Carbon Cycling in Carbonate-Dominated Benthic Ecosystems: Eddy Covariance Hydrogen Ion and Oxygen Fluxes

The pH of open ocean surface waters has decreased by 0.1 pH units since pre-industrial times and is projected to fall by a further 0.2 - 0.4 pH units within this century . In contrast to the open ocean, less is known about the impact of ocean acidification in coastal ecosystems, where biogeochemical cycling is strongly influenced by circulation, water residence times, terrestrial inputs, and two dominant sets of coupled processes; photosynthesis / respiration and calcification / dissolution. Benthic communities and processes are particularly important in coastal systems because they can have a large influence on the chemistry of coastal waters they reside in. In these settings, natural daily ranges of O2 and pH variability can dwarf the ranges of open ocean systems, with levels typically reaching those of >100 year future predictions for the open ocean, on a daily basis. With more than $10 trillion USD in annual resources or ~30% of the global ecosystem goods and services stemming from coastal marine ecosystems, understanding ecological dynamics in coastal regions is paramount to predicting and mitigating the effects of impending environmental change. 

Productivity and carbon exchange in coastal systems is often measured by the flux of O2, which is tightly linked to organic carbon cycling. However, this O2-based approach cannot quantify the production and oxidation of metabolites from anaerobic processes, and cannot constrain the rates of carbonate dissolution and precipitation.The O2 concentration and carbonate chemistry of the water column are primarily controlled by two dominant biogeochemical processes (photosynthesis / respiration and carbonate dissolution / calcification), through the exchange of O2 and CO2 between the atmosphere and the ocean, and by benthic fluxes. We have developed a new technology to comprehensively investigate these processes: the Eddy Covariance Hydrogen ion and Oxygen Exchange System (ECHOES) that estimates changes in total dissolved inorganic carbon (DIC) using eddy covariance O2 flux measurements . calculates the associated H+ flux, and directly measures the total H+ flux. The calculated and measured H+ fluxes are then compared, and the difference is used to estimate total alkalinity (TA) associated fluxes, which include carbonate dissolution / precipitation. This new ECHOES approach is analogous to, and complements, rate estimates from changes in water column DIC and TA concentrations. 

Figure 1. Generalization of photosynthesis, respiration, carbonate dissolution, and calcification and the resulting ECHOES fluxes. The change in water column carbonate chemistry (simplified equations, right) results in a hydrogen ion (H+) flux that is measured by eddy covariance. The differences between carbon cycling determined by O2 and H+ fluxes enables the quantification of photosynthesis/respiration and carbonate dissolution/precipitation.

This project will use the newly developed ECHOES to evaluate the coupled benthic processes of photosynthesis / respiration and dissolution / calcification at an ecosystem scale on the Bermuda platform, to quantify their contribution to carbon cycling, water column chemistry and the effects of adjacent benthic communities. The co-measured H+ and O2 exchange rates (in combination with discrete measurements of carbonate chemistry) will provide a comprehensive analysis of the total ecosystem carbon cycling, which is not well-described in global models, and will allow for a truly in situ examination of the natural drivers of carbon cycling, metabolism, and calcification. These results are expected to reveal new linkages and interactions between these processes through high frequency, ecosystem scale measurements that are now possible with the ECHOES. The influence of benthic processes on shallow coastal waters is expected to highlight the multiple parameter dependency (residence time, benthic community, light) of the local carbonate chemistry and will investigate the potential resilience of dynamic coastal environments to changing ocean conditions.