NITROGEN AND PHOSPHORUS DYNAMICS AND RETENTION IN ECOTONES OF LAKE TITICACA, BOLIVIA PERU

We are studying present conditions and consequences of material moveme nt from land to water in the Lake Titicaca basin, and how fluxes are a ffected by human activities. The principal objective of this research is to describe and explain the variability in the Andean Altiplano of (a) water, nutrient and sediment fluxes from land and (b) composition, nutrient limitation and other important features of nearshore lake co mmunities, and compare the effects of different agricultural practices (especially traditional and modern) on these factors. We are focusing on a comparison of the impacts of two forms of agriculture in this re gion: ancient raised fields currently under rehabilitation, and flat p astures and fields, which are more common. Results of the first year o f study indicate there is substantial variability in nitrogen and phos phorus dynamics in relation to ecotone complexity (simple vs. intermed iate vs. complex). Raised field sites have the beneficial effect of re ducing high available nutrient concentrations (nitrate and soluble rea ctive phosphorus) and sediment load (measured as turbidity) as the wat er passes through them enroute to the lake. Aquatic vegetation (algae and macrophytes) reflect well ambient total nitrogen and phosphorus co ncentrations. Experimental nutrient limitation bioassays indicate that nitrogen is the most important limiting nutrient, though there is imp ortant spatial variability within the landscape, and phosphorus as wel l as nitrogen can be limiting.