Structure changes in a planktonic food web: biotic and abiotic controls

We examined the response of the plankton community to strong changes in the volume of oligotrophic Lake La Caldera through the ice-free period of 1995-1997. As consequence of the changes in the precipitation regime, the lake volume diminished to 2 m depth during the extremely dry year of 1995. The increase in phosphorus availability for this year caused the diversification of the planktonic community, and the autotrophic:heterotrophic (A/H) ratio was below 1. In spite of the sporadic dust loads associated to atmospheric events, the quick recovery of the lake volumen in 1996 led to a severe phosphorus limitation. Phytoplankton community was dominated by Chrysophyceae (>90% of abundance) and large zooplankters were very scarce (<1 ind l-1). As a consequence, A/H ratio was over 1 and ciliates developed. Finally, towards the end of 1997, the zooplankton recovery led to a decrease of both A/H ratio and ciliates, and the lake returned to the conditions before drought. A decreasing tendency in both bacterial and phytoplankton densities, possibly as a consequence of top-down controls, was observed through the period of 1995-1997.