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

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.

Wind-Forced Submesoscale Symmetric Instability around Deep Convection in the Northwestern Mediterranean Sea

During the winter from 2009 to 2013, the mixed layer reached the seafloor at about 2500 m in the northwestern Mediterranean Sea. Intense fronts around the deep convection area were repeatedly sampled by autonomous gliders. Subduction down to 200–300 m, sometimes deeper, below the mixed layer was regularly observed testifying of important frontal vertical movements. Potential Vorticity dynamics was diagnosed using glider observations and a high resolution realistic model at 1-km resolution. During down-front wind events in winter, remarkable layers of negative PV were observed in the upper 100 m on the dense side of fronts surrounding the deep convection area and successfully reproduced by the numerical model. Under such conditions, symmetric instability can grow and overturn water along isopycnals within typically 1–5 km cross-frontal slanted cells. Two important hotpspots for the destruction of PV along the topographically-steered Northern Current undergoing frequent down-front winds have been identified in the western part of Gulf of Lion and Ligurian Sea. Fronts were there symmetrically unstable for up to 30 days per winter in the model, whereas localized instability events were found in the open sea, mostly influenced by mesoscale variability. The associated vertical circulations also had an important signature on oxygen and fluorescence, highlighting their under important role for the ventilation of intermediate layers, phytoplankton growth and carbon export.

Modulations in vertical stability trigger intraseasonal variations in phytoplankton bloom

<p>Seasonal surface chlorophyll (SChl) blooms are very chaotic in nature, but traditional bloom paradigms have climbed out of these intraseasonal/subseasonal variations. Here we quantify these intraseasonal fluctuations and explore the links between the surface bloom and vertical stability at intraseasonal timescales, along the spring bloom region of the northwestern Mediterranean Sea. We use 8-day merged ocean color SChl product distributed by the European Space Agency Ocean Color Climate Change Initiative (ESA OC-CCI), in situ data from the long-term time series BOUSSOLE, wind stress and heat flux from ERA-Interim reanalysis. The 8-day SChl product from ESA OC-CCI has excellent data coverage in the northwestern Mediterranean Sea. Comparison with the high frequency in situ data from the BOUSSOLE mooring shows that the satellite and in situ SChl products are reliable at the subseasonal timescale. Our results reveal that intraseasonal variability explains nearly half of the SChl variability in the bloom region and occur at smaller spatial scales (~100 km). We demonstrate that weekly SChl fluctuations are in phase with weekly changes in wind stress and net heat flux during the initial state of the bloom in winter and early spring, thus expanding the convection shutdown hypothesis of bloom onset to subseasonal timescales. We postulate that intermittency in vertical stability due to short-term episodes of calm weather in winter or to stormy conditions in early spring leads to short-term variations in light exposure or to events of vertical dilution, and to strong intermittency in phytoplankton bloom.</p>