Regional differences in the timing of the spring bloom in the Labrador Sea

The environmental factors that control the timing of the spring bloom in the Labrador Sea were investigated using ocean color data from satellites, historical oceanographic data and a primary production model. From the satellite data, the onset of the spring bloom was found to progress with the season from south to north, with the exception of the northern Labrador Sea. The onset for the northern Labrador Sea occurred at approximately the same time as in the southern Labrador Sea, Two possible factors for the regional differences were examined from the data: irradiance and mixed-layer depth (MLD). The early bloom in the northern Labrador Sea is related to the very shallow mixed layer associated with low-salinity water. The temporal variations of the mean chlorophyll concentration in 4 regions of the Labrador Sea (the Grand Banks, southern, central and northern Labrador Sea) during the growth phase of the spring bloom were modeled using a primary production model with MLD derived from historical temperature-salinity data. The temporal variations, in particular the early bloom in the northern Labrador Sea, were well reproduced by the model. A model sensitivity study showed that the onset of the spring bloom is highly sensitive to MLD. This confirms the finding from the data that MLD is a critical environmental factor for the spring bloom. The availability of the photosynthetically active radiation plays an important role, but is not a determining factor of the early bloom in the northern Labrador Sea. The sensitivity study also showed that pre-bloom oceanographic conditions and attenuation of light in water have a greater effect on the timing in a shallow mixed layer than in a deeper mixed layer.