Status of the health conditions of subtropical Atezca Lake

Abstract

Four indicators of ecosystem function and health were monitored monthly during two years (1981–82 and 1983–84) in Atezca Lake, Mexico: (a) physical and chemical parameters; (b) species diversity of the phytoplankton community; (c) phytoplankton community structure, and (d) rate of population increase. In addition, phytoplankton species composition was examined in April and July of 1986 and 1988 to evaluate possible changes in the health of the lake.In both the annual cycles high concentrations of nutrients were found at the beginning of the stratification period, decreasing toward the end. During the mixing period, the amount of nutrients increased. Diatoms, dinoflagellates and blue–green algae appeared mostly in spring; in summer, green algae, dinoflagellates and some blue–green algae bloomed; and in winter, green algae dominated. Two of the three possible classes of population increase were recognized during each cycle in relation to environmental conditions. Class 2 species (0.2 < k′ > 0.4day−1 average increase) exhibited maximum increment during stratification and recirculation. Most of the Class 3 species (k′ < 0.2day−1) appeared during the entire annual cycle, but reached their maximum increment during the end of the stratification period. The species composition found in 1986 was similar to that of two earlier annual cycles. Changes in dominance in 1988 from green algae to blue–green algae suggest that eutrophic processes in Atezca Lake increased from 1984 to 1988. Studies are urgently needed to investigate this situation in detail.