Linking estuarine nematodes to their suspected food. A case study from theWesterschelde Estuary (south-west Netherlands)

The present study investigates correlations between abundances of nematodes (at the genus level) and benthic microalgae on an intertidal mudflat in th e Westerschelde Estuary (south-west Netherlands), using both multi- and uni variate methods. Two sample series, covering surface areas of 10 cm(2) (mei oscale) and 1.25 cm(2) (microscale) per sample were analysed. Trophic type analysis indicated that an average of 31% of the nematode community were ca ndidate grazers of microalgae. Multivariate data analysis indicated that on ly a limited part of the variation in the nematode data could be explained in relation to pigments. Total nematodes did not show any correlation with the pigment data. On the meioscale, the genera Tripyloides and Calyptronema correlated negatively with chlorophyll concentration (chl-a and chl-c, res pectively), while Prochromadorella correlated positively with the ratio of fucoxanthin to chi-a, a ratio which at the present sampling site can be con sidered to be a measure of the proportion of diatoms in the total microalga l standing stock. On the microscale, up to ten genera, comprising 76% of to tal nematode numbers, were correlated with pigments. A majority (74%) corre lated specifically with the ratio of fucoxanthin to chi-a, while much fewer nematodes showed a direct correlation to pigment concentrations. Whereas m any of these correlations could be explained in terms of direct trophic lin ks, several others probably represented indirect relationships, trophic or other. Food densities may be less important structuring factors of nematode communities on tidal flats than relative abundances of particular food sou rces. It is suggested that nematodes actively migrate towards 'optimal' foo d patches, and that this dynamic aspect of nematode-microalgae correlations is best revealed at a spatial-scale small enough to allow a rapid response of nematodes to changes in adjacent patches. Apparently, the microscale us ed in the present study is more adequate for the study of such intricate in teractions than the meioscale.