Meiofaunal Organisms Research Articles

Validating the NemaSPEAR[%]-index for assessing sediment quality regarding chemical-induced effects on benthic communities in rivers

Fine, cohesive sediments provide a habitat for a very diverse fauna and considerably contribute to important ecosystem services of aquatic ecosystems. As fine sediments are often hotspots of chemical contamination, the benthic fauna has to be protected in order to maintain its ecological functioning and thereby the proper provision of important ecosystem services. However, in these habitats usually meiofaunal organisms prevail, which are, so far, neglected in biomonitoring studies, while routine benthos monitoring (e.g. according to the EU Water Framework Directive) is solely based on macroinvertebrates. The recently developed NemaSPEAR[%]-index filled this methodological gap by providing a monitoring tool using freshwater nematodes, which are one of the most abundant and species rich invertebrates in fine sediments. In the present study the NemaSPEAR[%]-index was revised and validated based on a larger data set of nematode species and physico-chemical properties in river sediments in order to increase its applicability. The larger data set led to a similar categorization of nematode species at risk (NemaSPEAR). Validation of the NemaSPEAR[%] with an independent test data set, as well as external field data published by other authors and experimental microcosm data confirmed its usefulness as a specific index detecting chemical induced changes in benthic communities, whereas the index also worked at a higher taxonomic level (genus; NemaSPEAR[%]genus). A separation of the index for metal and organic pollution (NemaSPEAR[%]metal, NemaSPEAR[%]organic) showed no benefits for assessing sediments with mixed contamination, allowing the use of only one NemaSPEAR[%] for overall pollution. Moreover, based on its variance in lowly polluted reference sediments, class boundaries were set up for categorizing samples according to their ecological status, with a NemaSPEAR[%] higher or lower 30% indicating an acceptable or not acceptable ecological status, respectively. Overall, this study confirmed the robustness and relevance of the NemaSPEAR[%]-index for assessing the quality of fine sediments and using it as a line of evidence in a weight-of-evidence framework. Thus, the index can be a valuable tool for classification and prioritization of fine sediments supporting risk managers and regulators in making sediment management decisions.

Influence of the ornamental red cherry shrimp Neocaridina davidi (Bouvier, 1904) on freshwater meiofaunal assemblages

Meiofauna serves as a link between microbial production and higher trophic levels, such as macroinvertebrates and juvenile fish. However, the role of meiofauna in the freshwater food web has not been accurately evaluated, and the influence of shrimp predation on freshwater meiofaunal assemblages is unknown. In this study, the predation effects of Neocaridina davidi , an ornamental freshwater shrimp native to inland water bodies in Asia, on meiofaunal density, biomass, structure, and secondary production were examined using model ecosystems (microcosms) that were sampled repeatedly over 42 days. Shrimp predation altered the structure of the meiofaunal community over the course of the experiment, in particular the density, biomass, and secondary production of nematodes, microcrustaceans, and oligochaetes. An analysis of the stomach contents of N. davidi indicated a high degree of omnivory and the frequent consumption of meiofaunal organisms. The results indicated that predation by the freshwater shrimp N. davidi depresses the overall abundance, biomass, and secondary production of meiofaunal assemblages. Moreover, they also provide insights into food-web ecology and the first evidence of freshwater shrimp predation on meiofaunal assemblages.

Occurrence of epibionts associated with meiofaunal basibionts from the world’s largest mangrove ecosystem, the Sundarbans

Epibionts–basibionts interaction is an ecological relationship observed across terrestrial and aquatic environments. Ciliate epibionts associated with meiofaunal organisms are ubiquitous throughout marine ecosystems, but their distribution patterns remain largely unknown in mangrove habitats. We studied the occurrence of ciliate epibionts (peritrichids and suctorians) on meiofaunal organisms in the Sundarbans, the world’s largest mangrove ecosystem, during early monsoon of 2015. The present study revealed that 1.13 % of meiofaunal organisms (18 out of 2744 specimens) were infested by ciliate epibionts. The epibionts were found to be adhered to host organisms belonging to free-living nematodes and harpacticoid copepods. Four different species of ciliate epibionts (Cothurnia nova, Pleurophryodendron sp., Thecacineta cothurnioides and Trematosoma rotunda) were associated with four different host meiofaunal species (Chromaspirina parapontica, Dorylaimposis punctata, Stenhelia sp. and Distioculus sp.) as observed in this study. Among them, the epibionts Cothurnia nova and Thecacineta cothurnioides were found in July 2015, while the other epibiont species were encountered in August 2015. The detailed systematic and host frequency and remarks of each ciliate epibiont have been discussed in this study. To the best of our knowledge, this is the first study which reports ciliate epibionts on meiofaunal basibionts from a mangrove ecosystem in India. Our results indicate that the occurrence of epibionts in a mangrove ecosystem (even a human-disturbed site) could be due to habitat heterogeneity and the nature of anthropogenic disturbance.

