NITROGEN VS PHOSPHORUS LIMITATION OF ALGAL BIOMASS IN SHALLOW COASTALLAGOONS

The relative importance of nitrogen and phosphorus limitation in shall ow coastal lagoons was tested in replicate microcosms which were inocu lated in summer with three algal groups and subjected to four levels o f nutrient enrichment in five N: P ratios (20 treatments, n = 4). A su bset of 12 treatments was repeated in spring. Algal groups were green macrophytes, phytoplankton, and benthic mats of cyanobacteria with ass ociated epiphytes. Nutrients limited biomass when supply was less than the demand of the total community (supply < 0.270 mg N liter-1 d-1). For the wide range of N and P treatments, N limitation was much more c ommon than P in summer (14 of 20 treatments); differential limitation was not as clear in spring. Although biomass was never directly contro lled by P, there were indications that P supply was more important to the cyanobacterial mats, especially in spring. While N directly contro lled the biomass of macroalgae, macroalgae controlled the biomass of m ats and phytoplankton. From this we hypothesize that macroalgae effici ently sequestered low concentrations of N and outcompeted the other gr oups. Only when N supply exceeded macroalgal demand was N available to the other groups. The differences among algal groups and the temporal variation in N and P limitation help explain the conflicting evidence for nutrient limitation,