Nitrogen vs. phosphorus limitation of algal biomass in shallow coastal lagoons

The relative importance of nitrogen and phosphorus limitation in shallow coastal lagoons was tested in replicate microcosms which were inoculated in summer with three algal groups and subjected to four levels of nutrient enrichment in five N: P ratios (20 treatments, n = 4). A subset of 12 treatments was repeated in spring. Algal groups were green macrophytes, phytoplankton, and benthic mats of cyanobacteria with associated 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 common than P in summer (14 of 20 treatments); differential limitation was not as clear in spring. Although biomass was never directly controlled by P, there were indications that P supply was more important to the cyanobacterial mats, especially in spring. While N directly controlled the biomass of macroalgae, macroalgae controlled the biomass of mats and phytoplankton. From this we hypothesize that macroalgae efficiently sequestered low concentrations of N and outcompeted the other groups. 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.