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

Low-power, low-penalty, flip-chip integrated, 10Gb/s ring-based 1V CMOS photonics transmitter

Modulation with 7.5dB transmitter penalty is demonstrated from a novel 1.5Vpp differential CMOS driver flip-chip integrated with a Si ring modulator, consuming 350fJ/bit from a single 1V supply at bit rates up to 10Gb/s

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

The role of oxygen in the vertical distribution of nematodes: an experimental approach

The role of oxygen in the vertical distribution of nematodes was investigated by means of an experiment in which different oxygen conditions were imposed on sediment from an intertidal area of the Oosterschelde (The Netherlands). To test our hypothesis that the vertical distribution of the nematode assemblages was not influenced by changing oxygen conditions (e.g. nematodes do not migrate to favourable oxygen conditions), 5 cm sediment was inversed and incubated for 5 days at the lab. In a first treatment, food (diatoms) was added to the bottom; in a second treatment oxygen and food were added to the bottom. For each case and a control treatment, fresh, well-aerated Oosterschelde water was added on top of the sediment. The analysis of the field situation showed that nematodes were the most abundant taxon. Highest densities were observed in the subsurface sediment layer (1-2 cm). The lower abundance in the oxygen and algae-rich superficial layer (0-0.5 cm) could be due to the time of sampling relative to the tides or to biotic factors (e.g. macrofaunal activity). The vertical distribution of the nematode assemblages in the experimental and control treatments proved to be significantly different. An obvious segregation existed between the nematode species assemblage from the superficial (0-0.2cm) and the deeper layers (0.2-1 cm and 4-5 cm). Characterising genera for the superficial sediment layers were Daptonema, Ptycholaimellus, Prochromadorella and Microlaimus; for the deeper layers Terschellingia and Microlaimus. The occurrence of the first species assemblage is determined by the presence of free oxygen. The second species assemblage is adapted to the reduced sediment; nevertheless, artificial addition of limited amounts of oxygen to the deeper sediment layers favoured the assemblage as higher abundances were recorded. In general, oxygen seems to be important in determining the vertical distribution of nematodes in this experiment

Predictability of marine nematode biodiversity

In this paper, we investigated: (1) the predictability of different aspects of biodiversity, (2) the effect of spatial autocorrelation on the predictability and (3) the environmental variables affecting the biodiversity of free-living marine nematodes on the Belgian Continental Shelf. An extensive historical database of free-living marine nematodes was employed to model different aspects of biodiversity: species richness, evenness, and taxonomic diversity. Artificial neural networks (ANNs), often considered as “black boxes”, were applied as a modeling tool. Three methods were used to reveal these “black boxes” and to identify the contributions of each environmental variable to the diversity indices. Since spatial autocorrelation is known to introduce bias in spatial analyses, Moran's I was used to test the spatial dependency of the diversity indices and the residuals of the model. The best predictions were made for evenness. Although species richness was quite accurately predicted as well, the residuals indicated a lack of performance of the model. Pure taxonomic diversity shows high spatial variability and is difficult to model. The biodiversity indices show a strong spatial dependency, opposed to the residuals of the models, indicating that the environmental variables explain the spatial variability of the diversity indices adequately. The most important environmental variables structuring evenness are clay and sand fraction, and the minimum annual total suspended matter. Species richness is also affected by the intensity of sand extraction and the amount of gravel of the sea bed