Initial Limnology of Laguna Pozo Verde, Costa Rica: Bathymetry, Water, Sediments, and Diatoms

ABSTRACT. Introduction: Costa Rica has hundreds of lakes, many of which have never been scientifically studied. Objective: To carry out a first, basic limnological study of Laguna Pozo Verde in Juan Castro Blanco National Park, Costa Rica (~1935m elevation), to provide baseline data for studying future changes. Methods: We measured water depths and temperatures, and Secchi depth; analyzed surface sediments; and examined maps and satellite imagery. Results: Though described by some as formed by volcanic processes, Laguna Pozo Verde likely formed in a landslide, which occur frequently in this rainy area on the steep south slope of the inactive Porvenir Volcano. Our soundings showed a maximum depth of 9,25m near the center of the lake. The water was moderately transparent (Secchi depth 2,6m), with circumneutral pH, and temperatures from 15,9–18,1°C, with weak stratification at 0,5m. Surface sediments contained 27% organic matter, and had C/N ratios and stable carbon isotope values consistent with lacustrine algae and C3 plants; Surirella angusta composed over 90% of the diatoms. Conclusion: The lake is significantly shallower than reported, and surface sediments have a diatom assemblage unique among 88 lakes examined in Costs Rica.

By chance, Christian Peréz was creating a video at Laguna Pozo Verde while we were sampling. You can see the lake, our inflatable raft, and drone images in his youTube video posted here: https://www.youtube.com/watch?v=DUBwIUPrMIs.