Physical and chemical characteristics of the Hooghly estuary during winter (December 1997–January 1998), summer (May 1998) and post-monsoon (November 1998) seasons have been studied. Salinity varied spatially and temporally and seasonally during ebb and flood tide conditions. Water temperature showed a difference of 10‡C in winter to summer. Temperature did not vary much vertically as it is a well-mixed estuary. Strong currents exceeding 100 cm S^-1 were observed during peak ebb and flood tide conditions irrespective of the season. Longitudinal eddy diffusion coefficient (K_x) was estimated as 757m S^-1 and 811m² S^-1 during summer and post-monsoon seasons, respectively. The vertical eddy diffusion coefficient (ε_v) was estimated as 0.0337 m² S^-1 during post-monsoon season. The salinity and current observations are compared with those obtained from models reported earlier. Values of pH, Dissolved Oxygen and Biological Oxygen Demand are within the threshold limits of the estuarine environment. Nutrients show seasonal variation in the estuarine environment. High values (160-2686 mg l^-1) of total suspended matter were noticed both at surface and bottom in the study region showing the impact of fresh water and sediment transportation.

Some chemical and biological parameters were analysed at sixteen stations in the mangrove ecosystem, of the neighbouring Gautami-Godavari (GG) river estuary and Kakinada (KKD) bay to understand the present status of water quality and the impact of external terrigenous inputs during southwest (SW) monsoon in the study areas. High concentrations of nutrients in the mangrove ecosystem compared to the bay and estuarine ecosystems reveal the importance of this zone as a source of nutrients to the adjacent coastal ecosystems. Low Si:N:P (29:4:1) ratios in these ecosystems are due to the enrichment of these nutrients through external anthropogenic inputs even after the utilization by phytoplankton in the biological cycle. The mean Chl b/Chla and Chl c/Chla ratios and high phaeopigments (Pp) concentrations compared to Chlb and high ratios of Chl a/Pp suggests the possibility of the potential growth of phytoplankton populations in lower light intensity and low turbulent areas of these mangrove ecosystems.

The Bay of Bengal is considered to be a low productive region compared to the Arabian Sea based on conventional seasonal observations. Such seasonal observations are not representative of a calendar year since the conventional approach might miss episodic high productive events associated with extreme atmospheric processes. We examined here the influence of extreme atmospheric events, such as heavy rainfall and cyclone Sidr, on phytoplankton biomass in the western Bay of Bengal using both in situ time-series observations and satellite derived Chlorophyll 𝑎 (Chl 𝑎) and sea surface temperature (SST). Supply of nutrients through the runoff driven by episodic heavy rainfall (234 mm) on 4–5 October 2007 caused an increase in Chl 𝑎 concentration by four times than the previous in the coastal Bay was observed within two weeks. Similar increase in Chl 𝑎, by 3 to 10 times, was observed on the right side of the cyclone Sidr track in the central Bay of Bengal after the cyclone Sidr. These two episodic events caused phytoplankton blooms in the western Bay of Bengal which enhanced ∼40% of fishery production during October–December 2007 compared to that in the same period in 2006.