Nutrient dynamics of northern Bay of Bengal (nBoB)—Emphasizing the role of tides

Abstract

The variability of nutrient concentrations (dissolved inorganic nitrogen, DIN; dissolved inorganic phosphorus, DIP; and dissolved inorganic silicate, DSi) along with salinity, total suspended matter (TSM), chlorophyll-a (chl-a) and dissolved oxygen (DO) were investigated in the shallow continental shelf waters of the northern Bay of Bengal. The sampling was conducted during the high tide and low tide conditions (day time) of the spring and neap phases throughout two annual cycles (February, 2013 to January, 2015). All analyses were carried out to evaluate the hypothesis—tidal processes principally regulate the nutrient variability in short-term scale. Results portrayed that DIN, DIP and DSi concentrations varied significantly between spring and neap phases as well as between high and low tides (p<0.05). Short-term spring–neap and high tide-low tide contrast was most prominent in pre-monsoon and post-monsoon season. Considering the entire dataset, DIN, DSi and DIP concentrations were 8.83±3.45mg l−1, 8.43±3.32mg l−1 and 0.61±0.36mg l−1 during the spring phase and 9.79±3.61mg l−1, 9.24±3.19mg l−1 and 0.73±0.41mg l−1 during the neap phase respectively. TSM exhibited strongest positive correlation with DIN and DSi (R2=0.9) followed by a strong negative correlation with salinity (R2=0.7), however, DIP showed a moderate correlation with both salinity and TSM. Salinity and TSM being a proxy of tidal variability, significant correlation between these two parameters and nutrient concentrations enabled the proposed hypothesis to be accepted. Moreover, this region exhibited low abundance of chl-a (∼0.1 to 3.5 mgm−3 throughout the study period) though nutrient values were quite high, as this region was phosphate limited in the post-monsoon, and for the rest of the year it was light limited.