Nutrient regulation of phytoplankton productivity in Monterey Bay, California

A series of nutrient enrichment grow-out experiments were conducted in Monterey Bay, California, to assess the relative importance of nutrient availability on growth rates and biomass accumulation of the natural phytoplankton assemblage. During a series of four cruises, enrichments with nitrogen (as nitrate and ammonium), silicate, and Guillardʹs “f ” medium consistently demonstrated that the phytoplankton were nitrogen limited, and that the addition of nitrate provided the most potential for growth and biomass accumulation. Contrary to previous reports for Monterey Bay, silicate was not found to limit the accumulation of biomass in this diatom-dominated system, although there was evidence that silicate additions can modify the uptake rates of the biomass-limiting substrate (nitrogen). We conclude that silicate is a regulating, but not limiting, nutrient in this study site. Our results are consistent with both the “shift-up” and “detrital” explanations for changes in specific uptake rates. During upwelling periods (May, September) when the biomass was dominated by phytoplankton, a shift-up type physiological response was observed that was not dependent on the uptake normalization procedure (e.g. chlorophyll versus PN). During the winter months (March, November), characterized by deep mixing, low light, and higher detrital N levels, the apparent shift-up response could be attributed to a change in the PN : Chl ratios and alleviation of light limitation due to the stable light regime provided by the enclosures.