Distribution and abundance of meiofauna in intertidal sand substrata around Iceland

Iceland is situated in an important subarctic transition area where complex oceanographic dynamics occur. The intertidal, subtidal, and deep-sea faunal communities of Iceland are being intensively studied, as a critical resource for continued sustainability of fisheries and the preservation of northern littoral ecosystems. However, the meiofaunal communities and the environmental factors affecting them are still relatively poorly known. The meiobenthic metazoan community was studied with core sampling in 23 sandy beaches along the intertidal zone of the Iceland coast in a campaign developed in September 2003. Small-scale variation in meiofauna composition (major taxa) was explored and related to biotic and abiotic factors at different scales, such as beach exposure, granulometry, and organic matter content. Differences in meiofaunal community structure at a low taxonomic resolution appeared among beaches located within wide biogeographical zones of hydrobiological significance (NE and SW Shelf regions) and exposure degrees. Seventeen major taxa were recorded. In contrast with more local and taxon-focused studies, oligochaetes were the dominant group all around Iceland, followed by nematodes, turbellarians, gastrotrichs, and copepods (mainly harpacticoids). Acari, ascidians, bivalves, cnidarians, collembolans, gastropods, isopods, kinorhynchs, insects, nemerteans, ostracods, and polychaetes were relatively scarce groups, together being less than 1.6% of the meiofauna. There was a large variation in meiofaunal abundance between sites. Maximum abundances (>500 ind. cm^?2) were found in Sau?arkrökur, Hraunhafnartangi, and Skálaness, whereas minimum abundances (<40 ind. cm^?2) were recorded in Magnavík, Jokülsárlón (glacier beach site), Vikurnúpur, Breidalsvík, and Stokknes. We did not find a clear pattern in overall meiofaunal abundance regarding the degree of exposure of beaches. Oligochaetes, nematodes, and copepods were relatively more abundant in sheltered beaches, whereas turbellarians and gastrotrichs tended to be more abundant in exposed beaches. The best correlates of meiofaunal composition and abundance within beaches were the proportion of gravels and the content of utilizable organic matter in the sediment. We should consider factors operating at wider scales (importantly beach exposure and overall situation in the complex oceanographical context of Iceland) to find a pattern in the local structure of intertidal meiofaunal assemblages.     

Development of a meiobenthic nematode community following the intertidal placement of various types of sediment

A field experiment was conducted at an intertidal mudflat in SE England to investigate processes affecting the development of meiobenthic nematode assemblages in response to the placement of different types of sediment, simulating the deposition of fine-grained maintenance dredged material for habitat creation. Natural assemblages were exposed to different types of substrate, including sediment with natural sand and organic content (defaunated control), sediment with elevated sand content (high sand content treatment) and sediment with elevated organic content (high organic content treatment) for a period of 12 months. Differences between colonising assemblages and those from ambient control sediment in terms of total nematode abundance, taxonomic membership (i.e. number of species, species composition) and size (i.e. total biomass, biomass size spectra) were most marked in the plots with elevated organic content. Nematode density, number of species and total biomass in the defaunated control and high sand content treatment came to resemble those in the ambient control sediment after 3 months. In the organically enriched sediment, values only attained background level at the end of the experiment. Multivariate analyses revealed that, although assemblage structure in all treatments, and in particular the defaunated control and the high sand content treatment, became increasingly similar to the ambient sediment over time, recovery to ambient condition had not occurred 1 year after the set-up of the experiment.     