Spatiotemporal variation and sediment retention effects on nematode communities associated with Halimeda opuntia (Linnaeus) Lamouroux (1816) and Sargassum polyceratium Montagne (1837) seaweeds in a tropical phytal ecosystem

Nematodes play an important role in ecological processes and are one of the most abundant meiofaunal organisms associated with seaweeds. Yet, knowledge on seaweed bed ecosystems is limited. Nematodes associated with Sargassum polyceratium and Halimeda opuntia were compared in two transects, 80 m apart and parallel to the beach line in Cupe Beach, Brazil. The temporal variation during the dry and rainy seasons and the effect of sediment retention by the seaweed on nematode density and composition were investigated. The differences in nematode communities between the two seasons were mainly caused by the increase in density of the most abundant genera in the rainy season. A significant difference was observed between the nematode communities of the two transects for H. opuntia. The nematode communities of both seaweed species did not differ significantly in the same transect. The genus Euchromadora was dominant in both seaweed species. The amount of sediment retained by the seaweeds did not affect the overall nematode density. However, it was positively correlated with the density of Draconema and Euchromadora in both seaweeds, and both genera were exclusively found associated with seaweeds. This result opposes the idea that the more sediment retained by the seaweed, the higher the nematode overall density and the higher the number of nematodes originally coming from the sediment.

At least some meiofaunal species are not everywhere. Indication of geographic, ecological and geological barriers affecting the dispersion of species of Ototyphlonemertes (Nemertea, Hoplonemertea).

Most meiofaunal species are known to have a broad distribution with no apparent barriers to their dispersion. However, different morphological and/or molecular methods supported patterns of diversity and distribution that may be different among taxa while also conflicting within the same group. We accurately assessed the patterns of geographic distribution in actual genetic species of a marine meiofaunal animal model: Ototyphlonemertes. Specimens were collected from several sites around Europe, Northern and Central America, Southern America, Pacific Islands and Asia. We sequenced regions of two mitochondrial and two nuclear genes. Using single-gene, a concatenated data set, multilocus approaches and different DNA taxonomy methods, we disentangled the actual diversity and the spatial structures of haplotypes and tested the possible correlation between genetic diversity and geographic distance. The results show (i) the importance of using several genes to uncover both diversity and highlight phylogeographic relationships among species and that (ii) independent genetic evolutionary entities have a narrower distribution than morphological species. Moreover, (iii) a Mantel test supported a positive correlation between genetic and geographical distance. By sampling from the two sides of Isthmus of Panama, we were additionally able to identify lineage divergence times that are concordant with vicariance mechanisms caused by the geological closure of the seaway across the Isthmus. We therefore propose that in addition to distance, other geological and ecological conditions are also barriers to the dispersion of and gene flow in marine meiofaunal organisms.

Vertical distribution of meiofauna on reflective sandy beaches

Abstract Extreme physical conditions usually limit the meiofauna occurrence and distribution in highly hydrodynamic environments such as reflective beaches. Despite sediment grains of the upper layers being constantly resuspended and deposited, the high energy of the swash zone besides depositing coarse sediments allows an ample vertical distribution of meiofaunal organisms. The effect of physical, chemical and sediment variables on the vertical distribution of meiofaunal organims and nematodes was analysed on two reflective exposed beaches. Sampling was conducted at three sampling points on each beach in the swash zone. The sediment collected was divided into four 10-cm strata (0-10 cm, 10-20 cm, 20-30 cm, 30-40 cm). The statistical differences between strata due to factors previously established (i.e. meiofaunal composition, density of most abundant taxa) were tested using a hierarchical PERMANOVA applied under similarity and euclidian distances. An inverse relation among average grain size, content of organic matter and sediment sorting was evident. Coarser sediment characterized the upper layers, while at deeper layers the sediment was very poorly sorted and presented a higher content of organic matter. A similar pattern in the vertical distribution of meiofaunal and nematofaunal composition and density was detected. The lowest densities were associated with the first stratum (0-10 cm), highly affected by hydrodynamics. The vertical distribution of organisms was statistically different only when the interaction among factors was considered. This result suggests that zonation and vertical distribution of meiofaunal organisms are determined by the within-beach variability.

