Profundal benthic invertebrates in an oligotrophic tropical lake: different strategies for coping with anoxia

The deep benthic communities of tropical lakes are poorly understood with respect to their composition, abundance, biomass and regulatory factors. Whereas the hypolimnia of temperate oligotrophic lakes remain oxygenated, the higher temperatures in tropical lakes frequently lead to the rapid development of hypolimnetic anoxia independent of trophic status. The deep benthic communities of tropical lakes must therefore develop strategies to respond to anoxic conditions. The dynamics of the deep benthic community of Lake Alchichica were studied over 15 months. We hypothesized that the sedimentation of the winter diatom bloom constitutes an input of high-quality food that contributes to the establishment and development of the deep benthic community. However, the remineralization of this organic matter leads to the prompt development of hypolimnetic anoxia, thus limiting the establishment and/or persistence of the deep benthic community. In contrast with the diverse littoral benthic community (50 taxa) in Lake Alchichica, only two species constitute its deep benthic community, the ostracod Candona cf. patzcuaro and the chironomid Chironomus cf. austini, which combined exhibit a low density (1197±1976 ind m^-2) and biomass (16.13±30.81 mg C m^-2). C. patzcuaro is dominant and is present throughout the year, whereas Ch. austini is recorded only when the bottom water of the lake is oxygenated. A comparison with the analogous but temperate Lake Mergozzo in Italy illustrates the role that anoxia plays in tropical lakes by diminishing not only taxonomic richness (13 versus 2 spp. in temperate versus tropical lakes, respectively) but also abundance (1145 versus 287 ind m^-2, respectively). C. patzcuaro is found throughout the annual cycle of the lake’s profundal zone, entering into diapause during the anoxic period and recovering as soon as the profundal zone reoxygenates. Ch. austini has adjusted its life cycle to use the habitat and available resources while the bottom of the lake is oxygenated and emerges en masse at the onset of anoxia. The presence of oxygen and abundant fresh food favors the development of the deep benthic community and yields increases in density and biomass. However, the anoxia that rapidly occurs during the stratification period and the presence of hydrogen sulfide are the key factors that limit deep benthic community colonization and/or permanence in the profundal zone of the tropical Lake Alchichica.