Recent nutrient enrichment and high biological productivity in the Labrador Sea is tied to enhanced winter convection

Abstract

The Labrador Sea is known for strong surface heat losses and deep ocean ventilation in the wintertime as well as high biological productivity and carbon export associated with extensive spring blooms. Using satellite-derived estimates of chlorophyll-a and net primary productivity, this study documents that the Labrador Sea has recently become more productive, evident in the form of more intense and spatially extensive phytoplankton blooms. The spatial and temporal variability in chlorophyll-a was compared against trends in nutrient concentrations across the Labrador Sea. Nutrient concentrations were obtained on a repeat hydrography line running from the Labrador to Greenland shelves. The 25-year long time series of nitrate, phosphate and silicate concentrations along the AR7 W line reveal decadal trends in the upper Labrador Sea. A comparison of these nutrients with basin-averaged annual mean surface chlorophyll-a concentrations reveals positive correlations in the upper 1000 m of the Labrador Sea. Furthermore, nutrient concentrations in the upper 1000 m were strongly correlated with wintertime convection depth. During years of strong winter convection, more nutrients are entrained from the deeper Labrador Sea, which then can be distributed through the Western Greenland and Irminger Currents and eddy activity, allowing sustained phytoplankton growth further north in the Labrador Sea.