Nutrients and mixing, chlorophyll and phytoplankton growth

Abstract During a cruise to the Sargasso Sea in April 1985, we observed an event of wind-induced mixing during a 4-day sequence of observations while operating in a Lagrangian sampling mode. The sharp increase in wind stress was followed by a sharp increase in nitrate concentration in the euphotic zone. The nitrate declined rapidly, and over the next 2 days the quantity of chlorophyll a in the euphotic zone increased by a factor of three. The phytoplankton community was dominated by diatoms; this and other evidence indicates that the events observed were part of the spring bloom in the north Sargasso Sea. These observations are interpreted in terms of laboratory models for nutrient-dependent phytoplankton growth. The cell-quota model of Caperon and Droop provides an internally consistent explanation of the observed data. The data also suggest the notion of “nutrient switching” (rather than a multiplicative form of nutrient interaction) in the interaction of nitrate and silicate, although this could not be verified.