Epilithic diatom assemblages in rivers draining into Golfo Dulce (Costa Rica) and their relationship to water chemistry, habitat characteristics and land use

Abstract

Epilithic diatom assemblages from neotropical streams draining into Golfo Dulce (Costa Rica) were studied to determine how diatoms were related to measured environmental conditions. Twenty-three sites, affected by different land use practices, were selected and monitored for 21 months. Water chemistry was characterized by moderate specific conductivity (100 - 300 µS/cm), neutral to alkaline pH and moderate to high values of soluble reactive phosphorus (10-70 µg/L). Anthropogenic impact was reflected in lower oxygen saturation, elevated nitrogen compounds and measured sedi- ment load. A diverse diatom flora of 212 taxa was found. Patterns in the diatom distri- bution in relation to environmental variables, as revealed by canonical correspondence analysis, showed that riparian shading, pH, conductivity, current velocity and turbidity were the most important measured factors in determining species composition. The re- lationship between conductivity and pH was sufficiently strong to develop weighted- averaging regression and calibration models. Optima and tolerances of the most com- mon diatom taxa are presented.