Experimental evidence for selective settlement of meiofauna from two distinct environments after sediment suspension

The cosmopolitan distribution of many meiofaunal organisms raises questions about their dispersal. The small size and the lack of a planktonic life stage of most meiofauna taxa including free-living nematodes suggest that passive dispersal is a main factor determining their distribution. This study investigates the settling behaviour of meiofauna in a water column under no-flow conditions. Two ex situ settling experiments were conducted with (1) macrophyte associated meiofauna from an intertidal salt marsh and (2) meiofauna of sulphidic sediments associated with bacterial mats from the deep-sea Håkon Mosby mud volcano. Cilyndrical containers filled with sieved seawater were used as settling chambers and five different substrates, placed on the bottom of the containers, were offered to the descending meiofaunal assemblage. The substrates used in experiment 1 were agar with bacteria, agar with Fucus spiralis, sulphidic agar medium, bare agar and an empty Petri dish. For experiment 2 azoic sediment with algae, azoic sediment with bacteria, a sulphidic medium, bare azoic sediment and an empty Petri dish were used. The intertidal experiment revealed that nematodes and nauplius larvae showed four- to tenfold higher densities in the Fucus treatment compared with the controls whereas deep-sea nematode and harpacticoid copepod densities in the sulphide treatment were more than three times higher compared with all other treatments. In both experiments nematode composition did not differ from the reference samples whereas proportions of harpacticoid copepods were increased in the treatments of the deep-sea experiment compared with the reference sample suggesting a better ability to select settlement sites than nematodes. In both experiments meiofauna abundance was highest in substrates with similar characteristics as their original habitat. These findings indicate that some meiofaunal organisms can selectively settle once they are suspended in the water column in the absence of water flow and therefore actively contribute to their dispersal at small spatial scales.

Seagrass vegetation and meiofauna enhance the bacterial abundance in the Baltic Sea sediments (Puck Bay).

This study presents the first report on bacterial communities in the sediments of eelgrass (Zostera marina) meadows in the shallow southern Baltic Sea (Puck Bay). Total bacterial cell numbers (TBNs) and bacteria biomass (BBM) assessed with the use of epifluorescence microscope and Norland’s formula were compared between bare and vegetated sediments at two localities and in two sampling summer months. Significantly higher TBNs and BBM (PERMANOVA tests, P < 0.05) were recorded at bottom covered by the seagrass meadows in both localities and in both sampling months. The relationships between bacteria characteristics and environmental factors (grain size, organic matter, photopigments in sediments), meiofauna and macrofauna densities, as well as macrophyte vegetation characteristics (shoot density, phytobenthos biomass) were tested using PERMANOVA distance-based linear model (DISTLM) procedures and showed that the main factors explaining bacteria characteristics are bottom type (vegetated vs. unvegetated) and meiofauna density. These two factors explained together 48.3 % of variability in TBN and 40.5 % in BBM, and their impacts did not overlap (as indicated by DISTLM sequential tests) demonstrating the different natures of these relationships. The effects of seagrass were most probably related to the increase of organic matter and providing habitat while higher numbers of meiofauna organisms may have stimulated the bacterial growth by increased grazing.