Top-down control of a meiobenthic community by two juvenile freshwater fish species

Top-down control of prey assemblages by fish predation has been clearly demonstrated for zooplankton and macroinvertebrates. However, in the benthic communities of freshwater ecosystems, the impact of fish predation on meiofaunal assemblages is nearly unknown. In this study, the predation effects of juvenile carp (Cyprinus carpio) and gudgeon (Gobio gobio) on meiofaunal abundance, biomass, community structure, and the diversity of nematodes were examined using microcosms that were sampled repeatedly over 64 days. Significant differences in abundance and biomass were found between the two fish treatments (carp and gudgeon) and their respective controls for nematodes, oligochaetes, and crustaceans (copepods, harpacticoids, ostracods, and cladocerans), but not for rotifers. These changes were consistent with top-down control of the freshwater meiofaunal assemblages in the microcosms over time. By contrast, small-bodied meiofauna was more abundant, suggesting indirect facilitation. Neither the species richness nor the diversity of the nematode community was affected by fish predation. The results indicate that predation by juvenile freshwater fish depresses the overall abundance and biomass of meiofaunal assemblages, except for rotifers, and alters the size structure of the meiofaunal community. Therefore, the meiofaunal assemblages of freshwater ecosystems may be influenced by bottom-feeding juvenile fish, e.g., carp and gudgeon, through top-down control of meiofaunal populations.     

The use of meiofauna in freshwater sediment assessments: Structural and functional responses of meiobenthic communities to metal and organics contamination

Because soft sediments are often hotspots of chemical contamination, their assessment can aid in identifying the causes of environmental stress and the implementation of measures to improve the health of the respective ecosystems. Achieving a “good ecological status” of surface waters, as required by the European Water Framework Directive, strongly depends on recognition of the chemical status of sediments. Meiobenthic organisms are important, but widely neglected components of the ecologically relevant fauna of a wide variety of ecosystems. In the present study, microcosms containing freshwater sediments were used to investigate the effects of eight different metals and polycyclic aromatic hydrocarbons (PAHs), in single and mixed applications, on natural meiofaunal assemblages. Structural (abundance and biomass) and functional (secondary production) parameters of the investigated assemblages were measured as ecologically relevant endpoints. Their sensitivity in revealing both the differential effects and the responses of meiofaunal taxa was evaluated to assess the general suitability of meiofauna and, in particular, of individual tested taxa, as bioindicators of soft sediment contamination. Structural parameters were found to be more valuable indicators than functional measurements, with more pronounced effects observed on the taxon level than on total meiofauna. Among the meiofaunal taxa considered in this study, nematodes were of particular utility as early indicators of chemical stress in freshwater soft sediments. Overall, this study provides new insights into the impact of toxicants on soft freshwater sediments and demonstrates the suitability of meiofaunal communities, especially nematodes, in assessing contamination of this ecosystem.     

Meiobenthic response to river-borne benthic particulate matter – a microcosm experiment

1. Benthic communities inhabiting river–lake interfaces are confronted with temporal variation in the import of river‐borne material to the associated littoral zone. The primary objective of this study was to evaluate the exposure to river‐borne benthic particulate matter as a force shaping the meiofauna of the river–lake interface. 2. In a 2 × 2 factorial design, combinations of strongly and weakly river‐influenced communities (near or far from the river mouth) with and without river‐borne benthic particulate matter enrichment were studied in laboratory microcosms to assess the response of the heterotrophic meiobenthos to river‐borne matter. 3. Nematodes dominated both habitat types and were the only meiofaunal taxon that responded to the enrichment. 4. A strong interactive effect between experimental enrichment and location of site was observed. Whereas nematodes from the littoral habitat close to the river mouth responded to the enrichment only marginally, nematodes from the remote site were strongly affected in the presence of river‐borne particulate matter. 5. In the non‐enriched samples of the remote site, the number of nematodes increased greatly over time. Among nematode feeding types, deposit feeders dominated over chewers at the beginning of the experiment but both types were equally abundant at the end. In the presence of river‐borne matter the increase of chewers ceased and a shift in feeding type composition did not occur. 6. It was shown that for river–lake interface meiofaunal assemblages in relative proximity, the same taxon might respond differently if exposed to an external pressure, e.g. river‐borne components. These components may have a stronger influence on taxa with longer life cycles.     

