Impact of open-ocean convection on nutrients, phytoplankton biomass and activity

• Phytoplankton and nutrients were studied during a deep convection episode.
• Nutrients supplies were of 8.0 and 0.4×108 mol of nitrate and phosphate.
• Nutrients supplies were equivalent to the annual rivers discharges.
• Convection episodes counterbalance the decreasing silicate to nitrate surface ratio.
• We proposed new hypotheses explaining the triggering of the intense spring bloom.

We describe the impact of an open-ocean convection event on nutrient budgets, carbon budget, elemental stoichiometry, phytoplankton biomass and activity in the Northwestern Mediterranean Sea (NWM). In the convective episode examined here we estimated an input of nutrients to the surface layer of 7.0, 8.0 and 0.4×108 mol of silicate, nitrate and phosphate, respectively. These quantities correspond to the annual nutrient input by river discharges and atmospheric depositions in the Gulf of Lion. Such nutrient input is sufficient to sustain new primary production from 46 to 63 g C m−2 y−1, which is the same order of magnitude found in the NWM open waters. Our results together with satellite data analysis, propose new scenarios that explain the origin of the spring phytoplankton bloom occurring in NWM.