Nitrogen limitation of phytoplankton in a shallow embayment in northern Puget Sound

The effect of nutrient enrichments on natural phytoplankton assemblages was examined in six experiments conducted from June to October 1992. Short-term (4 d to 7 d) nutrient enrichment bioassays were incubated in situ in Padilla Bay, a slough-fed estuary in northern Puget Sound, Washington. Ammonium additions (15 μM) significantly (p<0.001) stimulated phytoplankton biomass accumulation during all six experiments. In two experiments, nitrate additions (15 μM) significantly stimulated accumulation of phytoplankton biomass during October, but not September. Addition of phosphate (1.0 μM) or silicate (15 μM) alone did not stimulate phytoplankton biomass accumulation during any of the experiments. In most experiments, phytoplankton response was greatest in combination treatments of ammonium and phosphate. Dissolved inorganic nutrient concentrations in the containers decreased during all incubations, but showed the greatest reduction in treatments receiving nitrogen. Dissolved inorganic nitrogen (DIN) to phosphate (PO₄ ^3?) ratios were below 16∶1 during all experiments, suggesting the potential for nitrogen limitation. In three experiments, the response of photosynthetic nanoplankton (<20 μm) to ammonium additions was compared to that of the total phytoplankton assemblages. Accumulation of nanoplankton biomass exceeded that of the total phytoplankton during two experiments in August but showed no significant response to ammonium additions in October. Results from the bioassays, the low DIN∶PO₄ ^3? ratios, and the reduction in nutrient concentrations in the containers provide evidence for potential nitrogen limitation of phytoplankton production during summer in Padilla Bay.