The influence of tides on bacterial populations in a monsoon influenced tropical estuary was assessed through fine resolution sampling (1 to 3 h) during spring and neap tides from mouth to the freshwater end at four stations during pre-monsoon, monsoon and post-monsoon seasons. Higher abundance of total bacterial count (TBC) in surface water near the river mouth, compared to the upstream, during pre-monsoon was followed by an opposite scenario during the monsoon When seasonally compared, it was during the post-monsoon season when TBC in surface water was highest, with simultaneous decrease in their count in the river sediment. The total viable bacterial count (TVC) was influenced by the depth-wise stratification of salinity, which varied with tidal fluctuation, usually high and low during the neap and spring tides respectively. The abundance of both the autochthonous Vibrio spp. and allochthonous coliform bacteria was influenced by the concentrations of dissolved nutrients and suspended particulate matter (SPM). It is concluded that depending on the interplay of riverine discharge and tidal amplitude, sediment re-suspension mediated increase in SPM significantly regulates bacteria populations in the estuarine water, urging the need of systematic regular monitoring for better prediction of related hazards, including those associated with the rise in pathogenic Vibrio spp. in the changing climatic scenarios.
Estuaries are hotspots of anthropogenic activities. The deposition of pathogenic bacteria in the sediment and their re-suspension into the water column are influenced by riverine discharge and tides. The abundance of Escherichia coli O157:H7, Shigella spp., Salmonella spp., total coliforms (TC) and Vibrio spp. (Vibrio cholerae (VC), Vibrio parahaemolyticus (VP), Vibrio alginolyticus (VA)) was assessed along with the total bacterial count (TBC) and total viable count (TVC) in surface sediments along the banks of the Zuari estuary, Goa, India. The study was carried out fortnightly for a period of 17 months covering three seasons, i.e. pre-monsoon (PreM), monsoon (MON) and postmonsoon (POM). The spatial and temporal changes in the quality of organic matter were also assessed. The organic matter content was high and rich in carbohydrates and proteins towards upstream sites. The quality of organic matter was influenced by the seasons. E. coli O157:H7 was detected only during MON towards the upstream stations. A negative correlation between TC and TBC with salinity was evident indicating the influence of land run-off. The Shigella spp. and VA were high towards the mouth of the estuary during PreM. However, during POM, the TVC, TC and VP were abundant towards the upstream and VC were abundant at the mouth of the estuary. Among the Vibrios, VP and VA were the most frequently occurring bacteria whereas TC and Shigella spp. were dominant among allochthonous pathogens in the sediments irrespective of space and time. In addition to influence of seasons, the sampling time influenced by tidal condition also played an important role in the population dynamics of pathogenic bacteria in the sediments. Future studies should address the interaction of pathogenic bacteria with suspended particles, their transport and survival in the sediments.
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