The viral dynamics have rarely been investigated in estuarine environments of India. The present study brings out a first hand information on the distribution of virus and bacteria in an eutrophic estuary (Cochin, India). Thirteen stations were selected for monthly monitoring of bacterial abundance (BA), total viable bacterial count (TVC), viral abundance (VA), chlorophyll a (Chl a) and other water quality parameters (temperature, salinity, pH, dissolved oxygen and inorganic nutrients such as NO3–N, NH4–N, PO4–P and SiO4–Si) for 1 year. There was significant variations in the VA [0.59–4.48 × 107viral like particles (VLPs) mL−1] and BA (0.49–8.12 × 106 cells mL−1) in the estuary. The variation in the viral to bacterial ratio (3–22) indicated marked seasonality. Statistical analysis showed bacteria as a major factor controlling the distribution of viruses (60 % variability) in Cochin estuary (CE). The viruses also showed positive correlations with Chl a, pH and salinity. The distribution of virus followed a distinct pattern of three different zones in the estuary controlled by salinity in zone I, Chl a in zone II and salinity, Chl a in zone III. The zonal distribution suggests that the factors that control the viral‐host systems vary in different areas due to the complex hydrography of the estuary and environmental changes are very sensitive to the viral population in CE.

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An interesting physiological response of phytoplankton to large fluctuations in underwater photosynthetically active radiation (PAR) levels in the northern Bay of Bengal has been presented here. This study is primarily based on a 12-day time series observation in the northern Bay of Bengal during the peak Southwest Monsoon (July 2012), when the study region was recurrently exposed to alternating cloudy and sunny sky conditions. On overcast days, the PAR available underwater at the time series location (TSL) drastically decreased, with the noontime PAR at the surface water (2 m) usually being ~600 µmol m−2 s−1 on sunny days and declining to ~50 µmol m−2 s−1 on heavily overcast days. Closely linked with the sunny and cloudy days at TSL, chlorophyll a concentration in the water column showed noticeable features; it increased in the upper water column (surface-40 m) and decreased in the lower water column (41–80 m) on cloudy days, while the reverse was the case on sunny days. Based on in-situ and laboratory experimental data, it was observed that these temporal changes in the vertical distribution of chlorophyll a in the northern Bay of Bengal were due to the short-term physiological acclimation of phytoplankton to large changes in underwater PAR.

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