NITROGEN LIMITATION OF PHYTOPLANKTON IN A SHALLOW EMBAYMENT IN NORTHERN PUGET-SOUND

The effect of nutrient enrichments on natural phytoplankton assemblage s 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 mu M) significantly (p < 0.001) sti mulated phytoplankton biomass accumulation during all six experiments. In two experiments, nitrate additions (15 mu M) significantly stimula ted accumulation of phytoplankton biomass during October, but not Sept ember. Addition of phosphate (1.0 mu M) or silicate (15 mu M) alone di d not stimulate phytoplankton biomass accumulation during any of the e xperiments. In most experiments, phytoplankton response was greatest i n combination treatments of ammonium and phosphate. Dissolved inorgani c nutrient concentrations in the containers decreased during all incub ations, but showed the greatest reduction in treatments receiving nitr ogen. Dissolved inorganic nitrogen (DIN) to phosphate (PO43-) ratios w ere below 16:1 during all experiments, suggesting the potential for ni trogen limitation. In three experiments, the response of photosyntheti c nanoplankton (<20 mu m) to ammonium additions was compared to that o f the total phytoplankton assemblages. Accumulation of nanoplankton bi omass 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:PO43- ratios, and the reduction in nutrient concentrations in the containers provide eviden ce for potential nitrogen limitation of phytoplankton production durin g summer in Padilla Bay.