Eddy Driven Chlorophyll Bloom in Northeastern Red Sea during early Spring , 2015

Few in situ observations of phytoplankton biomass and productivity exist for the northern Red Sea. Typically, remotely sensed chlorophyll concentrations are low in the northern Red Sea except between November and April (Raitsos, et al., 2013). Acker et al. (2008) showed patches of chlorophyll greater than 1-2 µg/L in March 2000. Towed undulating vehicle (TUV) mapping in late April 2015 provided insight into both horizontal and vertical structure of the region. The TUV map showed eddy related chlorophyll features consistent with the observations of Acker et al. (2008). A MODIS-Aqua chlorophyll image for 26 April confirms chlorophyll concentrations >1.0 mg m^-3 associated with a cyclonic eddy centered around 27.05°N/34.7°E. TUV measurements through the eastern flank of the eddy showed vertical entrainment of cooler, lower dissolved oxygen sub-thermocline water in the vicinity of the eddy. Chlorophyll maxima ranged between 30 and 80m depth with concentrations as high as 1.6 to 2.4 mg m^-3. Mean chlorophyll in the upper 20m is 1.8 mg m^-3 while the column-integrated chlorophyll is 29 mg m^-2 at this location. Chlorophyll fluorescence in subsurface maxima reached ~2.73 mg m^-3 around 27°N/35°E. Our CTD profiles showed well developed near homogeneous layers (>100m) of temperature, salinity and dissolved oxygen in this region. Wintertime convection and cyclonic circulation may contribute to the increased nutrient flux and productivity in this region. Studies from summer show strong stratification and oligotrophic conditions. Since this survey occurred during the waning of the winter chlorophyll peak, we may have witnessed the decay phase of an eddy driven spring bloom. Several thick and widespread patches of Trichodesmium spp were visible in the vicinity of the bloom. Integrated chlorophyll concentrations south of the above region, showed an order of magnitude lower chlorophyll concentration. The most northern region of the Red Sea was colder, saltier and denser (σ_t >27.6) compared to the southern portion of the study region.