The effects of predation by juvenile fish on the meiobenthic community structure in a natural pond

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Macrofaunal recolonization on intertidal mudflats: effect of sediment organic and sand content

Following concerns over how differences in dredged material properties affects recovery following dredged material deposition on mudflats, a field manipulation experiment was conducted to investigate the effects of increased sediment organic content (from 0.9% to 2.8%) and sand content (from 12.0% to 47.0%) on macrofaunal recolonization and sediment properties on an intertidal mudflat, Crouch Estuary, Essex, UK. The manipulated sediments were placed in 1-m² plots, 10 cm deep, in a randomised block design and macrofauna and sediments sampled after 1 week, 1, 3, 6, and 12 months. The results suggested that while increased sand content had no discernible effect on sediment property changes, increased organic content resulted in marked reductions in redox potential at 1-, 2- and 4-cm sediment depths and increased shear strengths. These sediments exhibited a distinct ‘cracking’ appearance on their surfaces. Similarly, while macrofaunal recovery was only affected by increased sand content in the short-term (1 month), total numbers of individuals, number of species and diversity took longer to recover in sediments with increased organic contents. Recolonization proceeded via a gradual increase in the abundance of those taxa present in ambient sediments rather than via a distinct successional sequence. Recovery was species-specific with two species (Hediste diversicolor (Müller) and Hydrobia ulvae (Pennant)) recovering to ambient levels within the first week of the experiment. Multivariate analyses indicated that the community assemblages of the low-organic content treatments had recovered after 12 months while those of the high-organic content treatments were still significantly different to those of the controls at the end of the experiment. We propose that the relatively rapid recovery and lack of successional sequence generally observed in all sediment types of this study can be explained by the opportunistic nature of the ambient assemblage and by the dispersive nature of the dominant species. Reduced sediments and/or increased shear strengths led to a slow recovery in high-organic content treatments. These results indicate that a careful consideration of the organic content of fine-grained dredged material must be made during the licensing process of beneficial use schemes.     

Meiofaunal distribution in Hornsund fjord,Spitsbergen

The meiofaunal community structure at 32 stations in Hornsund fjord (77°N) was investigated, and results were compared with data from another Spitsbergen fjord, Kongsfjorden (79°N). Steep environmental gradients of sedimentation, organic matter content, and salinity from the inner to the outer basin of the fjord are present due to intensive glacial discharges of meltwater and ice. As the natural environmental disturbances were described for macrofauna benthic communities before, we aimed to check whether the same pattern occurs among meiofauna. A total of 12 higher meiofaunal taxa were recorded, with nematodes predominating at all stations. Non-parametric multivariate analyses demonstrated clear differences in meiofaunal abundance and composition between stations in the glacial bay and in the outer part of the fjord. Meiofaunal abundance increased with increasing distance from the source of disturbance, which in our study is tidal glaciers. Therefore, the current study demonstrates that the spatial structure of meiofauna is affected by the natural environmental disturbance, and analysis of meiofaunal assemblages can be used to assess the effect of such disturbances.     

Meiofaunal (re)colonization of the Arctic intertidal (Hornsund,Spitsbergen) after ice melting: role of wrack deposition

Intertidal meiobenthos of Hornsund—the southernmost fjord of Spitsbergen—was investigated between July and September 2005. Mean total meiofaunal densities ranged between 4.3 and 328 ind. 10 cm⁻². Nematode assemblages were impoverished in terms of the number of genera when compared with those from the western Spitsbergen coast (11 vs. 25–28 genera in total, respectively). It is suggested that severe environmental conditions in the southern part of Spitsbergen overcome the adaptation skills of many nematode species and hamper the establishment of a diverse community. Comparatively high nematode numbers on a beach subject to heavy macroalgal wrack input contrast sharply with numerically poor communities in sparse-wrack beaches (up to 315 vs. 31 ind. 10 cm⁻², respectively). It is suggested that the wrack input to the Arctic beach may substantially influence the richness and composition of the intertidal meiobenthic community. Nematode assemblages were dominated by extreme colonizers: Geomonhystera disjuncta and rhabditids. Their relatively high densities in beach sediments recorded at the beginning of July indicate their ability to recover rapidly after the winter period and to effective (re)colonization of the intertidal habitat just after the ice melt. Average concentration of rhabditids and monhysterids associated with macroalgal wrack deposited on the upper shore was as high as 52 × 10³ individuals per gram of the substrate. It is hypothesized that nematodes can play a substantial role in the wrack decomposition in the Arctic intertidal.     

