Biophysical Interactions in Driving the Summer Monsoon Chlorophyll Bloom Off the Somalia Coast

다국어 초록 (Multilingual Abstract)

The biophysical interactions along the Somali coast are discussed during summer monsoon based on a numerical model study. The Somali coast is known to be the fifth largest upwelling region of the world ocean and one of the most productive regions of the globe. Based on scattered observations, mostly dated back to 1960s, it is widely accepted that the strong chlorophyll bloom along the Somali coast during summer is driven by upwelled nutrient flux. We show that the upwelling‐driven productivity is primarily limited to the northern part of the coast when examined along the shelf break off the Somalia coast. In contrast, productivity in the south of 9°N is driven by weaker upwelling in the early half of the summer monsoon but later dominated by the wind‐based mixing induced entrainment and therefore shows much weaker chlorophyll concentration than the north. Further, the strong poleward alongshore currents advect the upwelled nutrients away from the southern and central parts of the coast to the north, thereby controlling the biological community over the Somali region. This abundant locally upwelled and remotely advected nutrients support the enhanced growth of diatoms in the northern part of the transect as earlier observed from in situ measurements. However, limited nutrients allow smaller phytoplankton communities to grow in the southern and central sections. We show that nitrate is the primary limiting nutrient for the phytoplankton growth in the central and southern parts of the Somalia coast.
The intense chlorophyll bloom along the Somali coast during summer monsoon is attributed to the upwelled nutrients from the subsurface. Recently, Chatterjee et al. (2019) reported that the strong upwelling along the Somali coast is limited to the northern part only and is much weaker than previously thought. In this study, with the help of satellite‐derived chlorophyll data and model‐simulated chlorophyll, we find that the intense chlorophyll bloom is restricted toward the northern part of the Somali coast and the chlorophyll concentration decreases gradually toward the south. In the central part of the coast, the entrainment of nutrients from the subsurface driven by wind‐induced mixing results in a weaker chlorophyll concentration. Our analysis indicates that, during the course of monsoon, large phytoplankton grow rapidly in the northern region due to the adequate availability of nutrients, whereas in the south (south of 9°N) small phytoplankton communities take advantage over the large phytoplankton owing to the limited availability of nutrients. Also, analysis of various model‐derived limitation terms suggests that nitrate is the most limiting nutrient that controls the growth of the phytoplankton communities in the central and southern parts of the Somali coast.


The Somali coast shows heterogeneous phytoplankton distribution with intense bloom exhibited only in the northern part of the coast
The northern part of the Somali region is dominated by large phytoplankton, whereas south of 9°N, small phytoplankton community dominates
Productivity in the southern and central parts of the Somalia is heavily limited by nitrate availability