Nutrients and harmful algal blooms in Kenya's coastal and marine waters: A review

Nutrients and Phytoplankton composition are important indicators of coastal water quality used for fisheries, tourism, and recreational purposes, which are the cornerstone of the blue economy. This is a comprehensive review and synthesis of the outcome of previous empirical studies conducted on anthropogenic nutrients (nitrogen and phosphorus) and phytoplankton productivity and community composition with a particular focus on harmful algal blooms (HABs) in Kenya's coastal and marine waters. Our results indicate that localized nutrient pollution occurs in estuaries and creeks adjacent to urban areas. Nitrogen speciation was characterized by high ammonium relative to nitrate concentrations with evidence of nitrogen limitation (N: P < 16). Based on all studies' average chlorophyll a concentration, seawater in coral and seagrass ecosystems was classified as mesotrophic, mangroves and nearshore as higher mesotrophic, and estuarine and creeks as eutrophic. Additionally, 44 species of potential toxin-producing HAB-forming species belonging to 22 genera were established in low abundances (3±2 - 210±11 cells/l) below the standards of HAB's warning thresholds. Their drivers, spatial and temporal trends are, however, unclear. This review revealed the existence of localized nutrient pollution and a low abundance of toxin-producing HABs species in Kenya's coastal waters. The existing data is, however, very fragmented and inadequate to guide coastal managers in addressing the problem of anthropogenic nutrient pollution and HABs in the area. We, therefore, recommend the inclusion of anthropogenic nutrients and toxic HABs in routine water quality monitoring in the area, especially in potential fisheries, aquaculture, tourism, and recreational sites.