COSTA-RICAN LAKES - PHYSICAL AND CHEMICAL-PROPERTIES OF COSTA-RICAN LAKES

Volcanic eruptions, landslides, rivers, glaciers, and human activity i n Costa Rica have formed hundreds of lakes. We surveyed 30 natural and artificial lakes ranging in elevation from near sea level to 3520 m. All are fresh (mean alkalinity, 62 ppm CaCO3; standard deviation, 55) and circumneutral in pH (mean, 6.5+/-1.3), regardless of location or e levation; the strongest correlate with elevation was temperature. Nati onwide, lake chemistries show low diversity, but we found that neighbo ring lakes can be surprisingly different in physical structure and che mistry, even when similar in morphology and setting. Ten lakes, all be low 1700 m, were stratified, albeit weakly; all are fairly deep or wel l-protected from wind. These 10 lakes are probably oligomictic, althou gh 1 of them (Laguna Rio Cuarto) tends towards meromixis. Most lakes i n Costa Rica are probably polymictic.