Nutrient-rich saltwater and its influence on the phytoplankton of the patos lagoon estuary, Southern Brazil

Five cruises carried out in the main navigation channel of the Patos Lagoon estuary, southern Brazil, between June and December 1989, showed high concentrations of ammonium (up to 3.0 μM), nitrate (up to 13.0 μM), and phosphate (up to 3.0 μM) in the surface saltwater close to the lagoonʹs mouth. High values of chlorophyll a were also measured in euhaline waters. The influence of nutrient-rich saltwater on phytoplankton production was found in a sequence of weekly measurements conducted at a shallow fixed station between September and November 1989. Peaks of carbon uptake coincided with high nitrate concentration measured during the intrusion of saltwater. Possible causes for the positive relationship between salinity and nutrients such as sewage input, sediment resuspension and return of previously exported estuarine water to the lagoon are discussed.