Vertical and temporal distribution of free-living nematodes dwelling in two sandy-bed streams fed by helocrene springs

We monitored for 1 year the sediment nematofauna of two headwater streams located near water resurgence from typical helocrene springs. Nematode community composition, benthic organic matter (BOM) and chlorophyll a ( BChl a) were assessed in two sediment layers (0-5 and 5-10 cm depth), providing insights into nematode vertical distribution. Globally, the density, diversity and functional richness of nematodes were lower in the upper sediments, although important amounts of BChl a were available there. The assemblages differed markedly between springs, with only 41 shared species from a total of 120 species, suggesting stochastic horizontal distribution even at small geographical scales(<5 km). Bacterial-feeding nematodes dominated in the upper sediments, whereas omnivorous, fungal-and plant-feeding nematodes thrived in the deeper sediments. Reproduction by the dominant algal-feeding species, Prodesmodora arctica, was detected in August. The substantial presence of juveniles throughout the year highlighted the importance of parthenogenetic reproduction.