How Winter Vertical Mixing Affects the Seasonal Surface Chlorophyll-a Signal in Oligotrophic Regions?

In oligotrophic regions of the global ocean, the seasonal cycle of surface chlorophyll-a concentration (Chl-a), which can be observed from space, consistently shows an increase in Chl-a in autumn and then, a winter maximum. This pattern was often attributed to an increase of winter phytoplankton production supported by an increase of the upward nutrient flux coinciding with the deepening of the mixed layer. However, with the development of autonomous observation platforms, which measure the vertical variability of physical and biogeochemical parameters at rather high temporal resolution, this last hypothesis is questioned.

Analysing data derived from Bio-Argo floats and collected in oligotrophic regions of the global ocean, we explored the interactions between mixed layer depth, Chl-a and nitrate concentration, especially during the autumn-winter period. We also attempted to explain the seasonal variability in the surface Chl-a signal. Comparing situations with and without vertical nutrient supplies, we tried to determine if consistent differences exist in surface winter Chl-a concentration between these two situations.

In oligotrophic regions of the North Atlantic and the Mediterranean Sea, the autumn-winter increase in surface Chl-a could be essentially attributed to changes in the vertical Chl-a distribution (i.e. shallowing of the deep chlorophyll maximum and homogenisation of the surface layer by vertical mixing). Only in some cases, the increase in winter surface Chl-a is clearly caused by a nitrate supply associated to vertical mixing. In those cases, the increase in the surface Chl-a signal is maintained up to late winter-early spring and could then be interpreted as a winter-spring bloom.