Andrea J. McEvoy

The specificity of meiobenthic community responses to different pollutants: Results from microcosm experiments☆

In preliminary microcosm experiments we have been able to demonstrate a clear differential response of meiobenthic assemblages to zinc, copper and cadmium contamination. Sediment and natural meiobenthic communities were collected from two estuaries, the Lynher (mud with a high organic content) and Exe (sand with a low organic content). The sediments were dosed separately with zinc, copper and cadmium at three different dose levels. The meiobenthic community structure from both sites was unaffected by cadmium at any of the dose levels. The communities in the zinc and copper treatments were significantly different (ANOSIM p < 0.05) from each other and from the controls (and cadmium treatments). Although the differential response occurred in both sediment types, its intensity was greater in the sand than the mud. The Exe sand communities were much more strongly affected by the contaminants, even at the lowest dose, copper having a more severe effect than zinc. The Lynher mud meiobenthos was most strongly affected by the zinc treatment and there was a graded response to both zinc and copper with significant differences in the communities at different doses (ANOSIM p < 0.05). These differences between sediments could be due to the binding of metals onto organic material, thus reducing their availability to the fauna.

The use of offshore meiobenthic communities in laboratory microcosm experiments: response to heavy metal contamination

Abstract A microcosm, originally developed for intertidal, estuarine meiobenthic communities, has been used to determine the effects of the heavy metals copper, zinc, cadmium and lead on offshore meiobenthic nematode communities. Significant differences were observed in community structure between controls and all metals except cadmium. The dose response of the offshore meiofauna to experimental contamination was rather confusing as copper and zinc low doses appeared to have much more drastic effects than the high doses. We speculate that at the highest copper and zinc dose levels the metals acted as preservatives such that animals died but did not decompose. This indicates that metals will affect the microbial component of the sediment as well as the meiobenthos in this type of experimental design. The response to the contaminants of offshore sediment biota differed from that previously observed in intertidal estuarine biota. This may be because fauna in the estuarine environment are subjected to greater levels of natural physicochemical stress and are therefore more generally tolerant. This suggests that environmental impact assessments should bioassay communities which naturally inhabit the environment to be assessed. The methods used have potential in the development of a community level bioassay particularly since it appears that the dominant nematode component of meiobenthic communities, from a wide range of habitats, can be easily maintained in simple microcosms.

The influence of Atrina zelandica Gray on meiobenthic nematode diversity and community structure

The influence of the large suspension feeding horse mussel Atrina zelandica Gray on meiobenthic diversity and community structure has been studied at two sites in New Zealand where Atrina beds form discrete patches. Community attributes inside (IN samples) and outside the beds (OUT samples) in the bare sediment have been compared. At both a sheltered muddy harbour site and a more exposed coastal sandy site, significant effects were found. These were stronger, however, at the sheltered site: The presence of Atrina resulted in (i) a greater reduction in several univariate measures (including Shannon diversity), (ii) more pronounced changes in the form of k-dominance curves, (iii) greater differences in species composition as revealed by the R-statistic of ANOSIM, (iv) a higher value for average Bray Curtis dissimilarity between IN and OUT samples and, finally, (v) greater variability among the IN samples compared with the OUT samples as measured by the Index of Multivariate Dispersion. Variations in density of different nematode functional groups indicate that the detrimental effects of Atrina on meiobenthic biodiversity may be attributed to its activity (production of biodeposits) and physical presence, resulting in reduced oxygen concentrations in the sediment. At the exposed coastal sandy site, the mussels were older and probably produced less biodeposits per unit biomass, and also the wave-induced flows over the bed are more likely to result in sediment mixing, which will tend to blur local effects.

EXPERIMENTAL EFFECTS OF TRIBUTYLTIN (TBT) CONTAMINATED SEDIMENT ON A RANGE OF MEIOBENTHIC COMMUNITIES

Microcosm experiments have been carried out with whole natural meiobenthic communities to look at the effects of TBT sediment contamination on the community structure of the dominant nematode component of the meiobenthos. TBT has a high affinity for aquatic sediments, yet this is the first study of the effects of this contaminant in sediment on natural benthic communities. Three communities were studied from contrasting locations in south-west England: the intertidal of the Lynher estuary (muddy sediment) and the Exe estuary (sandy sediment) and the subtidal (50m depth) at Rame Head off Plymouth (muddy sand). Fresh sediment with natural meiobenthic communities was incubated for 2 months with TBT-contaminated sediment (three dose levels) in bottles. Nematodes were identified and enumerated and subjected to multivariate data analysis. The sandy Exe estuary fauna was significantly affected by TBT-contaminated sediment at all three doses (0.3, 0.6 and 0.9 microg g(-1) dry wt (as Sn) sediment), whereas the offshore fauna from Rame Head was significantly affected only at the highest dose. The muddy Lynher estuary meiofauna was affected (somewhat peculiarly) at the medium dose level only. Meiobenthic nematodes may not be as sensitive to TBT-contaminated sediment as other infaunal benthos but exhibited responses to levels of contamination still persisting in some UK estuaries and harbours. Comparing the effects of TBT with those of copper and zinc in the same laboratory experiments, our observations suggest that the relative impact of TBT on meiobenthic community structure is not as great as these contaminants in marine sediments. Although there are very few observations of TBT toxicity in sediment, it appears that TBT is toxic at much lower concentrations in seawater (ppb) than it is in sediment (ppm).

Metabolically active, non-nitrogen fixing, Trichodesmium in UK coastal waters during winter

Trichodesmium, a colonial cyanobacterium typically associated with tropical waters, was observed between January and April 2014 in the western English Channel. Sequencing of the heterocyst differentiation (hetR) and 16S rRNA genes placed this community within the Clade IV Trichodesmium, an understudied clade previously found only in low numbers in warmer waters. Nitrogen fixation was not detected although measurable rates of nitrate uptake and carbon fixation were observed. Trichodesmium RuBisCO transcript abundance relative to gene abundance suggests the potential for viable and potentially active Trichodesmium carbon fixation. Observations of Trichodesmium when coupled with a numerical advection model indicate that Trichodesmium communities can remain viable for >3.5 months at temperatures lower than previously expected. The results suggest that Clade IV Trichodesmium occupies a different niche to other Trichodesmium species, and is a cold- or low-light-adapted variant.

Seasonality of Oithona similis and Calanus helgolandicus reproduction and abundance: contrasting responses to environmental variation at a shelf site

The pelagic copepods Oithona similis and Calanus helgolandicus have overlapping geographic ranges, yet contrast in feeding mode, reproductive strategy, and body size. We investigate how these contrasting traits influence the seasonality of copepod abundance and reproductive output under environmental variation, using time series data collected over 25 years at the Western Channel Observatory station L4. The proportional change in Egg Production Rate (EPR, eggs female-1 d-1) over the annual cycle was ~10-fold and similar for both species, although EPR of O. similis was only ~ 11% that of C. helgolandicus. The timing of EPR maxima for O. similis coincided with increased Sea Surface Temperature (SST) in summer, likely due to a temperature-dependent brooding period. Conversely, EPR of broadcast spawning C. helgolandicus was more strongly related to Net Heat Flux (NHF) and diatom biomass, both parameters associated with the spring phytoplankton bloom. In both species, female body mass negatively correlated with SST, with a 7.5% reduction in body mass per °C in C. helgolandicus compared to just 2.3% in O. similis. Finally, seasonality of EPR and adult and copepodite abundance was strongly decoupled in both species, suggesting that optimum conditions for reproduction and abundance occur at different times of the year.