Ocean response to attenuation of visible light by phytoplankton in the Gulf of St. Lawrence

Abstract The presence of phytoplankton affects the attenuation of the visible range of solar radiation in the upper ocean. An optically-coupled circulation model is used to examine the impact of phytoplankton on sea-surface temperature, stratification, circulation and air–sea non-solar heat fluxes in the Gulf of St. Lawrence. The attenuation coefficients are calculated by a spectral model of irradiance in the visible range, using the SeaWiFS chlorophyll concentration as input. The circulation model is forced by climatological atmospheric fields and sea-ice distribution. The impact of phytoplankton is examined by comparing model results from two numerical experiments using a phytoplankton-dependent attenuation of solar radiation and a constant attenuation corresponding to clear seawater, respectively. The presence of phytoplankton increases the sea-surface temperature by up to 2 °C, and thus enhances the heat loss from the ocean to the air by up to 40 W m− 2. Phytoplankton cools the subsurface temperature by up to 2 °C. The associated increase of the vertical gradient of temperature leads to higher density stratification by up to 0.4 kg m− 3. Phytoplankton also enhance the general cyclonic circulation, causing upwelling in the central Gulf of St. Lawrence. These oceanographic changes and their feedback to the atmosphere indicate that the optical effects of phytoplankton can be significant, and should be included in models of ocean dynamics, especially those developed in the context of climate change. The negative feedback on sea-surface temperature by non-solar air–sea heat fluxes is investigated using a 1-d analytical model. An analysis of the model results shows that the feedback mechanism limits the effect of bio-optical heating to a short period and thus significantly reduces the impact of heating by the absorption of the solar radiation by phytoplankton. The net temperature increase is determined by the total absorption of light in the mixed layer, accumulated over a period of a few weeks.

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