Phytoplankton biomass cycles in the North Atlantic subpolar gyre: A similar mechanism for two different blooms in the Labrador Sea

Abstract

An analysis of seasonal variations in climatological surface chlorophyll points to distinct biogeographical zones in the North Atlantic subpolar gyre. In particular, the Labrador Sea appears well delineated into two regions on either side of the 60°N parallel, with very different climatological phytoplankton biomass cycles. Indeed, north of 60°N, an early and short spring bloom occurs in late April, while south of 60°N, the bloom gradually develops 1month later and significant biomass persists all summer long. Nevertheless, at climatological scale, the first-order mechanism that controls the bloom is identical for both bioregions. The light-mixing regime can explain the bloom onset in both bioregions. In the Labrador Sea, the blooms seem to rely on a mean community compensation irradiance threshold value of 2.5molphotonm-2d-1 over the mixed layer. Key Points Bioregionalization reveals two distinct regions in the Labrador Sea Both bioregions present very different bloom phenology The light-mixing regime can explain these two bloom phenologies.