Processes of Carbon Dioxide Flux in the Fanning Island Lagoon

Abstract

Carbon dioxide follows a variety of transfer pathways through marine systems, and a budget of CO2 can be used to enumerate those pathways. In a coral reef system, the biogeochemical pathways of organic carbon production-consumption and calcification are likely to be prominent. We measured the temperature, salinity, pH, and total alkalinity of about 400 water samples to describe the CO2 budget of the lagoon at Fanning Island, Line Islands, during July and August 1972. Mean lagoon salinity was about 31.5 %, or 3.3 % below the open ocean salinity there, as a result of heavy rainfall and groundwater seepage. This salinity depression, together with data on rainfall and tides, was used to calculate a mean lagoon water residence time of about 1 month. The CO2 budget of the lagoon water can be described in terms of processes altering the ocean water composition. Gas exchange accounted for a netCO2 evasion of less than 1.4 moles m-2 month-1. Freshwater dilution lowered the CO2 content by about 0.5 moles m-2 month-1. Calcification lowered CO2 by 0.9 moles m-2 month-1; and CO2 changes attributable to organic carbon transfer lay between - 0.1 and +1.3 moles m-2 month-1. Net organic carbon transfer in the lagoon is near zero, suggesting a close balance between organic carbon production and consumption. Calcification is about 1 kg CaCO3 m-2 yr-1 , much lower than the rate which can be estimated from the standing crop and expected growth rate of corals there. It seems possible that the CO2 system of Fanning Lagoon, and perhaps that of other coral reefs as well, may be potentially limiting to biological activity.