Spatio-temporal variation of CO2 emission from Chilika Lake, a tropical coastal lagoon, on the east coast of India

Biogeochemical Carbon cycling was studied in Asiaʹs largest brackish lagoon, Chilika on the east coast of India. Systematic time-series observations were made at 35 hydrologically different stations over the entire lagoon. The first of these kinds of measurements reveal, inter and intra annual variability of partial pressure of CO2 (pCO2). A comparative analysis of pCO2, CO2 flux over four years (2005, 2009, 2010 and 2011) showed that the northern part of the lagoon maintained the highest levels of pCO2, with maximum CO2 efflux to the atmosphere associated with peak monsoon period. The high pCO2 corresponded to a significant decrease in pH (∼0.8) from the low to high flow periods. Higher bacterial abundance (1.55 ± 0.28 × 109 cells L−1) and bacterial respiration (185.31 ± 105.37 μg C L−1 d−1), suggested high levels of organic carbon decomposition during the high flow period. In contrast, the southern sector was least affected by river discharge, with low pCO2 values and CO2 flux as in the dry period. The central part and outer channel of the lagoon had intermediate characteristics. During high flow, the air–water CO2 flux from the entire lagoon was estimated to be 31.2 mol C m−2 y−1, which was comparatively very high with respect to the mean CO2 emission from the entire subtropical and tropical estuaries. Highest CO2 flux (65.98 mol C m−2 y−1) was observed in the northern sector, followed by the outer channel (17.61 mol C m−2 y−1), central sector (15.69 mol C m−2 y−1) and southern sector (14.44 mol C m−2 y−1). The northern part of Chilika lagoon being an important river-influenced region was responsible for the biological transformation of organic carbon to inorganic carbon and emitting CO2 to the atmosphere. Such a sectoral approach would reveal zonal influences of CO2 within a water body along with conditions leading to sink or a source.