LAUSR

Abstract The role of allochthonous nutrient inputs in governing phytoplankton distribution and abundance were assessed from the estuarine regions of the Amba River, west coast of India. A total of 35 species belonging to 24 genera were recorded, where the diatom Coscinodiscus oculus-iridis (99%) dominated the estuarine mouth with a density of 3.5 × 105 cells l−1. Community analyses indicate that diversity (H’) decreased towards the estuarine mouth (0.002 ± 0.001) compared to the middle (0.38 ± 0.06) and inner estuary (1.83 ± 0.14) due to the diatom outbreak. Chlorophyll a reached an average of 12.51 μ g l−1 at the estuarine mouth, which is over three times the value determined in the middle estuary (4.25 μ g l−1). The key sources of these land-based nutrients are identified as agricultural land and urban runoff for nitrogen (N) and phosphorus (P), extensive sand mining for silica (Si) and aeolian deposition of iron minerals from industrial conveyor belts. A linear correlation between cell density and chlorophyll a with chemical variables indicated that silicate coupled with excess nitrogen input was crucial causative factors for the bloom. The nutrient enrichment towards the estuarine mouth is due to the dispersal of these land derived nutrients by complex hydrological forces.