The contribution of water hyacinth (Eichhornia crassipes) and zooplankton to the internal cycling of phosphorus in the eutrophic Pampulha Reservoir, Brazil

The contribution of zooplankton and the water hyacinth, Eichhornia crassipes, to the internal cycling of phosphorus was investigated in Pampulha Reservoir, in the shallow and eutrophic Pampulha Reservoir. In the last 20 years, algal blooms, as well as outbreaks of the macrophyte, have been observed with increasing frequency. Previous investigations have suggested that phosphorus is the limiting nutrient controlling the plant production in this ecosystem. A restoration program is currently underway aiming at the complete removal of external nutrient input. Therefore, the knowledge of the magnitude of some internal metabolic processes would be desirable in order to estimate the time lag that would be necessary for the recovery of water quality. The production and loss of macrophyte biomass was monitored in limnocurrals between May 1994 and April 1995. The zooplankton was also monitored during this period at a central sampling point. The biomass losses of macrophytes were followed in marked plants during their growing season. The P-content of macrophyte biomass was also measured monthly. Excretion rates of zooplankton collected in the reservoir were determined using short time experiments at the laboratory. This study demonstrated that zooplankton was more important for the P-cycling during the dry season when it was able to recycle a maximum of 26% d^sup -1^ of total phosphorus in lake water. The macrophyte contribution was more important during the rainy season (especially at the end of the rainy season). In April 1995, the macrophytes were able to recycle as much as 26% d^sup -1^ of the total phosphorus present in the water. At this time of the year, both communities recycled more than 40% d^sup -1^ of the total phosphorus available in water. Therefore, it was demonstrated that zooplankton and macrophytes play a key role in the internal cycling of a limiting nutrient in this reservoir.[PUBLICATION ABSTRACT]