A Multi-decadal Comparative Analysis of a Set of Physicochemical and Nutrient Parameters in the Tropical Tidal Creeks of Indian Sundarban Mangrove Biosphere Reserve

Salinity, pH, dissolved oxygen (DO), dissolved inorganic nitrate (DIN), dissolved inorganic phosphate (DIP), and reactive silicate (DSi) concentrations were measured in three tidal creeks in the Sagar Island (situated within the Sundarban Biosphere Reserve, India) during 2016–17, once in a month throughout an annual cycle. The present observations were compared with a study carried out three-decade ago in the same locations during 1988–89. There was no significant difference in salinity between the years 1988-89 and 2016-17. Compared to the 1988–89 scenario, pH reduced considerably during 2016–17, indicating the dominance of heterotrophic activities over autotrophic processes. The comparative analysis shows that DO varied between 3.8 mg l− 1 and 5.4 mg l− 1 during 1988–89, which went to hypoxic levels (< 2 mg l− 1) during 2016–17. Such hypoxic conditions can lead to direct mortality of several marine biotas that thrives in these creeks. DIN and DSi concentrations reduced significantly. Compared to the 1988–89 scenario, DIN and DSi concentrations reduced by three times and ten times, respectively, in the year 2016-17. DIP concentration showed an increase in annual mean concentration, which was statistically not significant. Despite that DIN:DIP ratio reduced significantly during 2016–17 compared to that of 1988–89, which can lead to changes in the primary productivity potential of the phytoplankton community. Such changes in concentrations of the physicochemical and nutrient parameters can adversely affect the sustenance of several aquatic life forms and lead to changes in species composition or total abandonment.