Evaluation of hydro-biological parameters in response to semi-diurnal tides in a tropical estuary

One of the significant aspects of estuarine water quality assessment is to discern the effect of tide on the environmental parameters. The variability of different physical, chemical, and biological parameters, in response to semi-diurnal tides, over a tidal cycle was investigated at contrasting salinity regimes of Mahanadi estuary. This estuary is formed at the mouth of Mahanadi River (the third largest river in peninsular India) that meets the Bay of Bengal on the east coast of India. Significant variability was observed in salinity, phosphate concentration, picophytoplankton biomass between upper and lower estuary irrespective of tides. Salinity and ammonium concentration varied significantly between high and low tide, irrespective of sites. A low salinity regime was observed in the upper estuary. Dissolved oxygen levels were higher at the lower estuary and relatively decreased at the upper estuary during high tide with increased tidal amplitude. The biochemical oxygen demand levels < 4 mg.l(-)(1) signified estuarine water quality in between fairly-clean and moderately polluted. Ammonium and phosphate distribution suggested anthropogenic influx at the lower/middle part of the estuary. Higher silicate content in upper estuary regardless of tides indicated prominent riverine source. Inorganic macronutrient molar ratios indicated a potential nitrogen limiting condition. Smaller phytoplankton size classes contributed predominantly to total phytoplankton biomass, irrespective of tides. This study highlighted the significance of continuous sampling over the tidal cycle to understand the effect of tides on estuarine water quality, which will aid in developing forecast models. (C) 2021 European Regional Centre for Ecohydrology of the Polish Academy of Sciences. Published by Elsevier B.V. All rights reserved.