Pulse of marine subsidies: the role of surf diatom Asterionellopsis glacialis accumulations in structuring the meiofauna of sandy beaches

Resource subsidies are flows of biologically fixed energy and nutrients from one ecosystem to another. Oceanic sandy beaches are typical examples of systems constantly subjected to subsidies. A distinctive feature of some exposed sandy beaches is the brown discoloration of surf zone waters due to rich accumulations of diatoms housed on the sediments seaward of the breaker zone. Large amounts of the rich diatom accumulations may end up on the sediments of surf and the intertidal zones of sandy beaches. In this study, we described the seasonal frequency and compared the strength the subsidies among different beaches by measuring sediment chlorophyll a and total organic content. We hypothesized that large amount of fresh diatom subsidies to beaches sediment will fuel meiofauna from the surf zone to the upper intertidal and change the structure of these communities. Our results supported that pulsed subsidies by the deposition of A. glacialis represented an important resource for the meiofauna. The meiofauna responded quickly to the increase in concentrations of labile organic matter in the sediments along different tidal levels of the beach. This subsidized input, however, is not an isolated process but linked with the passage of cold fronts and significant physical changes in the surf zone. Subsides did not fuel surf and intertidal meiofauna similarly. At the surf zone, subsidies were mainly used by those forms that cope with increasingly turbulent flows that paralleled the accumulation events. Yet in the intertidal zone, the densities and richness of the most abundant meiofauna organisms, the nematodes, thrived together with other meiofaunal organisms. Due to their frequency over the time and differences on space, A. glacialis patch deposits most certainly are not the main food item for high-energy sandy beach consumers, but accumulations do represent a window of opportunity for meiofauna during the year, particularly in periods of food shortage during winter.

Trawling exerts big impacts on small beasts

Throughout the world, there is concern about the impact of trawling on deep-sea bottom communities (1). For the most part, this concern has been directed toward deep-sea coral reefs and sponge beds (2, 3). Reef structures, formed by the scleractinian coral, Lophelia pertusa, for example, can be up to 8,000 y old and can provide habitat for more than 700 smaller species (2). In contrast, there has been the sense that trawling on open mud bottoms has little or no impact on the resident species or at least that, because the smaller species have higher turnover rates, the recovery from trawling will be rapid (4). In PNAS, however, Pusceddu et al. show that trawling on deep-sea muddy bottoms strongly alters the biochemical nature of the sediment and reduces the species diversity and abundance of meiofaunal organisms (animals ∼40–500 µm in size), and, in particular, the nematodes (5).

The meiofauna of artificial water-filled tree holes: colonization and bottom-up effects

The meiofaunal community of artificial water-filled tree holes was determined, and the bottom-up effects of different amounts of leaf litter on abundance and diversity were estimated. We assume a positive impact of leaf litter on meiofaunal abundances, species diversity, and trophic links. Plastic cups with different amounts of leaf litter were placed in a beech forest (Teutoburg Forest, Bielefeld, Germany) for 24 weeks. As early as 1 week later, the artificial tree holes were colonized by bdelloid rotifers, tardigrades, and nematodes. Rotifers were dominant throughout the experiment, followed by nematodes and tardigrades. The 29 nematode species that were identified included bacterial and hyphal feeders, with common species such as Plectus cirratus/accuminatus and Aphelenchoides parietinus predominating. Impacts of water volume (up to complete desiccation), pH, and O2 on the meiofaunal community were not detected, whereas the addition of leaf litter resulted in bottom-up effects. Nematode abundance, especially that of bacterial feeders, and species number increased with increasing leaf input. The predatory nematode Prionchulus muscorum was found only in treatments containing high leaf content. Rotifer abundances were partly negatively affected by the amount of added leaves and, like tardigrades, showed a reversal in their correlation at higher leaf inputs. Our study revealed the fast colonization of small water bodies by meiofaunal organisms and the importance of passively distribution. Furthermore, the results provide a comparison with the meiofaunal community in lakes and soil.

Grazing effects on nematodes and other components of the periphyton at three depths in Lake Erken

The periphyton of stony hard substrates in the littoral zone of lakes is inhabited by an abundant and diverse algae and meiofauna assemblage that is regulated by grazing. The objective of our study was to examine the effect of macroinvertebrate grazers on periphytic algae and meiofaunal organisms at different depths in the littoral zone of Lake Erken (Sweden). To this end, a 2-factorial field experiment was carried out in which macrofaunal access to the periphyton was manipulated at 3 different water depths (30, 70, and 200 cm). Grazing effects on algal biomass varied among depths and were highest in both deeper zones. Compared to algal biomass, the abundance of meiofaunal organisms was less affected by grazing because macrograzers significantly reduced meiofauna only at the 200-cm depth. The organism mainly responsible for the depth-dependent differences in grazer effects was the snail Theodoxus fluviatilis, which dominated macrofaunal assemblage at depths where grazing effects were the strongest (70 and 200 cm). The abundance of T. fluviatilis was positively correlated with the proportions of reduced algal biomass and reduced meiofaunal abundance. Meiofaunal groups and individual nematode species responded differently to grazing, so our results further revealed that grazing macroinvertebrates affected the composition of the meiofaunal assemblage. However, meiofaunal and nematode assemblages differed more across depths than across grazing treatments, suggesting that other depth-related factors (e.g., hydrodynamics or light) also were important for structuring periphytic meiofaunal assemblages.

