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

[摘要] Several trigger mechanisms have been proposed for the onset of thephytoplankton spring bloom. Among these is that phytoplankton cells begin tobloom when they experience higher average light levels in shallower mixedlayers, a result of the surface net heat fluxes becoming positive and windstrength decreasing. We study the impact of these two forcings in thenorthwestern Mediterranean Sea. We take advantage of hourly measurements ofoceanic and atmospheric parameters collected at two neighbouring mooringsduring the months of March and April in the years 2016 to 2019, combinedwith glider data in 2016. We identify the onset of the surface phytoplanktongrowth as concomitant with the start of significant biological activitydetected by a sudden decrease in dissolved inorganic carbon derivedfrom measurements in the upper 10 m of the water column. A rapid reductionin wind stress following high-wind events is observed at the same time. Aresulting shallow mixing layer favours carbon uptake by phytoplankton lastinga few days. Simultaneously, the air–sea net heat flux switches from negativeto positive, linked to changes in the latent air–sea heat flux, which isproportional to the wind speed. This results in an increased thermalstratification of the ocean's surface layers. In 2016, glider data show thatthe mixing layer is significantly shallower than the mixed layer at theonset of the surface phytoplankton bloom. We conclude that decreases in themixing- and mixed-layer depths lead to the onset of the phytoplankton growthdue to the relaxation of wind speed following storms. We estimate net dailycommunity production in the mixing layer over periods of 3 d between 2016and 2019 as between 38 and 191 mmol C m −2 . Theseresults have important implications, as biological processes play a majorrole in the seasonal evolution of surface p CO 2 and thereby the rate ofreduction in atmospheric CO 2 by exchange at the air–sea interface.

[学科分类] 大气科学