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

Abstract

Nematode biomass spectra (NBS) for different nematode communities —subject to dif- ferent 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 inter- preting 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 regres- sions 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 undis- turbed sediments proved to be conservative: no differences in size distribution were found for com- munities 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 com- munities 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 corre- sponded 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.