Plankton carbon metabolism and air-water CO2 fluxes at a hypereutrophic tropical estuary

Multiple biotic and abiotic drivers regulate the balance between CO2 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 CO2 fluxes (based on abiotic parameters) during 1 year (2008) in a hypereutrophic tropical estuary (Recife Harbor, NE Brazil - 08 degrees 03S, 34 degrees 52W) to test the hypothesis that high productivity leads to a net CO2 flux from the atmosphere. The calculated CO2 fluxes through the air-water interface (FCO2) were negative throughout the year (FCO2: -2 to -9 mmol C.m(-2).day(-1)), indicating that Recife Harbor is an atmospheric CO2 sink. Respiration rates of the plankton community ranged from 2 to 45mmol C.m(-2).hr(-1). Gross primary production ranged from 0.2 to 281 mmol C.m(-2).hr(-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 CO2 flux studies, in order to better understand their role as a sink of atmospheric CO2.