Delineating CO2 and chlorophyll-a variability of an urban estuary with respect to changes in available dissolved nutrients: A microcosm study

Abstract

Changes in dynamics of nutrient with respect to partial pressure of dissolved CO₂ [pCO₂(water)] from tropical lotic ecosystem, namely diamond harbor an end member of Hugli estuary, were analyzed by way of a microcosm experiment. Though surface waters behaved as an extensive CO₂ source under <em>in-situ</em> condition [<em>in-situ </em>pCO₂(water) ranged from ~1000 to ~20,000 ppm], it was capable of becoming a sink in post-monsoon while pH as well as primary productivity was significantly high (monitored in microcosm). In pre-monsoon and monsoon, internal generation of nutrients by means of re-mineralization was pre-dominant and the CO₂ source character of the water was maintained for eight consecutive days (the duration of the microcosm). Amongst the five nutrients, dissolved nitrate-nitrogen (NO₃-N) followed by phosphate-phoshorus (PO₄-P) were the most utilized nutrients during higher photosynthetic activities and conditions conducive for optimum photosynthesis can lower the magnitude of CO₂ source character. Substantial nutrient utilization efficiency especially during post-monsoon enabled photosynthetic CO₂ utilization because of low turbidity and optimum light availability. It infers that controlled freshwater discharge from the Farakka Barrage along with proper replenishment with freshwater borne nutrients could minimize the re-mineralization process and lessen the CO₂ source potential of this estuary.