Biological evidence of a winter convection event in the South Adriatic: A phytoplankton maximum in the aphotic zone

An unusual phenomenon was recorded in the deep South Adriatic in February 2008: phytoplankton reached their maximum abundance in the aphotic zone. This was associated with strong downward flow induced by (1) surface cooling during bura (the region's cold, dry north wind) events and (2) ingression of more saline Eastern Mediterranean Water. Maximum abundance of microphytoplankton (MICRO) and nanophytoplankton (NANO), 1.4×104 and 6.8×105 cells L−1, respectively, was at 400 m in the South Adriatic Pit. Diatoms dominated deep microphytoplankton samples. Atypically, autotrophic picoflagellates were found throughout the water column, with high abundance in the aphotic layers. Zooplankton did not show such dramatic deviations from their usual vertical distribution, but the abundance and relative contribution of some species between 200 and 800 m layer were markedly different.

Research highlights
► High phytoplankton abundance in the aphotic zone is the evidence of deep convection.
► Convection affects nano- and microphytoplankton distribution the most.
► General pattern of zooplankton distribution is not affected by convection.
► The whole mixing layer should be sampled for real estimation of primary production.