Distribution of dissolved inorganic carbon and net ecosystem production in a tropical brackish water lagoon, India

• DIC mass balance approach revealed the trophic status of the lagoon.
• Lagoon behaves autotrophic and heterotrophic during lean and active flow, respectively.
• Significant spatio-temporal variation of DIC in both water and benthic community.
• NCP=14 mg C m−2 d−1and −148 mg C m−2 d−1 during lean and active flow, respectively.
• DIC=1109−18.1 Temp+137 pH+19.2 Sal−14.5 chlorophyll-a.

Chilika, the second largest lagoon of the world was studied for 2 years on monthly basis to understand the dissolved inorganic carbon (DIC) dynamics through mass balance approach. The study revealed that during the low flow period the lagoon is net autotrophic (Net ecosystem production (NEP)=33 Mg d−1) resulting in a sink of DIC whereas remains net heterotrophic during the high flow period (NEP=−3099 Mg d−1). DIC concentration significantly varied with respect to seasons as well as sectors owing to fresh water discharge and biological activity of both pelagic and benthic communities. Sectoral mixing model showed negative residuals (sink of DIC) in all sectors during low flow, attributable to primary production (PP) as evidenced by strong correlation with residuals (r=−0.51, p<0.05). During high flow, DIC residuals of the entire lagoon (except southern sector) were positive (source of DIC) possibly due to bacterial mineralization as evidenced by δ13CDIC isotopic signature. The estimated net community production (NCP) in pelagic compartment was 14 mg C m−2 d−1 during the low flow period. and −148 mg C m−2 d−1 during high flow indicated dominating community respiration. The lagoon sediment acted as a net source of DIC throughout the study period.