pdf

Seagrass landscapes: a terrestrial approach to the marine subtidal environment.

The intertidal marsh community comprises both benthic and natant faunal components. The benthic components are primarily small invertebrates residing within or on the soft sediments of the vegetated marsh surface. The natant components include larger, fully aquatic organisms (e.g., fish and shrimp) that inhabit the shallow waters adjacent to the marsh at low tide but interact with the benthic components of the community when the marsh is tidally inundated. In this structurally complex and often expansive intertidal environment, patterns of invertebrate distribution and abundance are not apparent to the casual observer. Benthic core samples taken along an intertidal marsh transect on Sapelo Island, Georgia, USA show that many of the inconspicuous infaunal organisms, which numerically dominate the macrofaunal elements of this soft-substrate community, exhibit zonal distribution patterns along a tidal gradient. Patterns of invertebrate distribution in the intertidal salt marsh are often attributed to the activities of aquatic predators. The results of most predator exclusion experiments have left little doubt that predation/disturbance can be an important determinant of invertebrate abundance in soft-substrate communities; but a growing number of experiments, in both freshwater and marine environments, have produced results that apparently conflict with this, general tenet. Dismissed by some as “failed” experiments, these investigations have exposed our lack of knowledge about the effects of specific predators and the importance of complex interactions which involve more than two trophic levels. Although the importance of predation has been stressed in many recent experimental investigations, there are many other factors that, alone or in combination, may also influence the structure of salt marsh invertebrate assemblages. Included among these are: (1) various density-dependent processes (e.g., adult-larval interactions, agonistic behavior, interspecific competition), (2) selective larval settlement or mortality, (3) the influence of physical factors expressed through habitat preferences, and (4) unpredictable or cyclic physical disturbances. Many questions concerning the spatial and temporal patterns of invertebrate distribution and abundance in the salt marsh are unresolved and remain as challenges to our understanding of soft-substrate community dynamics.

Patterns of Invertebrate Distribution and Abundance in the Intertidal Salt Marsh: Causes and Questions

Accurate measures of intertidal benthic microalgal standing stock (biomass) and productivity are needed to quantify their potential contribution to food webs. Oxygen microelectrode techniques, used in this study, provide realistic measures of intertidal benthic microalgal production. By dividing a salt-marsh estuary into habitat types, based on sediment and sunlight characteristics, we have developed a simple way of describing benthic microalgal communities. The purpose of this study was to measure and compare benthic microalgal biomass and production in five different estuarine habitats over an 18-mo period to document the relative contributions of benthic microalgal productivity in the different habitat types. Samples were collected bimonthly from April 1990 to October 1991. Over the 18-mo period, tall Spartina zone habitats had the highest (101.5 mg chlorophyll a (Chl a) m−2±6.9 SE) and shallow subtidal habitats the lowest (60.4±8.9 SE) microalgal biomass. There was a unimodal peak in biomass during the late winter-early spring period. The concentrations of photopigments (Chl a and total pheopigments) in the 0–5 mm of sediments were highly correlated (r2=0.73 and 0.88, respectively) with photopigment concentrations in the 5–10 mm depth interval. Biomass specific production (μmol O2 mg Chl a−1 h−1) was highest in intertidal mudflat habitats (206.3±11.2 SE) and lowest in shallow subtidal habitats (104.3±11.1 SE). Regressions of maximum production (production at saturating irradiances) vs. biomass (Chl a) in the upper 2 mm of sediment by habitat type gave some of the highest correlations ever reported for benthic microalgal communities (r2 values ranged from 0.43 to 0.73). The habitat approach and oxygen microelectrode techniques provide a useful, realistic ranged from 0.43 to 0.73). The habitat approach and oxygen microelectrode techniques provide a useful, realistic method for understanding the biomass and production dynamics of estuarine benthic microalgal communities.

Biomass and Production of Benthic Microalgal Communities in Estuarine Habitats

Spatial distribution of meiofauna at minimum and maximum populationdensities on a sand- and mudflat in North Inlet, South Carolina, was determined. Using four core sizes ranging in area from 0.3 to 32.0 cm2, plots of Green's Index of Dispersion vs. area sampled usually showed maximum aggregation at 3–5 cm2. Nematodes were slightly patchy at the 5 cm2 scale. The degree of aggregation of harpacticoid copepods was influenced primarily by dispersion patterns of the dominant species. Differences among species spatial distributions were detected. Naupliar and adult distribution was very similar on the mudflat but not on the sandflat. Gastrotrichs were patchy on a smaller scale (3–5 cm2) than had been previously known for this site. The primary patch-generating mechanism was thought to be heterogeneously distributed resources (probably food). Macrofauna and their structures (tubes, etc.), inter-specific competition and social (breeding) behavior were also possible causes. Dispersion patterns of some taxa (e.g. gastrotrichs, mudflat copepods) changed seasonally, presumably due to changes in population density or in the spatial distribution of resources. There were no consistent differences between degree of aggregation on the sandflat vs. the mudflat, two distinctly different meiofaunal habitats.

Small-scale spatial distribution ofmeiofauna on a mud- and sandflat

The effect of sampling scale on apparent distribution of meiofauna was demonstrated by simulating the sampling of a “known” distribution with three different sampling schemes. Small-scale sampling gave better reproduction of the original pattern than larger scales, which tended to homogenize patchiness. My results suggest that for intracommunity meiofaunal research, roughly fifty 0.5–1.0 cm2 cores is a reasonable first guess “optimum” sampling scheme. The position of the cores should be random but their position relative to each other should be noted so the spatial information may be utilized in analysis.

Influence of sampling scale on apparent distribution of meiofauna on a sandflat

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Small-scale spatial distribution ofmeiofauna on a mud- and sandflat

Influence of sampling scale on apparent distribution of meiofauna on a sandflat