Effect of enhanced salinity on the chlorophyll-a concentration and partial pressure of CO2 of a sewage-fed freshwater aquaculture pond: a microcosm experiment

Numerous fresh and saline-water tropical aquaculture ponds that often can be significant sources super-saturation of CO2 towards the atmosphere are gradually increasing throughout the world. The role of salinity in regulating the chlorophyll-a and dissolved pCO2 in a freshwater aquaculture pond, situated in East Kolkata Wetlands, Eastern India, was tested using a microcosm conducted during summer (April-May) and winter (December���January) months. The effects of enhanced salinity to microcosm sets of 5, 10 and 15 ppt were monitored for nine consecutive days. pH, dissolved oxygen (DO), pCO2(water), chlorophyll-a, and gross primary productivity (GPP) were measured in-situ and every day during the entire microcosm. Enhanced salinity was hypothesised to alter the pCO2(water) and chl-a dynamics of this system significantly. Increasing salinity to 5 ppt would enhance the photosynthetic potential of the existing primary producers and reduce the pCO2(water) significantly, thus boosting the CO2 sink strength. This enhanced CO2 sink strength, accompanied with increased GPP and DO, was also observed in 10 ppt, only during summer; however, low temperature was found to abate the photosynthetic response during winter. Finally, in 15 ppt treatments, drastic chlorophyll-a reduction and extreme CO2 super-saturation showed the existing phytoplankton community cannot cope with such high salinity.