Shutdown of convection triggers increase of surface chlorophyll

The MIT Faculty has made this article openly available. Please share how this access benefits you. Your story matters. a b s t r a c t a r t i c l e i n f o The long-standing explanation of the triggering cause of the surface increase of phytoplankton visible in spring satellite images argues that phytoplankton biomass accumulation begins once the mixed layer depths become shallower than a 'critical depth'. However, a series of recent studies have found evidence for phytoplankton increase in deep mixed layers, and several hypotheses have been proposed to explain this early increase. In this manuscript it is suggested that the surface concentration of phytoplankton increases rapidly in a 'surface bloom' when atmospheric cooling of the ocean turns into an atmospheric heating at the end of winter. The hypothesis is supported by analysis of satellite observations of chlorophyll and of heat fluxes from atmospheric reanalysis from the North Atlantic. Satellite images show that the subpolar North Atlantic turns green every spring in response to an explosive surface increase of freely drifting microscopic algae, called phytoplankton. The primary production during this 'spring bloom' is of considerable interest to oceanographers, because it is the first link of the area's food chain and contributes significantly to global photosynthesis and ocean carbon uptake (Takahashi et al., 2009). It is generally believed that the increase in surface chlorophyll coincides with the onset of the spring bloom, when growth from photosynthesis first outweighs losses, driving primary production (e.g. Siegel et al., 2002). However Behrenfeld (2010) cautioned that net biomass increase may start earlier in the season without a signature in the surface phyto-plankton concentration, if ocean turbulence rapidly mixes the new phy-toplankton down into the deep ocean. We will therefore refer to changes in ocean color as the surface spring bloom to distinguish them from the proper spring bloom which represents the net increase of phytoplankton biomass throughout the entire water column. In this paper we test the hypothesis that surface spring blooms are associated with a change in air–sea heat fluxes and begin when winter cooling of the ocean switches to spring warming, thereby reducing vertical mixing of phytoplankton. The prevailing view is that the surface greening coincides with the spring bloom (e.g. Follows and Dutkiewicz, 2002; Siegel et al., 2002) and its onset can be explained with the " critical depth " hypothesis production, because the euphotic …