DIATOM ASSEMBLAGES FROM COSTA-RICAN LAKES - AN INITIAL ECOLOGICAL ASSESSMENT

We compared the distributions of 59 diatom species in surface sediment s of 25 Costa Rican lakes with 21 environmental variables using canoni cal correspondence analysis (CCA). The distribution of taxa was relate d to the chemical and physical characteristics of the lakes. The most influential chemical variables were cation concentrations (especially magnesium) and related variables such as water hardness, pH, and tempe rature. Lake area and lake depth were among the most important physica l variables. A number of taxa were identified as potential environment al indicators. The diatoms Brachysira serians var. brachysira and Frus tulia rhomboides seem to be associated with low Values of alkalinity, hardness, Ca, Mg, and SiO2. Cymbella minuta var. silesiaca is associat ed with low to moderate values of alkalinity, hardness, Ca, and Mg. Ni tzschia cf. amphibia may be an indicator of moderate-to-high concentra tions of Mg. Pinnularia braunii var. amphicephala seems to prefer low values of hardness, Ca, Mg, and SiO2. In many closed lakes, these envi ronmental variables (Mg/Ca/hardness/alkalinity) increase with effectiv e evaporation. Consequently, these diatoms may be indirectly tracking P:E ratios. Results from this initial, small data set indicate the pot ential of diatoms for inferring lake paleochemistry, and perhaps P:E r atios, in Costa Rica.