Biogeochemical Drivers of Productivity in Kenyan Territorial Waters During the Southeast Monsoon Season

The Kenyan coastal and offshore waters of the Western Indian Ocean undergo marked seasonal shifts in physical forcing and biogeochemical cycling driven by the bimodal monsoon system. This presentation examines the mechanisms linking southeast monsoon (Kusi, April–October) wind forcing to patterns of nutrient supply and primary productivity along the Kenyan coast. During this period, intensified southeasterly winds strengthen the East African Coastal Current and enhance vertical mixing and localized upwelling near headlands, reef passages, and the continental shelf break, lifting nutrient-rich subsurface waters into the euphotic zone. Concurrently, increased rainfall over the catchment elevates terrestrial nutrient and sediment loading from the Tana and Sabaki rivers, altering nearshore nitrogen, phosphorus, and silicate ratios and influencing phytoplankton community composition. These combined physical and terrestrial inputs are examined alongside dissolved oxygen dynamics, water column stratification, and chlorophyll-a distribution to characterize spatial and temporal variability in productivity across nearshore, shelf, and offshore zones. Comparisons with the northeast monsoon (Kaskazi) season highlight contrasts in upwelling intensity, current direction, and nutrient residence time, underscoring the southeast monsoon as a period of elevated biological productivity with implications for fisheries recruitment, coral reef nutrient stress, and carbon export. Findings from resent surveys conducted onboard Dr. Fridtjof Nansen compiles insitu data on currents and CTD measurements providing critical insights on localized processes. Understanding these monsoon modulated biogeochemical processes is critical for predicting fisheries productivity windows, assessing ecosystem resilience to climate variability, and informing marine spatial planning within Kenya's Exclusive Economic Zone. This work contributes to a broader regional effort to characterize Western Indian Ocean biogeochemical seasonality under increasing anthropogenic and climatic pressures.