Changes in and slow recovery of a meiobenthic nematode assemblage following a hypoxic period in the Gullmar Fjord basin,Sweden

Meiobenthos samples were collected from Gullmar Fjord, on the Swedish west coast, for just over 2 yr (December 1978–February 1981, inclusive), spanning a short period of severe hypoxia. Previously published analysis indicated no meiofaunal response at the major taxon level. Nematode samples were further analysed to the genus level. Data were analysed with both univariate and multivariate statistics. Although there was no immediate response to the hypoxic period, changes in assemblage structure at both genus and family level during the subsequent year may have been caused by it: diversity was reduced, multivariate assemblage structure was altered, and within-site variability was considerably increased. These changes may have resulted directly from the influence of hypoxia on reproductive success and juvenile survival and/or indirectly as a consequence of the complete disappearance of the macrobenthos (due to the hypoxia) which would alter macrobenthos-meiobenthos interactions. A year after the hypoxia there was no evidence that the nematode assemblage structure was returning to its pre-hypoxia state. This suggests that in physically undisturbed, deep sites, such as the Gullmar Fjord basin, recovery of nematode assemblages after environmental disturbance may be a lengthy process.     

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).     

Evaluation of combined morphological and molecular techniques for marine nematode (<Emphasis Type="Italic">Terschellingia</Emphasis> spp.) identification

Marine nematodes, which play an important role in many ecosystems, include a number of apparently cosmopolitan taxa that exhibit broad biogeographic ranges even though there is no obvious dispersal phase in their lifecycle. In this study, standard taxonomic approaches to marine nematode identification in conjunction with multivariate statistical analysis of morphometric data were compared with molecular techniques. Specimens of the marine nematode Terschellingia longicaudata that had been identified by their morphological features were investigated from a range of localities (East and West Atlantic, Bahrain, Malaysia) and habitats (estuarine, intertidal, subtidal) using molecular approaches based on the amplification and sequencing of the small subunit ribosomal RNA (18S rRNA). The study revealed that the majority of the morphologically defined T. longicaudata specimens share a single 18S rRNA sequence and apparently belong to a single taxon distributed from the British Isles to Malaysia. In addition, 18S rRNA analysis also revealed two additional sequences. One of these sequences was found in both the British Isles and Mexico, the other was recorded only from British waters. Individuals collected in Bahrain and identified from their morphology as T. longicaudata had two highly divergent 18S rRNA sequences. Separate morphological and morphometric approaches to identification of specimens from the same sites that had been formalin-preserved did not support evidence of multiple genotypes revealed previously by molecular analysis. Current taxonomy based on morphological characters detected using light-microscopy may be unable to discriminate possible species complexes. Biodiversity of marine nematodes may often be underestimated due to the presence of morphologically cryptic species complexes. High-throughput techniques such as DNA barcoding would aid in species identification but may require thorough analysis of multiple nuclear and mitochondrial molecular markers.     

Effects of physical disturbance on an estuarine intertidal community: field and mesocosm results compared

The response of an estuarine macrobenthic community to different intensities of physical disturbance was examined using comparable field and mesocosm experiments. In the field study, areas of sediment (2500 cm²) were disturbed by surface-raking at a range of disturbance intensities: no disturbance, once every 2 mo, once every month, once every 2 wk, once or three times every week. These disturbance regimes were also imposed on buckets of sediment (855 cm²) collected from the field study site and maintained within a mesocosm system. In the univariate data analysis, neither field nor mesocosm communities displayed any trends in relation to the frequency of disturbance. Diversity indices were generally reduced in the mesocosm communities compared with the field communities. Multivariate analysis indicated significant differences between the field communities subjected to the most intense disturbance and those communities disturbed every ±2 wk or less. These differences were attributed to reductions in the five numerically dominant species in the high- disturbance treatments. More statistically significant differences were observed between the different disturbance intensities in the mesocosm experiment than in the field study. The implications of differences between field and mesocosm experiments results are discussed. Received: 14 June 1999 / Accepted: 3 December 1999     

Methods for estimating peak physiological performance and correlating performance measures

Estimates of animal performance often use the maximum of a small number of laboratory trials, a method which has several statistical disadvantages. Sample maxima always underestimate the true maximum performance, and the degree of the bias depends on sample size. Here, we suggest an alternative approach that involves estimating a specific performance quantile (e.g., the 0.90 quantile). We use the information on within-individual variation in performance to obtain a sampling distribution for the residual performance measures; we use this distribution to estimate a desired performance quantile for each individual. We illustrate our approach using simulations and with data on sprint speed in lizards. The quantile method has several advantages over the sample maximum: it reduces or eliminates bias, it uses all of the data from each individual, and its accuracy is independent of sample size. Additionally, we address the estimation of correlations between two different performance measures, such as sample maxima, quantiles, or means. In particular, because of sampling variability, we propose that the correlation of sample means does a better job estimating the correlation of population maxima than the estimator which is the correlation of sample maxima.