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An effort has been made for the first time in Asia's largest brackish water lagoon, Chilika, to investigate the spatio-temporal variability in primary productivity (PP), bacterial productivity (BP), bacterial abundance (BA), bacterial respiration (BR) and bacterial growth efficiency (BGE) in relation to partial pressure of CO2(pCO2) and CO2air–water flux and the resultant trophic switchover. Annually, PP ranged between 24 and 376 µg C L−1d−1with significantly low values throughout the monsoon (MN), caused by light limitation due to inputs of riverine suspended matter. On the contrary, BP and BR ranged from 11.5 to 186.3 µg C L−1d−1and from 14.1 to 389.4 µg C L−1d−1, respectively, with exceptionally higher values during MN. A wide spatial and temporal variation in the lagoon trophic status was apparent from BP/PP (0.05–6.4) and PP/BR (0.10–18.2) ratios. The seasonal shift in net pelagic production from autotrophy to heterotrophy due to terrestrial organic matter inputs via rivers, enhanced the bacterial metabolism during the MN, as evident from the highpCO2(10,134 µatm) and CO2air–water flux (714 mm m−2d−1). Large variability in BGE and BP/PP ratios especially during MN led to high bacteria-mediated carbon fluxes which was evident from significantly high bacterial carbon demand (BCD >100% of PP) during this season. This suggested that the net amount of organic carbon (either dissolved or particulate form) synthesized by primary producers in the lagoon was not sufficient to satisfy the bacterial carbon requirements. Lagoon sustained low to moderate autotrophic–heterotrophic coupling with annual mean BCD of 231% relative to the primary production, which depicted that bacterioplankton are the mainstay of the lagoon biogeochemical cycles and principal players that bring changes in trophic status. Study disclosed that the high CO2supersaturation and oxygen undersaturation during MN was attributed to the increased heterotrophic respiration (in excess of PP) fuelled by allochthonous organic matter. On a spatial scale, lagoon sectors such as south sector, central sector and outer channel recorded “net autotrophic,” while the northern sector showed “net heterotrophic” throughout the study period. [ABSTRACT FROM PUBLISHER]

Abstract: Biogeochemical Carbon cycling was studied in Asia''s 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. [Copyright &y& Elsevier]