Response of Red Sea phytoplankton biomass to marine heatwaves and cold-spells

In tropical oceans, phytoplankton experience significant alterations during marine heatwaves (MHWs), yet the consequences of reduced or absent marine cold-spells (MCSs) on these microscopic algae are currently overlooked. Synergistically combining in situ measurements, Argo-float data, remotely-sensed observations, and hydrodynamic model outputs, we explore such relationships in the Red Sea. Results show a long-term (1982 to 2018) gradual increase in MHW days (5–20 days/decade) and a clear decrease in MCS days (10–30 days/decade). Compound extreme temperature and chlorophyll-a events (Chl-a – an index of phytoplankton biomass) exhibit consistently lower Chl-a concentrations during MHWs and higher ones during MCSs, particularly in the northern and southern Red Sea. In these regions, during the main phytoplankton-growth period, the presence of MHWs/MCSs leads to respective Chl-a anomalies in 94% of the cases. Yet, phytoplankton responses in the central Red Sea are more complex, most likely linked to the region’s highly dynamic circulation (e.g., mesoscale anti-cyclonic eddies), and multiple nutrient sources. In the naturally warm and stratified ecosystem of the Red Sea, where deeper mixed layers enhance the transfer of nutrient-rich waters to the lit zone, the substantial reduction of MCSs could be more impactful for phytoplankton than the gradual rise of MHWs. 

Biomass Chlorophyll Cold Temperature Ecosystem Hot Temperature Indian Ocean Phytoplankton