Characterizing the influence of tide on the physico-chemical parameters and nutrient variability in the coastal surface water of the northern Bay of Bengal during the winter season

The spatial distribution of physico-chemical parameters (sea surface temperature (SST), pH, sea surface salinity (SSS), dissolved oxygen (DO) and Secchi depth) along with filterable nutrients (dissolved inorganic nitrate (DIN), dissolved inorganic phosphate (DIP) and reactive silicate (DSi)) are measured in the winter months of November, December, January and February for four consecutive years from 2009-2010 to 2012-2013 on the shallow continental shelf (<20 m bathymetry) of the coastal waters (up to 18 km away from shoreline) of the northern Bay of Bengal (nBoB) during the highest high tide (HHT) and lowest low tide (LLT) hours for the first time. The variability of the coastal biogeochemical environment is assessed during the HHT and LLT hours and for this purpose, seawater samples are collected from seven different locations of a transect in the coastal region. Physicochemical parameters (except SST) show significant difference in magnitude during the HHT and LLT hours respectively. pH, SSS and DO are found to increase in the HHT hours and vice-versa. The data reveal that during the LLT hours, a relative increase of freshwater input in the nBoB can have elevated the nutrient concentration compared with that observed during the HHT hours. The ratio of nutrient concentration is found to deviate significantly from the Redfield ratio. The abundance of DIP is much higher compared with that of DIN and DSi. The anthropogenic sources of DIP from the upstream flow (especially the domestic effluent of several metropolises) can be mainly attributed behind such an observation. In order to characterize and establish the trend of such variation in such an important bio-climatic region, long-term and systematic ecosystem monitoring in the coastal water of the nBoB northern Bay of Bengal should be carried out throughout the year.