A story of famine in the pelagic realm: temporal and spatial patterns of food limitation in rotifers from an oligotrophic tropical lake

Food limitation is one of the most important extrinsic factors influencing population dynamics in aquatic ecosystems, but the spatial patterns of resource limitation and their influence on zooplankton species have not been investigated. We tested the hypothesis that the occurrence and intensity of food limitation may vary both spatially and temporally, and that the irregular population dynamics of the rotifers Brachionus group plicatilis and Hexarthra jenkinae in a saline, warm-monomictic, oligotrophic lake (Alchichica, Central Mexico) can be attributed to resource limitation. We estimated plankton abundance and composition and performed 19 laboratory experiments to assess the intensity of food limitation (Delta r) at three selected depths in the water column. Most of the phytoplankton biomass consisted of algae inedible to rotifers, and good-quality food (phytoplankton <= 20 mu m + small heterotrophic flagellates) was frequently below the threshold food concentration for Brachionus species. The vertical distribution and abundance of rotifers were correlated with the availability of food resources. Food limitation was observed during mixing and late stratification; however, the intensity of limitation was rather similar throughout the water column owing to the overall scarcity of resources in the lake. Hexarthra seldom appeared. We conclude that the irregular pattern of rotifer abundance we observed is a consequence of the interplay between the harsh conditions of scarce food availability, resulting in intense resource competition and opportunities for rapid exploitation of opportunities when food outbursts occur.