Phytoplankton Growth and Regulation in the Labrador Sea: Light and Nutrient Limitation

The importance of light and nutrients in regulating phytoplankton growth in the Labrador Sea was evaluating using climatological data, 12 years of measurements made as part of the Labrador Sea Monitoring Program (LSMP) and physiological information from the literature. Light limits primary production and phytoplankton growth much of the year, even during summer when surface irradiance is at its seasonal peak. Nutrients, nitrogen (nitrate) and silicate, are reduced to low levels in surface waters in summer/autumn and can limit phytoplankton production and growth at this time of year. Nitrate appears to be the nutrient in shortest supply on the Labrador Shelf while silicate is in shortest supply in the central Labrador Basin. Multiyear trends in regional hydrography (increases in water temperatures, decreases in mixed layer depth) and changes in nitrate and silicate supply over the past decade (i.e. increases in nitrate and decreases in silicate) may be linked to changes in phytoplankton community composition and structure (i.e. decreases in large forms such as diatoms and increases in small picoplankton and nanoplankton forms).