Diversity and composition of macro- and meiofaunal carapace epibionts of the hawksbill sea turtle (Eretmochelys imbricata Linnaeus, 1822) in Atlantic waters

The presence of macro-epibionts on turtle carapaces is a well-known phenomenon, whereby carapaces are occupied by dynamic and fully functional epibiont communities. However, meiofaunal organisms have been largely ignored in turtle shell studies despite their omnipresence and higher abundances and diversity than the macrofauna. Epifauna from the hawksbill sea turtle Eretmochelys imbricata was investigated during summer 2010 with the aim to advance our knowledge on meiofaunal epibiont communities on turtle carapaces and gain insights into their interaction with settled macrofauna. Eighteen epibiont higher taxa were found (17 meiofauna, 5 macrofauna), 5 of which are common for macro- and meiofauna. Meiofauna was present on all turtle carapaces, but macrofauna occurred on only 8 out of 19 investigated carapaces, suggesting that carapace colonization by meiofauna precedes macrofauna recruitment. In addition, the macrofauna embedded on the carapaces increased the microhabitat complexity, favoring richer and more abundant meiofauna communities. The significant positive correlations between meiofauna and macrofauna taxa (up to 90 %) suggests the presence of mutual facilitating processes and indicates the positive effects between meio- and macrofaunal epibionts important for their recruitment and establishment. The hawksbill sea turtle carapaces were occupied by fully functional and active epifaunal communities, with adult and reproductive stages for most meiofaunal and macrofaunal taxa. Turtle carapaces can therefore be seen as a biological substrate that can serve as a platform for faunal dispersal, as has been observed for barnacles, enhancing the geographical distribution of several species through sea turtle migration. In addition to the main focus of this paper on meio- and macrofaunal epibiont communities, we provide an updated list of taxa found on carapaces of the hawksbill sea turtle and discuss the geographical scope and dispersion potential of some of these taxa.

Variación espacio-temporal de la meiofauna submareal en una playa arenosa nororiental de Venezuela

Meiofauna organisms that play an important role in the trophic ecology of soft bottom benthos, have short life cycles and they respond quickly to disturbance and pollution. The present study shows the spatio-temporal variation ofsubtidal meiofauna (metazoans passing a 500im sieve but retained on meshes of 40-63micro m) in four shallow subtidal stations. Samples were taken in the sandy beach of San Luis, in the Northeastern coast of Venezuela, from October 2005 until September 2006. For this, three replicate sediment core samples (4.91cm2), were collected monthly to a depth of 10cm into the sediment, and preserved in 6% formalin stained with rose Bengal. Specimens of 14 meiofaunal groups (Foraminifera excluded) were collected, being the nematodes, ostracods and harpacticoid copepods the most abundant. Monthly density was comprised between 64 and 503ind./10cm2, and mean density of stations between 173 and 449ind./10cm2. There is a trend of low densities from October to February (end of the rainy season until the middle of the dry season). The San Luis beach control of the meiofaunal community is shared by climatic conditions and by the biology of the species found. The meiofauna mean density in San Luis beach (263ind./10cm2) was low when compared to other studies in tropical areas.