Long-term fluctuations of soft-bottom intertidal community structure affected by ice cover at two small sea bights in the Chupa Inlet (Kandalaksha Bay) of the White Sea

The effect on bottom organisms of periodic freezing of the upper layer of intertidal sediments during abnormal cold winters has been studied for a long time in seas of a moderate climatic zone. However, the effect of ice cover every year on intertidal communities in polar seas is still poorly investigated. Seasonal and long-term variation in the structure of intertidal soft-bottom communities in two small bights in the White Sea with annual ice cover was studied for over two decades. Sampling was carried out four times a year, in the hydrological spring, summer, autumn, and winter. It was found that bottom macrobenthic communities at upper and lower horizons of the intertidal distinctly differed in the studied sites. Periodic changes caused by the effect of abnormal ice conditions, including the partial removal by ice of sediment with in situ organisms, were discovered. Recovery of communities after disturbance normally took less than half a year. Communities at the lower and upper horizons of the intertidal zone were more stable than intermediate communities, which led to periodic shifts of the biological border between lower and upper intertidal assemblages.     

Distribution and abundance of soft-sediment meiofauna and a predatory goby in a coral reef lagoon

Spatial patterns in the abundance of the softsediment meiofauna and a predatory goby, Valenciennia longipinnis, were examined in the lagoon of One Tree Reef (Great Barrier Reef). The study provided a quantitative framework to assess the importance of physical factors on and predator prey interactions between the meiofauna and V.longipinnis. Patterns of abundance were examined at two spatial scales: among four habitats (100's of m apart) and among sites (10's of m apart) within habitats. Of the four major constituents of the meiofauna (harpacticoid copepods, nematodes, polychaetes and oligochaetes), gut analyses showed that harpacticoid copepods were the primary prey of V.longipinnis. Spatial patterns of meiofaunal abundance in the lagoon were taxon specific. Polychaetes and harpacticoid copepods exhibited significant differences among habitats. Within habitats, however, polychaetes exhibited significant differences between sites whereas copepods were uniformly distributed. Abundances of nematodes and oligochaetes did not differ between habitats. Densities of nematodes differed significantly between sites while the number of oligochaetes were similar at both spatial scales. V.longipinnis was more abundant in shallow habitats than in deep ones. This study suggests that sediment type may be an important factor influencing the distribution of both the goby and the meiofauna. V.longipinnis and two of the four meiofaunal taxa (harpacticoid copepods and polychaetes) were more abundant in the shallow habitat with fine-grained sediments. There was no significant difference between abundances of meiofaunal taxa in sites where V.longipinnis was present or absent. Overall, more fish occurred in the habitat which had the highest densities of harpacticoid copepods.     

Intertidal meiofauna of a high-latitude glacial Arctic fjord (Kongsfjorden, Svalbard) with emphasis on the structure of free-living nematode communities

