Coastal ecosystems’ health assessment in Kenya: Use of anthropogenic nutrients and phytoplankton diversity metrics

Coastal habitats offer a wide range of services that support human welfare and vital ecosystem functions utilized by various marine biodiversity during their life cycle. However, these habitats are becoming more vulnerable to anthropogenic nutrient pollution due to the growing human population and intense socio-economic activities in coastal areas. This study identified, mapped, and conducted a comprehensive ecological quality assessment in sea turtle foraging sites using physicochemical indices and phytoplankton assemblages between September 2021 and July 2022.

The study established a total of 154 turtle foraging sites spread over 264 km of the Kenyan coast with 70% of the sites located in marine unprotected areas. The benthic composition sites frequented by turtles were dominated by seagrass and corals while seaweeds dominated areas where turtles are no longer sighted. The study established an average concentration of dissolved inorganic nutrients - NO_X (Nitrate + Nitrite), ammonium (NH₄-N), phosphate, and silica of 9.98. 12.77, 0.67, and 6.40 µmol/L, respectively. The sites were well-oxygenated. The study also revealed the presence of 154 phytoplankton species belonging to 119 taxa with the species composition dominated by diatoms (59 %,), dinoflagellates (23%), and cyanobacteria (9%), respectively. The total relative abundance was dominated by three non-harmful diatom species and eight harmful species including six diatoms one dinoflagellate and one cyanobacterium. About half of the sites were in a ‘Moderate’ ecological state, estuarine and creeks were ‘Poor’ and ‘Bad’ while nearshore and oceanic environments were ‘Good’ based on both nutrients and phytoplankton (Chl a). Most ‘Poor’ and ‘Bad’ conditions occurred during the rainy season. The region provides important fisheries and tourism and foraging grounds to five of the seven endangered sea turtles species. These findings will help to conserve the remaining sea turtle population, and livelihoods of coastal communities and justify effective management of land-based activities to minimize eutrophication.