Examination of biogeochemical processes in tropical watersheds of Puerto Rico and Panama

U.S. Geological Survey, 3215 Marine Street, Boulder, CO 80303-1066, USA

Abstract

Research is concentrated on investigations of nutrient cycles, gas exchange, and weathering and erosion processes in small watersheds in northeastern Puerto Rico at the Luquillo Experimental Forest (LEF), and in central Panama at the Barro Colorado Nature Monument (BCNM). The LEF is the site of a recently initiated project funded under the Water, Energy, and Biogeochemical Budgets (WEBB) Program of the U.S. Geological Survey's Water Resources Division (WRD) and run through the Caribbean District Office of WRD. The LEF is readily accessible to researchers and is amenable to a complex research program. It is a region that climatologically and geologically resembles typical tropical environments more than any other area within U.S. territory. The BCNM includes Barro Colorado Island and surrounding mainland peninsulas in Gatun Lake, the largest artificial lake in the Panama Canal basin. The BCNM is under the stewardship of the Smithsonian Tropical Research Institute (STRI), which presently shares funding for the work.

Sampling is organized using a quasi-nested-basin approach. Micro-watersheds have been selected on contrasting lithologies [Puerto Rico--quartz diorite, basaltic volcaniclastics; Panama--andesite and basaltic volcaniclastics]. The micro-watersheds are used to identify and characterize processes in detail. Larger watersheds are used for the hydrologic and chemical budgets, because the spatial distribution of agricultural development and of many important hillslope processes, such as landslides, is sufficiently patchy and uneven that it is impossible to select representative micro-watersheds. In Puerto Rico, the larger watersheds are contrasted with geologically matched nearby watersheds that have been agriculturally developed. In Panama, the Panama Canal Basin, which has a detailed 80-year hydrologic record, is used as the reference watershed. Paired sites within the Panama Canal basin are used to determine the effects of development.

The primary objective of the studies of weathering and erosion is to use long-term chemical sampling and physical monitoring to characterize the processes that control the distribution and transport of major, important minor, and nutrient elements (Al, C, Ca, Cl, F, Fe, Ge, H, K, Mg, Mn, N, Na, O, P, S, Si, Sr, Ti, Zr) through soils, downslope, and out of the watershed. Phenomena of interest to global-change research include the fixation, storage, and export of carbon and nutrients as related to biogeochemical and geomorphic processes within the watersheds. The experimental design permits comparison between natural and developed environments. The exchange of methane and carbon dioxide with the atmosphere is being examined for different types of environments (forests, swamps, lakes) in light of variations in hydrologic conditions. The soil-gas work in Panama is completed and is among the first studies to quantify seasonalitv of methane emission in tropical habitats.

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