Abstract: We visited 51 lakes in Costa Rica as part of a broad-based survey to document their physical and chemical characteristics and how these relate to the mode of formation and geographical distribution of the lakes. The four oxbow lakes were low in elevation and tended to be turbid, high in conductivity and C[O.sub.2], but low in dissolved [O.sub.2]; one of these, L. Gandoca, had a hypolimnion essentially composed of sea water. These were similar to the four wetland lakes, but the latter instead had low conductivities and pH, and turbidity was often due to tannins rather than suspended sediments. The thirteen artificial lakes formed a very heterogenous group, whose features varied depending on local factors. The thirteen lakes dammed by landslides, lava flows, or lahars occurred in areas with steep slopes, and were more likely to be stratified than most other types of lakes. The eight lakes that occupy volcanic craters tended to be deep, stratified, clear, and cool; two of these, L. Hule and L. Río Cuarto, appeared to be oligomictic (tending toward meromictic). The nine glacial lakes, all located above 3440 m elevation near Cerro Chirripó, were clear, cold, dilute, and are probably polymictic. Cluster analysis resulted in three significant groups of lakes. Cluster 1 included four calcium-rich lakes (average 48 mg [l.sup.-1]), Cluster 2 included fourteen lakes with more Si than [Ca.sup.+2] and higher [Cl.sup.-] than the other clusters, and Cluster 3 included the remaining thirty-three lakes that were generally less concentrated. Each cluster included lakes of various origins located in different geographical regions; these data indicate that, apart from the high-altitude glacial lakes and lakes in the Miravalles area, similarity in lake chemistry is independent of lake distribution.

Key words: Costa Rica, limnology, lakes, tropical, lake ecology, lake biogeography.

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Costa Rica has long been a mecca for tropical biology, yet most visiting biologists are probably unaware of the number and diversity of its lakes. Only in the past two decades has much attention been directed toward its lakes. In 1985, the Instituto Geográfico Nacional prepared an unpublishcd list of all lakes shown on its 1:50 000 scale topographic maps. The surprisingly high total of 652 lakes underestimated the total number of lakes, because those smaller than about 0.1 hectares were often not mapped. However, by the time the list was prepared, many of the floodplain lakes shown on the maps had already been drained for agriculture. Other lakes have been drained or filled subsequently, such that the total number of lakes in Costa Rica today is probably lower than reported by IGN--but still very high for a country of its size.

A very complete, recent overview of limnological research in Costa Rica was compiled by Umaña et al. (1999). Much of the early aquatic biology was concerned with disease vectors and water as a resource. Most publications on Costa Rican lakes have focused on a single lake (or on a small number of similar lakes) and how they have changed over time (see review...