Meiofauna communities of four intertidal sites, two sheltered and two more exposed, in Kongsfjorden (Svalbard) were investigated in summer 2001 at two different tidal levels (i.e. the low-water line and close below the driftline, referred to as mid-water (MW) level). A total of seven meiofaunal higher taxa were recorded with nematodes, oligochaetes and turbellarians being numerically dominant. Mean total meiofaunal densities ranged between 50 ind. 10 cm⁻² and 903 ind. 10 cm⁻². Our data showed a clear decrease in total meiofaunal densities with increasing coarseness of the sediment. Total meiofaunal biomass varied from 0.2 g dwt m⁻² to 2 g dwt m⁻² and, in general, was high even at low meiofaunal densities, i.e. larger interstitial spaces in coarser sediments supported larger meiofauna, especially turbellarians. The results on the vertical distribution of meiofauna contrasted sharply with typical meiobenthic depth profiles on other beaches, probably in response to ice-scouring and concomitant salinity fluctuations. Oligochaetes were the most abundant taxon, with a peak density of 641 ind. 10 cm⁻² at Breoyane Island. They were mainly comprised of juvenile Enchytraeidae, which prohibited identification to species/genus level. Nematode densities ranged between 4 ind. 10 cm⁻² and 327 ind. 10 cm⁻². Nematodes were identified up to genus level and assigned to trophic guilds. In total, 28 nematode genera were identified. Oncholaimus and Theristus were the most abundant genera. The composition of the nematode community and a dominance of predators and deposit feeders were in agreement with results from other arctic and temperate beaches. Nematode genus diversity was higher at the more sheltered beaches than at the more exposed ones. Low-water level stations also tended to harbour a more diverse nematode communities than stations at the MW level. Differences in nematode community structure between low- and MW stations of single beaches were more pronounced than community differences between different beaches and were mainly related to resources quality and availability.     

Temporal Variations in Fine Sand Assemblages in the North Aegean Sea (Eastern Mediterranean)

Two sites were selected in the North Aegean Sea for the study of shallow fine sand assemblages, which are poorly known in the Eastern Mediterranean. The biomonitoring of these sites can provide useful information on the impact of human activities on the macrofaunal and meiofaunal composition of these assemblages. In order to examine this impact, sampling took place in five different periods between 2000 and 2001. The qualitative and quantitative compositions of the communities at the two sites showed certain differences, mostly between the sites and the year of sampling rather than between sampling periods, which would be the expected outcome. The communities seem to be affected by a combination of the human activities with the particular hydrodynamics of the studied sites.     

Immediate Effects of Experimental Human Trampling on Mid-Upper Intertidal benthic Invertebrates at the Asinara Island MPA (NW Mediterranean)

To assess potential risks of human visitation to ecological communities, the immediate effects of human trampling were investigated experimentally on small invertebrates inhabiting mid-upper intertidal hard bottoms covered by algae. Two different experimental intensities of trampling (60 and 120 footsteps) and controls (with no trampling) were applied to quadrats 20×20 cm in size (experimental area), within the two ‘no-entry, no-take’ zones of the Asinara Island MPA (Italy, Mediterranean Sea). One day after trampling ended, samples of benthic fauna were collected and the animals attributed to macrofaunal and meiofaunal components. Analyses of variance on the nine most common taxa of macrofauna identified significant higher abundance of bivalves, gammarid amphipods, polychaetes, isopods, oligochaetes in controls than in trampled plots. For nematodes, polychaetes, ostracods, oligochaetes, bivalves, acari, caprellid amphipods and tanaids a significant higher abundance of meiofaunal animals was found in controls than in trampled areas. Although no information on recovery is available, these results suggest that macrofaunal and meiofaunal taxa are vulnerable to this type of disturbance.     

Assessing the Response of Nematode Communities to Climate Change-Driven Warming: A Microcosm Experiment

Biodiversity has diminished over the past decades with climate change being among the main responsible factors. One consequence of climate change is the increase in sea surface temperature, which, together with long exposure periods in intertidal areas, may exceed the tolerance level of benthic organisms. Benthic communities may suffer structural changes due to the loss of species or functional groups, putting ecological services at risk. In sandy beaches, free-living marine nematodes usually are the most abundant and diverse group of intertidal meiofauna, playing an important role in the benthic food web. While apparently many functionally similar nematode species co-exist temporally and spatially, experimental results on selected bacterivore species suggest no functional overlap, but rather an idiosyncratic contribution to ecosystem functioning. However, we hypothesize that functional redundancy is more likely to observe when taking into account the entire diversity of natural assemblages. We conducted a microcosm experiment with two natural communities to assess their stress response to elevated temperature. The two communities differed in diversity (high [HD] vs. low [LD]) and environmental origin (harsh vs. moderate conditions). We assessed their stress resistance to the experimental treatment in terms of species and diversity changes, and their function in terms of abundance, biomass, and trophic diversity. According to the Insurance Hypothesis, we hypothesized that the HD community would cope better with the stressful treatment due to species functional overlap, whereas the LD community functioning would benefit from species better adapted to harsh conditions. Our results indicate no evidence of functional redundancy in the studied nematofaunal communities. The species loss was more prominent and size specific in the HD; large predators and omnivores were lost, which may have important consequences for the benthic food web. Yet, we found evidence for alternative diversity–ecosystem functioning relationships, such as the Rivets and the Idiosyncrasy Model.     

