Ordination of phytoplankton communities in the Chapala Lake, Jalisco-Michoacan, Mexico

Abstract

We investigated what environmental variables explained major axes of compositional variation in phytoplankton communities at the Chapala lake. We used presence-absence data for the ordination analysis, which included 96 phytoplankton samples and 116 species. The primary gradient of the phytoplankton community structure, obtained from Bray-Curtis variance-regression, was explained by sulfates (r=0.751) and inversely by total alkalinity (r=-0.635). The secondary compositional gradient of phytoplankton communities, which was independent from the primary one, was explained by hardness of water due to Calcium (r=-0.617) and by total hardness (r=-0.602). The remainder of environmental variables (16) showed no relationship to any of the ordination axis. These data suggest habitat preference for most communities of phytoplankton toward higher sulfate concentration and toward low total alkalinity levels. Only few species showed specialization toward low sulfate concentration and toward high total alkalinity. Two groups of community were shown, which are separated from each other on differences on sulfates and total alkalinity levels. In addition these data showed a relationship to the last sampling period. A decrease in sulfur, in the form of sulfate, and an increase of total alkalinity from 1996 to 1998, due in part to a decrease of the lake level, associated to the El Niño effect, explained also a decline in species diversity through this period. In conclusion, alkalinity and sulfates seem to be major environmental factors explaining relevant compositional gradients as well as species richness of the phytoplankton communities at the Chapala Lake.