Reprint Series No. 19

NUTRIENT LIMITATION OF PHYTOPLANKTON IN PADILLA BAY Anne Bernhard June 1993
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Abstract
The effect of nutrient additions on natural phytoplankton assemblages in Padilla Bay was examined from June to October, 1992 by short-term nutrient enrichment bioassays incubated in situ. Nitrogen additions (15 µM NH⁴) significantly (p<0.001) stimulated phytoplankton growth during all six experiments. Nitrate additions (15 µM) significantly stimulated phytoplankton growth in October, but not in September. Addition of silicate (15 µM SiO⁴), phosphate (1.0 µM P0⁴), or trace metals alone did not stimulate phytoplankton growth. In most experiments, phytoplankton growth was most enhanced by combined additions of nitrogen and phosphorus. In three of the experiments, the response of the nanoplankton to ammonium additions was compared to that of the total phytoplankton assemblages. Nanoplankton growth exceeded that of the total phytoplankton during August, but showed no response to ammonium additions in October. Additionally, DIN/P0⁴ ratios were below 16:1 during four out of six experiments, which would indicate the potential for nitrogen limitation. These results indicate that there is the potential for eutrophication in Padilla Bay as a result of nutrient inputs during the summer when light is less likely to be limiting phytoplankton growth.