Deep chlorophyll maximum (DCM) in the Red Sea

Presence, distribution, and importance of deep chlorophyll maximum (DCM) in the Red Sea were examined from the oceanographic data gathered during four cruises between 2012 and 2015. The data, which are new and extensive for this sea region, showed that the DCM is a permanent feature in the Red Sea, characterized by a south-north gradient and spatially alternating highs and lows in strength. The north-south gradient is possibly related to the progressive sinking of Gulf of Aden Intermediate Water (GAIW) layer which is the source of nutrients to the DCM and the alternating patterns of Chl a concentrations in DCM, to the entrainment of GAIW in eddy circulations across the basin. The flow patterns of GAIW in the Red Sea thus become a major mechanism of sustaining the DCM in the Red Sea. While the DCM is important in the overall primary production, the Chl a content of DCM and its percent contribution to column production were much less than in other studies. This could be attributed, based on the distribution of phaeopigments/Chl a ratios, to grazing pressure. The composition of phytoplankton of DCM (as size fractions and taxonomic groups) and patterns of changes in it in response to climate changes could be useful avenues of future research in Red Sea thanks to the latter being already an extreme environment.