Meiofaunal recruitment to mimic pneumatophores in a cool-temperate mangrove forest: spatial context and biofilm effects

A field experiment was devised to test whether meiofauna that colonised mimic pneumatophores (artificial substrates) resembled the assemblage on adjacent live pneumatophores in three randomly chosen intertidal, estuarine sites. The experiment showed that the close proximity of particular biota on living pneumatophores did not reliably influence subsequent development of assemblages upon mimic pneumatophores within a scale of 10 m during a colonisation period of less than 20 weeks. There was some convergence of the composition of the colonising assemblage of meiofauna on mimic pneumatophores with the local assemblages in sites dominated by barnacles, or where the natural pneumatophores were free from macroscopic epibionts. However, tychopelagic meiofauna from algal epiphytes did not significantly colonise mimic pneumatophores during the 20-week trial, probably due a lack of growing algae. During the conditioning phase suspended in water at a marine site 20 km from the mangroves, mimic pneumatophores acquired an assemblage of meiofauna different from the estuarine assemblage that colonised mimics following implantation in the estuarine mudflat. Enhanced colonisation rates of mimics in suspended bags at the conditioning site may be explained by the absence of benthic macroinvertebrates, and the lack of intertidal exposure. Biofilms aged 2, 7, and 11 weeks had no consistent, different effect on the subsequent colonisation of meiofauna. We conclude that divergence of phytal-based assemblages of meiofauna depends upon the amount of coverage, as well as the type, of fouling macro-epibionts on the pneumatophores. Meiofaunal assemblages on artificial substrates after 20 weeks colonisation displayed less intrinsic patchiness than mature phytal assemblages on natural pneumatophores, and so present a potentially useful way of improving the power of biomonitoring applications using meiofauna.     

Subtle top‐down control of a freshwater meiofaunal assemblage by juvenile fish

1. Top‐down control of prey assemblages by fish predators has been clearly demonstrated in lakes (for zooplankton prey) and rivers (for macroinvertebrate prey). Fish predation can have a significant impact on the body size of prey assemblages; often large‐bodied prey are reduced in abundance, and indirect facilitation of small‐bodied prey occurs potentially initiating a trophic cascade. 2. Benthic communities in aquatic ecosystems also include a numerous and functionally important meiofaunal‐sized component, but in freshwaters the impact of fish predation on meiofaunal assemblages is unknown. We used a laboratory microcosm study to explore the impact of juvenile fish predation on the abundance and size structure of a riverine meiofaunal assemblage. 3. The presence of fish in our microcosms had no significant effect on overall meiofaunal (temporary and permanent) abundance. However, for the Copepoda, we found the first evidence of top‐down control of freshwater meiofaunal assemblages; in microcosms with juvenile fish, the abundance of large‐bodied Copepoda was significantly reduced, whereas small‐bodied Copepoda were significantly more abundant suggesting indirect facilitation. 4. We conclude that predation by juvenile fish can alter the structure of freshwater meiofaunal assemblages, although we do not yet know whether these relatively subtle changes are overwhelmed by large‐scale events such as flow disturbances.     

