Population dynamics of a cavernicolous nematode community in a chemoautotrophic groundwater system

Movile Cave harbors a completely isolated groundwater ecosystem. Although it lacks any input of photoautotrophically based food, it is inhabited by a rich invertebrate community. Chemoautotrophic microbial mats float at the surface of the thermomineral cave water, which is rich in sulfide and methane. The mats are mainly inhabited by dense populations of five bacterivorous nematode species and one predacious copepod species. As extensive investigations of these mats are hampered by the fragility and isolation of this ecosystem, we developed a model system allowing long-term studies of their meiofaunal communities. Using the original cave water (same aquifer), we carried out experiments in a laboratory basin that allowed us to simulate the labile oxic and sulfidic conditions of Movile Cave. A few weeks after the start of the experiment, floating microbial mats developed within hypoxic enclosures and were quickly colonized by highly adapted organisms, including a meiofaunal community dominated by five nematode species (Poikilolaimus sp., Monhystrella sp., Panagrolaimus c.f. thienemanni, Udonchus tenuicaudatus, and Chronogaster troglodytes). The faunal composition of samples from the original mat material was identical. In the laboratory, peak densities of 193.2 nematodes cm(-2) were reached after three months. By monitoring the nematode community structure over one year, we observed a sequential shift in the relative importance of the nematode taxa, in which those that were fast growing and r-selected (P. c.f. thienemanni, Poikilolaimus sp.) were gradually displaced by more K-selected species (C. troglodytes, U. tenuicaudatus). Similar oscillations were determined in samples of native mat material, apparently reflecting a cyclic succession characterized by decreasing microbial food availability.