Phytoplankton community structure is governed by salinity gradient and nutrient composition in the tropical estuarine system

Primary production in the estuarine ecosystems is principally represented by phytoplankton. The presence or function of phytoplankton in estuarine production is mainly regulated by factors such as salinity, light, turbidity, temperature, nutrients, grazers and the dynamics of the water basin. In order to understand the impact of salinity and hydrological characteristics on phytoplankton community structure in tropical estuaries, surface water samples for phytoplankton and biogeochemical parameters were collected from 26 estuaries along the Indian coast during non monsoon/no discharge period. Large spatial gradient in salinity was observed within the estuary (upper, middle and lower regions of estuary) from 2 to > 15 due to sea water intrusion through tidal currents. A higher concentration of suspended particulate matter (SPM) is observed in the lower estuarine regions, such as Tapti, and Narmada due to higher tidal amplitude. Higher phytoplankton biomass/abundances in upper estuarine region is associated with higher concentration of nutrients, and lower salinity whereas higher SPM, grazers and salinity is noticed associating with lower phytoplankton abundance in lower estuarine region. Phytoplankton community structure is found to vary significantly with reference to salinity as green algae and diatoms were dominated in the low saline upper estuary, diatoms and blue-green algae were abundant in the moderate saline region of middle estuary and diatoms alone were prominent in the high saline lower estuarine region. Moreover, diatoms in the upper estuarine region is exhibiting high heterogeneity with Melosira spp., Thalassiosira spp., Navicula spp. and Nitzschia spp. while the middle estuary and lower estuary were homogenous, mainly dominated by Thalassiosira spp. Higher species diversity (lower species dominance) was observed in upper estuary due to the diverse taxa of green algae and blue-green algae along with diatoms, whereas lower diversity was noticed in the lower estuarine region. Present study reveals that availability of light and grazing pressure is predominately governing phytoplankton abundance, while variations in nutrient concentrations and salinity is more influencing phytoplankton community structure.