Plankton carbon metabolism and air-water CO sub(2) fluxes at a hypereutrophic tropical estuary

Multiple biotic and abiotic drivers regulate the balance between CO sub(2) assimilation and release in surface waters. In the present study, we compared in situ measurements of plankton carbon metabolism (primary production and respiration) to calculated air-water CO sub(2) fluxes (based on abiotic parameters) during 1 year (2008) in a hypereutrophic tropical estuary (Recife Harbor, NE Brazil - 08 degree 03'S, 34 degree 52'W) to test the hypothesis that high productivity leads to a net CO sub(2) flux from the atmosphere. The calculated CO sub(2) fluxes through the air-water interface (FCO sub(2)) were negative throughout the year (FCO sub(2): -2 to -9 mmol C.m super(-2).day super(-1)), indicating that Recife Harbor is an atmospheric CO sub(2) sink. Respiration rates of the plankton community ranged from 2 to 45 mmol C.m super(-2).hr super(-1). Gross primary production ranged from 0.2 to 281 mmol C.m super(-2).hr super(-1), exceeding respiration during most of the year (net autotrophy), except for the end of the wet season, when the water column was net heterotrophic. The present results highlight the importance of including eutrophic tropical shallow estuaries in global air-water CO sub(2) flux studies, in order to better understand their role as a sink of atmospheric CO sub(2).