Phytoplankton assemblage and chlorophyll a along the salinity gradient in a hypoxic eutrophic tropical estuary-Ulhas Estuary, West Coast of India.

Phytoplankton assemblage and chlorophyll a along the salinity gradient and oxygen zonation were studied in Ulhas Estuary. Oxic condition exist in euhaline region of the estuary that receives oxygenated coastal waters, while, poly-meso-oligohaline region is hypoxic due to the accumulation of voluminous amount of pollutants from industrial effluents, domestic/sewage and agricultural discharges. Additionally, anthropogenically generated nutrients enhanced phytoplankton biomass (chlorophyll a) and their senescence also led to hypoxia. The dominance of diatoms (>70%) was observed in the euhaline-oxic region, while a tremendous decrease in their contribution (18%) was observed in the oligohaline region. Meso to oligohaline region of the estuary is governed by cyanophytes and chlorophytes due to their affinity towards low salinity and high dissolved inorganic nutrients. Carlson's Trophic State Index (TSI) further substantiated the estuary as eutrophic. Additionally, the study could identify the adaptation of cyanophytes and chlorophytes in oxygen deficient water. [Display omitted] • Hypoxia occurs in the upper part of the estuary with decreasing salinity <28 and high nutrient discharges. • Chlorophyll a and phytoplankton abundance are likewise highest in the upper part of the estuary. • Phytoplankton species composition is shifting along the observed salinity-dissolved oxygen gradient. • Phytoplankton species which are organic pollutant indicators are encountered. • Based on the Trophic State Index, upper parts of the Ulhas Estuary are eutrophic. [ABSTRACT FROM AUTHOR]