ON THE CONTROLS OF PHYTOPLANKTON ABUNDANCE AND PRODUCTION IN COASTAL LAGOONS

The diversion of the Ebro River water to irrigate rice paddies and sub sequent drainage into coastal lagoons results in a shift from the seaw ater-dominated phase of the water column of the lagoons from December to May, to a freshwater-dominated phase for the remainder of the year. The species composition of phytoplankton within the lagoons changes a ccordingly. This ecosystem level shift provides a unique opportunity t o discuss the relationships between phytoplankton productivity and abu ndance and nutrient concentrations and the effect of water turnover on productivity of phytoplankton. During the freshwater phase, the nutri ent concentrations and ratios are such that nutrients seem unlikely to limit phytoplankton production. During the seawater phase, relatively more dissolved inorganic nitrogen (DIN) than phosphorus (SRP) is cons umed, the DIN:SRP ratio in the water is lowered below the Redfield rat io, the loss of nitrogen from the water is similar in magnitude and co ncurrent to the increase in particulate N, and changes of the DIN:SRP ratio and production rates are inversely related. All these relationsh ips point to the phytoplankton uptake as a mechanism lowering DIN. The assemblage of marine phytoplankton species prefers to use nitrate rat her than ammonium. The turnover rate of the water in the lagoons is no t slow enough for nitrogen recycling mechanisms to supply sufficient N for P to become the limiting element of the phytoplankton growth.