The effect of trophic state and depth on periphytic nematode communities in lakes

The aim of this study was to investigate whether nematode community patterns are shaped by nutrient and light availability. Accordingly, nematode communities inhabiting periphyton were investigated at gradual water depths (50, 150 and 250 cm) in three Swedish lakes showing graded trophic states. It was hypothesized that: (1) nematode density correlates positively with increasing nutrient availability and negatively with increasing depth; (2) increasing nutrient availability favors species and feeding type richness; (3) increasing depth favors deposit-feeders; and (4) differences in the algal composition of the periphyton affect the diet of algal-feeders. Our results showed that the biomass of periphytic algae increased with nutrient availability and decreased with increasing depth. Nematode density also increased with increasing trophic state. Species richness decreased with increasing depth in the investigated oligotrophic lake, while the opposite pattern was determined in the other two lakes. Lake trophic state significantly affected the trophic structure of the nematode community, with more algal-feeders occurring in the eutrophic lake whereas chewers were found only in the meso- and eutrophic lakes. Water depth was also shown to influence nematode feeding type structure, as in all lakes the abundance of deposit-feeders was greater at increasing depth. While diatoms dominated the periphytic algal community at all lakes and depths, analyses of the gut pigment content of nematodes showed that their diet shifted toward green algae in the oligotrophic lake and in shallow zones of the mesotrophic lake.