Standley, L. J., and B. W. Sweeney. 1995. Journal of the North American Benthological Society 14:38–49.
We investigated whether biota and sediments in river catchments consisting of primary forest in northwestern Costa Rica are contaminated by atmospheric transport of organochlorine pesticides from nearby regions where their use is intense. We measured organochlorine residues in stream mayflies, as well as in the bark and leaves of trees in catchments of an undisturbed dry tropical forest west of Volcán Orosí and Cerro Cacao (i.e., western catchments), and in tree leaves and bark in partially disturbed rain forest catchments east of Cerro Orosí (i.e., eastern catchments) in a region where agricultural activity is intense. Samples were solvent extracted, purified by silica gel clean-up, and concentrated before analysis by gas chromatography/mass spectrometry. Twelve organochlorine pesticides were detected in samples, including hexachlorocyclohexanes, heptachlor epoxide, endosulfans, DDT, DDE, dieldrin, endrin, endrin aldehyde, and aldrin. Endosulfans dominated the organochlorine signature of mayflies collected in the western catchments and of leaves and bark collected from eastern catchments. Leaves and bark collected in the undisturbed western catchments contained ten-fold lower concentrations of the endosulfans than those collected from the eastern catchments. Hexachlorocyclohexane isomers were dominated by the gamma isomer and were present at comparable levels in samples of leaves and bark collected from both sides of the volcanic ridge. An exception was high residues in bark collected from one eastern catchment, suggesting a local source. Stream mayflies and terrestrial vegetation in the undisturbed dry tropical forest contained substantial quantities of organochlorine pesticides, the most likely source being long- and short-range atmospheric transport.