Water, energy, and biogeochemical budgets in the Luquillo mountains, Puerto Rico

¹U.S. Geological Survey GSA Center 651 Federal Drive Guaynabo, Puerto Rico 00965-5703, USA

²U.S. Geological Survey, 3215 Marine Street, Boulder, CO 80303-1066, USA

Abstract

The Luquillo mountains, in eastern Puerto Rico, are the U. S. Geological Survey research site for the study of geomorphic and biogeochemical processes controlling the movement and transformation of water, energy, bedrock weathering products, and nutrients in the earth-surface environment. Two areas in the mountains will be studied: 1. the 11,300 hectare U. S. Forest Service-administered Luquillo Experimental Forest; and 2. the Río Grande de Loíza basin, an agriculturally-developed watershed. The principal research elements of the Water, Energy, and Biogeochemical Budget program (WEBB) in Puerto Rico are:

1. Biogeochemical Budgets: Calculation of water, energy, ionic, nutrient, sediment, and gas budgets in two micro-watersheds instrumented with meteorologic, soil, hydrologic, and ground-water monitoring equipment.
2. Physical and Chemical Weathering Processes in Undeveloped Watersheds of Contrasting Lithology: Comparison of chemical-weathering and mass-wasting processes in micro-watersheds within two dominant geologic terranes (volcaniclastic and quartz diorite). The effects of landslides on biogeochemical cycling in each region will be assessed by a temporal evaluation of physical, chemical, and mineralogic variation found in similar types of landslides of varying ages. This element will enable the development of models for hillslope hydrology and landform evolution.
3. Comparison of Agricultural and Forested Watersheds: Paired basins will be selected and gaged in the adjacent Rio Grande de Loiza watershed. Budgets of all aqueous constituents in flux (Ca, Mg, Na, K, Fe, Mn, Si, NH₄, HCO₃, Cl, SO₄, NO₃, NO₂, Br, F, PO₄, and dissolved organic carbon) will be compared and contrasted in impacted and undeveloped basins of similar lithology. Gas-flux differences between agriculturally developed and forested areas will be evaluated using chamber techniques and the results related to land-use differences.
4. Reservoir and Agricultural Pond Gas Fluxes: Most soils contain methanotrophic bacteria and consume methane to a greater degree than methane is produced at anaerobic microsites in the soil matrix. Methanogenesis is normally significant only in the sediments of shallow, standing water. In Panama, 1 hectare of shallow pond or lake produces approximately the same amount of methane as 400 hectares of pristine forest consumes. To calculate a budget for methane in northeastern Puerto Rico, methane production will be determined using bubble collectors in selected reservoirs and agricultural ponds.

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