Regionalization of the Red Sea based on phytoplankton phenology: a satellite analysis

The current average state of Red Sea phytoplankton phenology needs to be resolved in order to study future variations that could be induced by climate change. Moreover, a regionalization of the Red Sea could help to identify areas of interest and guide in situ sampling strategies. Here, a clustering method used 21 years of satellite surface chlorophyll-a concentration observations to characterize similar regions of the Red Sea. Four relevant phytoplankton spatiotemporal patterns (i.e., bio-regions) were found and linked to biophysical interactions occurring in their respective areas. Two of them, located in the northern part the Red Sea, were characterized by a distinct winter-time phytoplankton bloom induced by mixing events or associated with a convergence zone. The other two, located in the southern regions, were characterized by phytoplankton blooms in summer and winter which might be under the influence of water advected into the Red Sea from the Gulf of Aden in response to the seasonal monsoon winds. Some observed inter-annual variabilities in these bio-regions suggested that physical mechanisms could be highly variable in response to variations in air-sea heat fluxes and ENSO phases in the northern and southern half of the Red Sea, respectively. This study reveals the importance of sustaining in situ measurements in the Red Sea to build a full understanding about the physical processes that contribute to phytoplankton production in this basin.