Nitrate and phytoplankton production in southern California coastal waters1

The transport of nitrate into the euphotic zone appears to be a major factor regulating the standing stock and production of phytoplankton in southern California coastal waters. The evidence is as follows. 1) The rate of photosynthetic carbon assimilation is proportional to the rate of nitrate assimilation and to the ratio of nitrate:total N assimilated. 2) The phytoplankton standing stock (g C·m −3 ) and its production rate are related to the depth of the vertical nitrate concentration gradient. 3) The chemical composition of the particulate organic matter, as the POC:PON ratio, is related to the carbon:nitrogen assimilation ratio of the phytoplankton. 4) Regenerated production, measured as ammonium assimilation, is proportional to the nitrate assimilation rate, implying parallel and concurrent increases in the production of heterotrophic microplankton and phytoplankton attending new inputs of nitrate into the euphotic zone. 5) The vertical diffusion of nitrate, when calculated from the vertical nitrate concentration gradients and nitrate assimilation rates, appears to give reasonable estimates of the vertical eddy diffusivity coefficient for nitrate.