Meiobenthic copepod fauna of a marine cave (NW Mediterranean) closely resembles that of deep-sea communities

The 3PP marine cave near Marseille (NW Mediterranean) is at a depth of only 15 m. Nevertheless, the cave is a unique deep-sea-like habitat zone due to total darkness, lack of water circulation, oligotrophy, and cold homothermy, a thermal regime similar to that of the deep Mediterranean. We studied the meiofaunal community composition and diversity at 3 sampling stations in the cave (entrance, middle, and blind end). Major taxon composition, based on presence/absence data, did not vary over the transect; however, significant differences in community were found. Tardigrades were the only major taxon restricted to the inner parts of the cave. Copepod diversity decreased towards the inner parts of the cave. The 3PP Cave was characterized by very low abundances of meiofaunal organisms similar to abyssal sites elsewhere. A total of 405 individual copepods were assigned to 27 families and 90 species (75% of them new to science). Significant differences in copepod communities between stations were found at family, genus, and species level. Some harpacticoid taxa generally known from the deep sea, such as Marsteinia, Ancorabolina, Paranannopus, Nematovorax, and Argestidae, were represented in the 3PP Cave. A naive Bayes model was used for the first time to classify the communities as 'shallow' or 'abyssal'. The meiobenthic communities at the entrance and the end of the cave were unequi vocally classified as 'shallow' and 'abyssal' respectively, while the middle part of the cave presented a mixed community. Our study further highlights the strong faunal and community affinities between marine caves and the deep sea, indicating the existence of dispersal mechanisms from deep waters into the euphotic zone for benthic organisms.

Meiofauna Metabolism in Suboxic Sediments: Currently Overestimated

Oxygen is recognized as a structuring factor of metazoan communities in marine sediments. The importance of oxygen as a controlling factor on meiofauna (32 µm-1 mm in size) respiration rates is however less clear. Typically, respiration rates are measured under oxic conditions, after which these rates are used in food web studies to quantify the role of meiofauna in sediment carbon turnover. Sediment oxygen concentration ([O2]) is generally far from saturated, implying that (1) current estimates of the role of meiofauna in carbon cycling may be biased and (2) meiofaunal organisms need strategies to survive in oxygen-stressed environments. Two main survival strategies are often hypothesized: 1) frequent migration to oxic layers and 2) morphological adaptation. To evaluate these hypotheses, we (1) used a model of oxygen turnover in the meiofauna body as a function of ambient [O2], and (2) performed respiration measurements at a range of [O2] conditions. The oxygen turnover model predicts a tight coupling between ambient [O2] and meiofauna body [O2] with oxygen within the body being consumed in seconds. This fast turnover favors long and slender organisms in sediments with low ambient [O2] but even then frequent migration between suboxic and oxic layers is for most organisms not a viable strategy to alleviate oxygen limitation. Respiration rates of all measured meiofauna organisms slowed down in response to decreasing ambient [O2], with Nematoda displaying the highest metabolic sensitivity for declining [O2] followed by Foraminifera and juvenile Gastropoda. Ostracoda showed a behavioral stress response when ambient [O2] reached a critical level. Reduced respiration at low ambient [O2] implies that meiofauna in natural, i.e. suboxic, sediments must have a lower metabolism than inferred from earlier respiration rates conducted under oxic conditions. The implications of these findings are discussed for the contribution of meiofauna to carbon cycling in marine sediments.

Temporal changes in periphytic meiofauna in lakes of different trophic states

Meiofaunal organisms in the periphyton of stony hard-substrates (epilithon) were studied in three Swedish lakes with different trophic states (oligo-, meso- and eutrophic) with respect to seasonal successions in abundance, biomass, and production. Over a period of 2 years, the meiofaunal population of all three lakes fluctuated greatly, with densities varying up to nine-fold within a season. In the oligotrophic lake, a significant decrease in meiofauna in winter was striking, whereas in the other two lakes, richer in nutrients, there was a pronounced peak in early summer. Although the lakes, on average, did not differ in epilithic organic and inorganic material, the differences in meiofaunal abundance, biomass, and production were significant. Correlation analysis revealed that altogether the meiofaunal biomass was positively related to the lakes’ trophic state (total phosphorus), while the meiofaunal abundance and production along the trophic spectrum displayed a humped-shape distribution, with maximum values measured in the mesotrophic Lake Erken (1324 ind cm-2 and 2249 mg DW cm-2 y-1). Nematodes were the dominant meiofaunal group in the epilithon of all three lakes, accounting for up to 58% in abundance, 33% in biomass and 55% in production of the whole meiofaunal community. However, their relative importance tended to decrease with increasing trophic state. Beside nematodes, rotifers, oligochaetes, copepods and tardigrades were also found in large numbers in the epilithon. Overall, the results demonstrated that, due to their high abundance, biomass, and production, meiofaunal organisms play an important role in epilithic communities.

