Oxygen metabolism and pH in coastal ecosystems: Eddy Covariance Hydrogen ion and Oxygen Exchange System (ECHOES)

An aquatic eddy covariance (EC) system was developed to measure the exchange of oxygen (O-2) and hydrogen ions (H+) across the sediment-water interface. The system uses O-2 optodes and a newly developed micro-flow cell H+ ion selective field effect transistor; these sensors displayed sufficient precision and rapid enough response times to measure concentration changes associated with turbulent exchange. Discrete samples of total alkalinity and dissolved inorganic carbon were used to determine the background carbonate chemistry of the water column and relate the O-2 and H+ fluxes to benthic processes. The ECHOES system was deployed in a eutrophic estuary (Waquoit Bay), and revealed that the benthos was a sink for acidity during the day and a source of acidity during the night, with H+ and O-2 fluxes of +/- 0.0001 and +/- 10 mmol m(-2) h(-1), respectively. H+ and O-2 fluxes were also determined using benthic flux chambers, for comparison with the EC rates. Chamber fluxes determined in 0.25 h intervals co-varied with EC fluxes but were similar to 4 times lower in magnitude. This difference was likely due to suppressed pore-water advection in the chambers and changes in the chemistry of the enclosed chamber overlying water. The individual H+ and O-2 fluxes were highly correlated in each dataset (EC and chambers), and both methods yielded H+ fluxes that could not be explained by O-2 metabolism alone. The ECHOES system provides a new tool for determining the influence of benthic biogeochemical cycling on coastal ocean acidification and carbon cycling.