Pigment-based phytoplankton community composition and primary production were investigated for the first time in the Red Sea in February-April 2015 to demonstrate how the strong south to north environmental gradients determine phytoplankton community structure in Red Sea offshore regions (along the central axis). Taxonomic pigments were used as size group markers of pico, nano- and microphytoplankton. Phytoplankton primary production rates associated with the three phytoplankton groups (pico-, nano- and microphytoplankton) were estimated using a bio-optical model. Pico- (Synechococcus and Prochlorococcus sp) and Nanophytoplankton (Prymnesiophytes and Pelagophytes) were the dominant size groups and contributed to 49% and 38%, respectively, of the phytoplankton biomass. Microphytoplankton (diatoms) contributed to 13% of the phytoplankton biomass within the productive layer (1.5 Zeu). Sub-basin and mesoscale structures (cyclonic eddy and mixing) were exceptions to this general trend. In the southern Red Sea, diatoms and picophytoplankton contributed to 27% and 31% of the phytoplankton biomass, respectively. This result induced higher primary production rates (430 ± 50 mgC m-2 d-1) in this region (opposed to CRS and NRS). The cyclonic eddy contained the highest microphytoplankton proportion (45% of TChla) and the lowest picophytoplankton contribution (17% of TChla) while adjacent areas were dominated by pico- and nanophytoplankton. We estimated that the cyclonic eddy is an area of enhanced primary production, which is up to twice those of the central part of the basin. During the mixing of the water column in the extreme north of the basin, we observed the highest TChla integrated (40 mg m-2) and total primary production rate (640 mgC m-2 d-1) associated with the highest nanophytoplankton contribution (57% of TChla). Microphytoplankton were a major contributor to total primary production (54%) in the cyclonic eddy. The contribution of picophytoplankton (Synechococcus and Prochlorococcus sp) reached maximum values (49%) in the central Red Sea. Nanophytoplankton seem to provide a ubiquitous substantial contribution (30-56%). Our results contribute to providing new insights on the spatial distribution and structure of phytoplankton groups. An understanding and quantification of the carbon cycle in the Red Sea was made based on estimates of primary production associated with pico-, nano- and microphytoplankton.

The export and fate of organic carbon in the mesopelagic zone are still poorly understood and quantified due to lack of observations. We exploited data from a biogeochemical‐Argo float that was deployed in the Red Sea to study how a warm and hypoxic environment can affect the fate of the organic carbon in the ocean's interior. We observed that only 10% of the particulate organic carbon (POC) exported survived at depth due to remineralization processes in the upper mesopelagic zone. We also found that POC exported was rapidly degraded in a first stage and slowly in a second one, which may be dependent on the palatability of the organic matter. We observed that apparent oxygen utilization (AOU)‐based loss rates (a proxy of the remineralization of total organic matter) were significantly higher than the POC‐based loss rates, likely because changes in AOU are mainly attributed to changes in dissolved organic carbon. Finally, we showed that POC‐ and AOU‐based loss rates could be expressed as a function of temperature and oxygen concentration. These findings advance our understanding of the biological carbon pump and mesopelagic ecosystem.

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.

Formation of intermediate waters in the northern Red Sea was captured by Seagliders during the winter of 2016. The analysis is based on measurements collected by consequent deployments of Seaglider autonomous underwater vehicles along a 70 km transect, which was run repeatedly from the coast to midbasin during the period from October 2015 to May 2016. The observations revealed typical open ocean convection processes including a preconditioning phase followed by deep convection and spreading of the newly formed waters through lateral exchange. Winter conditions were characterized by surface cooling and enhanced evaporation, which were observed from late October until late February. During this period, the gradual surface cooling and salinity increase resulted in the weakening of the surface stratification. Recurrent cyclonic gyres forced by strong atmospheric events led to episodic convection and the formation of intermediate seawater typical of the upper overturning circulation cell of the Red Sea. Following deep mixing, a remarkable bloom of chlorophyll was detected at the top layer of the water column in response to the nutrients delivery from the deeper layers. The end of winter was marked by the intrusion of lower salinity water presumably advected from the south and the reestablishment of the general cyclonic circulation, typical for the northernmost part of the Red Sea. To the best of our knowledge, this is the first observation study of such convection events in the northern Red Sea.