서해 장봉도 인근 사질 퇴적물에서 서식하는 중형저서동물 군집 특성

The community structure of meiobenthos was studied at 16 stations within sandy tidal and subtidal zones in Jangbongdo in the Yellow Sea, Korea from Aug. 2006 to Jan. 2007. Meiobenthic organisms were collected by three core samples, with a 3.6 cm diameter, from each sediment sample taken with a Smith-McIntyre Grab. Mean grain size of study stations ranged from <TEX>$1.49{\phi}$</TEX> to <TEX>$3.55{\phi}$</TEX>. Composition of sand ranged from 80.38% to 99.89%. There was reduction in total abundance and biomass of meiobenthos from summer to winter. Total densities of meiofauna ranged between 17 inds./10 <TEX>$cm^2$</TEX> and 853 inds./10 <TEX>$cm^2$</TEX>. Nematodes, gastrotricha, nauplius and harpacticoids appeared as major taxa in decreasing order. This study shows that major taxa comprised 90 percent of total abundance. Most meiofaunal organisms are concentrated in the upper sediment layers and the total abundance and biomass of organisms in the tidal zone is higher than the subtidal zone.

Temporal patterns in macrograzer effects on epilithic algae and meiofauna: a comparative approach to test for single species and whole grazer community effects

Within the shallow littoral zones of lakes, periphyton is an essential component, representing an important source of primary production and a food resource for herbivores. Periphytic communities are abundantly inhabited by meiofaunal organisms, which are mostly dominated by nematodes. During the last 3 decades, consumer–resource interactions between herbivore consumers and periphytic components (mainly algae) have been intensively studied. Although whole grazer community and single species effects on periphyton are known from field and laboratory experiments, the importance of single, dominant grazer taxa in direct comparison to whole community impacts is unknown. To investigate the continuity of grazing effects of a single, dominant macrograzer (Theodoxus fluviatilis, Gastropoda, Prosobranchia) on epilithic meiofauna and algae with respect to the whole grazer community, a temporally structured field experiment was carried out in Lake Erken (Sweden). Grazer impacts on periphytic algae and meiofauna were tested by controlling macrograzer access to littoral periphyton communities for 8 weeks in an exclosure/enclosure experimental design. Overall, the results showed macrograzer presence to have temporally constant, strongly negative effects on algal biomass as well as meiofaunal abundance and community composition. Moreover, T. fluviatilis alone accounted for up to 80% of the grazing effects, indicative of their ability to regulate periphytic communities in lakes. The present study yields new insights into the effects of a single grazer species and stressed temporal patterns of consumer–resource interactions in freshwater lakes.

Nematode species at risk — A metric to assess pollution in soft sediments of freshwaters

Soft sediments are often highly polluted as many of the toxic chemicals introduced into surface waters bind to settling particles. The resulting accumulation of pollutants in the sediments poses a risk for benthic communities. However, pollution induced changes in benthic communities have been difficult to determine when using macro-invertebrates as bioindicators, as these organisms are often absent in soft sediment. The present study therefore examined the ability of meiofaunal organisms, specifically, nematodes, to assess the ecological status of soft sediments. Over a 9-year period, nematode communities present in sediments collected from large rivers and lake Constance in Germany were studied. These sediments showed a large range of physico-chemical properties and anthropogenic contamination. After the degree of metal and organic contamination was translated into ecotoxicologically more relevant toxic units (TUs), multivariate methods were used to classify nematode taxa in species at risk (NemaSPEAR) or not at risk (NemaSPEnotAR). This approach clearly distinguished the influence of sediment texture from that of the toxic potential of the samples and thus allowed classification of the nematode species according to their sensitivity to or tolerance of toxic stress. Two indices, expressing the proportion of species at risk within a sample (NemaSPEAR[%]metal, NemaSPEAR[%]organic), were calculated from independent data sets obtained in field and experimental studies and showed good correlations with the toxic potential (field data) or chemical concentrations (microcosm data). NemaSPEAR[%] indices for metal and organic pollution were therefore judged to be suitable for assessing the impact of chemical contamination of freshwater soft sediments.