Intertidal zonation and latitudinal gradients on macroalgal assemblages: Species,functional groups and thallus morphology approaches

Macroalgae are unavoidable biological elements when monitoring and assessing costal environments. However, these tasks can be difficult to address because macroalgae a) present a high natural variability across a range of spatial and temporal scales, b) they imply a high sampling and laboratory processing effort and good taxonomical expertise (as they are a very diverse group of species), and c) there is insufficient knowledge about their structural and functional characteristics. This work addressed how the vertical (intertidal zonation) and horizontal (latitudinal gradient) variability of macroalgae assemblages are structured across continental Portugal, as well as how some surrogates for species-level biodiversity measures (namely functional groups and thallus morphology approaches) respond to such large-scale variability. Particularly, it was tested if intertidal zonation patterns are higher than fine-scale horizontal variation, and however, if vertical variation decreases along broad-scale horizontal variation. To do so, cover per species was taken (using a photographical and GIS methodological approach) from five sites located along the shoreline and along respective upper- mid- and lower-intertidal zones. The work findings include that both intertidal and latitudinal gradients impose deep structural changes on assemblages patterns. That is, broad-scale processes along Portuguese latitudes act as strongly as vertical stress gradients on assemblages patterns. Functional groups and thallus morphology approaches were useful to generalize the latitudinal assemblages patterns, where some groups emerge at the expense of others, and may improve biodiversity understanding and ecological synthesis. Because these surrogates decrease taxonomical expertise needs and can provide insight into the functional structure of macroalgal communities, their patterns founded may be particularly useful as reference data for further monitoring, so that shifts in such patterns might represent early warning surrogate approaches to detect environmental impact changes. Ultimately, to generate broader databases on rocky shore assemblages diversity (from species-level to functional groups and thallus morphologies approaches) can be useful for large-scale comparisons and for establishing ecological reference conditions, including for monitoring programs and environmental impact studies.     

Spatial and temporal variations in nematode assemblages in tropical seagrass sediments

The changes in the community dynamics of infaunal nematodes associated with tropical Australian intertidal seagrasses at 4 estuarine sites were investigated through 3 seasons (autumn, winter and spring). Nematode densities were highest in winter in all but one of the sites, ranging from 1971 to 3084 inds./10 cm², with one site showing a highest density of 3411 inds./10 cm² in spring. Multiple regression revealed significant correlations between nematode density peaks and seasonal changes in temperature, salinity and surface seagrass cover. Non-metric Multi-Dimensional Scaling revealed that the communities were characterised by relatively low within-site spatial variability but relatively high temporal variability through the three seasons. This temporal variability was largely due to significant increases in abundance of epistrate-feeding species in winter and spring. An investigation of the dominant epistrate-feeding nematodes revealed that predominantly infaunal species were responsible for overall winter and spring density increases. This study provides further evidence of the role played by temperature in regulating tropical, intertidal meiobenthic communities but also indirectly provides evidence of micro-scale seagrass canopy effects (micro-algal supply and availability) that may further enhance the impacts of these larger-scale seasonal environmental changes on the infaunal nematode community.     

The influence of mangrove-derived tannins on intertidal meiobenthos in tropical estuaries

Summary Mangrove-derived tannins negatively effected laboratory-reared nematode populations and natural communities of meiobenthos in tropical mangrove forests along the northeastern coast of Australia. In the low and mid intertidal zones of five mangrove estuaries, nearly all of the dominant meiofaunal taxa correlated negatively with concentrations of sediment tannins. Only nematodes correlated with low tannin concentrations in the high intertidal zones. The negative exponential equation y=be ^-mx represented the best-fit for most of the meiofauna-tannin relationships. The mangrove-dwelling nematode, Terschellingia longicaudata did not grow (r=0.001) in the laboratory on fresh, tannin-rich leaves of the red mangrove, Rhizophora stylosa. Population growth of the nematode was significantly greater on fresh, tannin-poor leaves of the grey mangrove, Avicennia marina (r=0.081) with best growth (r=0.112) attained on a diet of tannin-free, mixed cereal. These preliminary field and laboratory results suggest that hydrolyzable tannins leached from mangrove roots and leaf litter are an important factor regulating intertidal meiobenthic communities in tropical mangrove forests along the northeastern Australian coast.