Nematode biomass spectra as descriptors of functional changes due to human and natural impact

Nematode biomass spectra (NBS) for different nematode communities—subject to different forms of stress and enrichment—from the Belgian continental shelf have been constructed and analysed. These analyses showed that non-normalised NBS yield better results for comparisons of nematode assemblages than normalised NBS (in which the biomass in a weight class is divided by its corresponding weight interval) since the ecologically relevant information is retained. Normalising the spectra caused elevated biomass values and peaks to disappear, introducing bias when interpreting the distribution of biomass over spectra. Cumulative nematode biomass spectra proved to be useful in evaluating statistical differences, using the slope of the regression line of the cumulative biomass to the nominal value of a log2-based size class. Interpreting Pareto-type graphs and regressions was not straightforward. We suggest a combined use of both NBS and the regression approach for the analysis of NBS. NBS and cumulative NBS constructed for nematode communities from undisturbed sediments proved to be conservative: no differences in size distribution were found for communities from different locations. Physical disturbance, introduced by sand extraction, did not affect the regression slopes of cumulative NBS. However, a shift in peak biomass values towards lower size classes was observed in the regular NBS. This was attributed to an alteration of the nematode communities due to the frequent physical disturbance of the sediments. At an oxygen-stressed site, we observed a single class biomass peak, due to the presence of a single nematode species well adapted to the impoverished sediment quality. Phytoplankton sedimentation during a spring bloom corresponded to shifts in peaks in NBS due to a change in age structure of the nematode communities. Biomass values probably increased as a result of a higher food supply to the benthos

Tidal migration of nematodes on an estuarine tidal flat (the Molenplaat, Schelde Estuary, SW Netherlands)

The vertical distribution patterns of the nematode community and of the 10 most dominant nematode species on an intertidal flat in the Schelde Estuary (the Molenplaat, The Netherlands) are described at specific time intervals over a tidal cycle. The observed distribution profiles indicate that vertical migrations occur and are species-specific. The predatory Enoploides longispiculosus and the deposit-feeding Daptonema normandicum migrated upwards at incoming tide and downwards when the flat became exposed, while another deposit feeder,Daptonema setosum, did the opposite. Several abiotic and biotic factors may contribute to the observed patterns. Hydrodynamics, pore water drainage and episodic steep increases in temperature upon low tide exposure as well as vertical movements of prey organisms may have been of particular relevance at the time and site of sampling. However, the impact of each of these factors needs further investigation. The present study corroborates the dynamic nature of vertical distribution profiles of nematodes in intertidal sediments, highlighting the importance of sampling time with respect to the tide, as well as a species approach