Response of biofilm-dwelling nematodes to habitat changes in the Garonne River, France: influence of hydrodynamics and microalgal availability

Lotic epilithic biofilms are submitted to seasonal disturbances (e. g. flood events, self-detachment), which influence the biomass, diversity and viability of their algal and bacterial communities. The objective of this study is to examine whether (1) biofilm-dwelling nematodes respond to such seasonal changes in terms of diversity and community structure, (2) nematode species and feeding-types distribution respond to the varied trophic situations within the biofilm, since variations in biofilm microalgal composition may represent a variation in available food. The biofilm-dwelling nematode community was monitored in a temperate river over an 18 month period with a high sampling frequency. These data were linked to environmental abiotic and biofilm biotic factors. Nematode density was positively correlated to biofilm and microalgal biomass, but was dampened by floods. A clear seasonal pattern of the community was detected (summer shift), so that two nematode groups stand out: (1) the epistrate-feeders Chromadorina bioculata (Schultze in Carus, 1857) and Chromadorina viridis (Linstow, 1876) were primarily related to diatom availability, and dominated the nematode assemblage most of the time, (2) seven species from various feeding types (deposit-feeders, suction-feeders and chewers) grew mainly under summer conditions concomitantly to a change of biofilm trophic status and microalgal composition. Overall, the results suggested that, in addition to abiotic disturbances, the availability of potential preys in the biofilm might represent an important driver of nematode community patterns.