Physical mechanisms for biological carbon uptake during the onset of the spring phytoplankton bloom in the northwestern Mediterranean Sea (BOUSSOLE site)

Abstract

Abstract. Several trigger mechanisms have been proposed for the onset of the phytoplankton spring bloom. Among these, that phytoplankton cells begin to bloom when they experience higher average light levels in shallower mixed layers, a result of the surface net heat fluxes becoming positive and wind strength decreasing. We study the impact of these two forcings in the northwestern Mediterranean Sea. We take advantage of hourly measurements of oceanic and atmospheric parameters collected at two neighboring moorings during the months of March and April in the years 2016 to 2019, combined with glider data in 2016. The onset of phytoplankton growth is concomitant with the start of significant biological activity detected by a sudden decrease in dissolved inorganic concentrations derived from measurements in the upper 10 m of the water column. A rapid reduction in wind stress following high-wind events is observed at the same time. A resulting shallow mixing layer favors carbon uptake by phytoplankton lasting a few days. Simultaneously, the air-sea net heat flux switches from negative to positive because of changes in the latent air-sea heat flux, which is proportional to the wind speed. This results in an increased thermal stratification of the ocean's surface layers. In 2016, glider data show that the mixing layer is significantly shallower than the mixed layer at the onset of the phytoplankton bloom. We conclude that decreases in the mixing and mixed layer depths lead to the onset of the phytoplankton bloom due to the relaxation of wind speed following storms. We estimate net daily community production in the mixing layer over periods of 3 days between 2016 and 2019 as between 38 mmol C m−2 and 191 mmol C m−2. These results have important implications on the oceanic carbon cycle and biological productivity estimates in the Mediterranean Sea in a scenario of climate-driven changes of the wind.