Alpha and beta components of diversity of freshwater nematodes at different spatial scales in subtropical coastal lakes

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Small organisms usually occur in large numbers and are easily dispersed by passive means. Accord- ingly, one could expect these assemblages to exhibit low spatial structure, at least at small (m) and intermediate (1-100 km) scales. We assessed the community structure of freshwater nematodes (0.063 - 0.5 mm) associated with five spatial scales (ranging from 10 cm to 10 - 35 km) in three coastal lakes in southern Brazil. We assessed whether dissimilarities among assemblages are structured according to spatial scales, using an NMDS ordination. We also tested the hypothesis that most of the total diversity is associated with the intermediate (2 - 2.5 km) spatial scale that reflected distinct habitats (lakeshores), using additive partitioning of total species richness and two null models. We obtained 13,358 individuals and 59 morphospecies of nematodes. The NMDS ordination indicated that the nema- tode assemblages were mostly structured according to the spatial hierarchical sampling design. Lakes harbored dis- tinct nematode assemblages, and within-lake ordination indicated that the type of lakeshore (protected or exposed) and areas within lakeshores were important. The mean number of morphospecies recorded for the core sampling units (α1) was 12.26 -16.90, and represented 30 - 33 % of the total species richness found in the three lakes. Accord- ingly, beta diversity accounted for most of the total diversity. The partitioning for individual-based randomization over all lakes revealed that the observed local richness (14.19 species) was significantly lower than expected. The magnitude of the observed beta diversities in relation to the expectation for the lowest scales depended on the null model used. The observed beta diversity at high spatial scales, reflecting distinct lakes and habitat types, tended to be larger than the expected. The nematode assemblages showed intraspecific aggregation and low beta diversity at fine spatial scales, but high beta diversity at coarse spatial scales. We conclude that despite their microscopic size, the spatial distribution of nematode assemblages in lakes is mostly similar to the general pattern observed for macroscopic organisms.