On two different opportunities the distribution and diversity of the benthic macrofauna was studied at different depths in the Laguna del Cerro Chato erater lake, Alajuela, Costa Rica. For both surveys, the deep samples had signifieantly lower species diversity and evenness than the shallow samples. There was a drop in species diversity and evenness at 6 m, followed by an inerease at 9 m. This pattem is associated with fluctuations in the lake's thermoc\ine. Chironomids were the most abundant organism in both surveys and the genus Chironomus dominaled at all depths. Analysis of the elements Ca, Cu, S, N, Fe, P and percentage of organic matter in the bottom sediments did not show a c\ear bathymetrical distribution. However, al a depth of 9 m the increase in both N content and percentage of organic matter could be related to the increase in abundance of organisms.
Keywords: volcanic lake, benthos, macrofauna, aquatic insects, chironomids, Costa Rica