El plancton de las lagunas de gravera y el fósforo: el enriquecimiento de las paradojas

Abstract

Several hypotheses have been outlined on the relationship of lake communities and phosphorus enrichment in recent years. Some of them are the following: i) a positive relationship between phosphorus and planktonic biomass, ii) species richness peaks at intermediate phosphorus concentrations, iii) the destabilisation of food web functional groups arising from phosphorus enrichment (called the paradox of enrichment), iv) the plankton biomass as the outcome of interactions between the limiting resource (phosphorus in most lakes) and the food web, and v) the remarkable influence of phosphorus enrichment and piscivorous fish on planktonic size spectra. To test these hypotheses, we carried out a seasonal study on plankton communities of sixteen gravel-pit lakes for fifteen months in the river Jarama plain (Madrid, Central Spain). These lakes showed a wide range of average phosphorus contents (36-2500 µg P/L), piscivorous fish lived in some of them and most harbour benthic, omnivorous ciprinids. Hypothesis i was only demonstrated for bacteria, phytoplankton and rotifers. Hypothesis ii was refuted since species richness peaks occurred in hypertrophic lakes. Hypothesis iii was not supported by our data, and we even found a stabilisation of bacterial and edible phytoplankton populations along with phosphorus enrichment, such a stabilisation being a likely result of omnivory by copepods and ciprinids. As expected, piscivorous fish influenced cladoceran and bacterial density whereas phosphorus enrichment increased phytoplankton biomass, but the top-down effect did not affect phytoplankton, rotifers and copepods, thus supporting the bottom-up:top-down model against the trophic cascade model. Both phosphorus and piscivorous fish