Impacts of ocean acidification on sediment processes in shallow waters of the arctic ocean

Despite the important roles of shallow-water sediments in global biogeochemical cycling, the effects of ocean acidification on sedimentary processes have received relatively little attention. As high-latitude cold waters can absorb more CO2 and usually have a lower buffering capacity than warmer waters, acidification rates in these areas are faster than those in sub-tropical regions. The present study investigates the effects of ocean acidification on sediment composition, processes and sediment-water fluxes in an Arctic coastal system. Undisturbed sediment cores, exempt of large dwelling organisms, were collected, incubated for a period of 14 days, and subject to a gradient of pCO2 covering the range of values projected for the end of the century. On five occasions during the experimental period, the sediment cores were isolated for flux measurements (oxygen, alkalinity, dissolved inorganic carbon, ammonium, nitrate, nitrite, phosphate and silicate). At the end of the experimental period, denitrification rates were measured and sediment samples were taken at several depth intervals for solid-phase analyses. Most of the parameters and processes (i.e. mineralization, denitrification) investigated showed no relationship with the overlying seawater pH, suggesting that ocean acidification will have limited impacts on the microbial activity and associated sediment-water fluxes on Arctic shelves, in the absence of active bio-irrigating organisms. Only following a pH decrease of 1 pH unit, not foreseen in the coming 300 years, significant enhancements of calcium carbonate dissolution and anammox rates were observed. Longer-term experiments on different sediment types are still required to confirm the limited impact of ocean acidification on shallow Arctic sediment processes as observed in this study.

The synergistic effects of ocean acidification and organic metabolism on calcium carbonate (CaCO3) dissolution in coral reef sediments

Ocean acidification (OA) is expected to reduce the net ecosystem calcification (NEC) rates and overall accretion of coral reef ecosystems. However, despite the fact that sediments are the most abundant form of calcium carbonate (CaCO3) in coral reef ecosystems and their dissolution may be more sensitive to OA than biogenic calcification, the impacts of OA induced sediment dissolution on coral reef NEC rates and CaCO3 accretion are poorly constrained. Carbon dioxide addition and light attenuation experiments were performed at Heron Island, Australia in an attempt to tease apart the influence of OA and organic metabolism (e.g. respiratory CO2 production) on CaCO3 dissolution. Overall, CaCO3 dissolution rates were an order of magnitude more sensitive to elevated CO2 and decreasing seawater aragonite saturation state (Omega Ar; 300-420% increase in dissolution per unit decrease in Omega Ar) than published reductions in biologically mediated calcification due to OA. Light attenuation experiments led to a 70% reduction in net primary production (NPP), which subsequently induced an increase in daytime (115%) and net diel (375%) CaCO3 dissolution rates. High CO2 and low light acted in synergy to drive a 575% increase in net diel dissolution rates. Importantly, disruptions to the balance of photosynthesis and respiration (P/R) had a significant effect on daytime CaCO3 dissolution, while average water column ?Ar was the main driver of nighttime dissolution rates. A simple model of platform-integrated dissolution rates was developed demonstrating that seasonal changes in photosynthetically active radiation (PAR) can have an important effect on platform integrated CaCO3 sediment dissolution rates. The considerable response of CaCO3 sediment dissolution to elevated CO2 means that much of the response of coral reef communities and ecosystems to OA could be due to increases in CaCO3 sediment and framework dissolution, and not decreases in biogenic calcification.