Abstract:

Limiting nutrients, nitrogen and phosphorus, are of crucial importance in the structure and dynamics of freshwater plankton. Oligotrophic lakes are nutrient limited environments and therefore their plankton dynamics depends mostly on internal mechanisms of recycling. The plankton dynamics and food web structure of Andean lakes were analysed linking the biogeochemical cycles and classic food web ecology. As in other oligotrophic waters, the producer trophic level of Andean lakes is formed by a phytoplankton dominated by small cells. A particular feature of these lakes is that the herbivorous zooplankton is restricted to rotifers and small crustaceans with body length < 1 mm. The direct effect of grazing decreases nanoplanktonic cells abundance but, the indirect effect of zooplankton (through nutrient recycling) results in an increase in netphytoplankton, particularly diatoms. Changes ín the zooplankton constitution over the annual cycle may change the nutrient supply ratio. The copepod Boeckella gracilipes decreased P limitation lowering the N:P ratio whereas the dominance of the cladoceran Bosmina longirostris increased the N:P ratio and therefore increased P limitation during summer. In addition, predation by larvae of the authoctonous fish Galaxias maculatus change the quality and quantity of grazers since their predation on Boeckella nauplii provokes the failure of the summer cohort of the copepod. Consequently, zooplankton composition and the elemental ratios of recycled nutrients change. These results suggest that top-down and bottom-up effects of pelagic food webs cannot be easily separated.