Century-long acidification reveals possible consequences of coral reef sediment dissolution

Artur Fink1, Christiane Hassenrueck1, Katja Guilini2, Anna Lichtschlag3, Sergey Borisov4, Katharina Fabricius5 and Dirk de Beer1, (1)Max Planck Institute for Marine Microbiology, Microsensor Group, Bremen, Germany, (2)Ghent University, Biology Department -Marine Biology Research Group, Gent, Belgium, (3)National Oceanography Centre, Geochemistry, Southampton, United Kingdom, (4)Technical university of Graz, Institute of Analytical chemistry and Radiochemistry, Graz, Austria, (5)Australian Institute of Marine Science, Water quality, Townsville, Australia
Coarse permeable carbonate sediments play a key role as biocatalytical filters in element cycling on coral reefs, but are subjected to increased dissolution due to ocean acidification (OA). We investigated coral reef sediment properties and remineralization rates along a pH gradient in an area of volcanic CO2 seeping within a fringing coral reef (Papua New Guinea). In coarse carbonate-rich sediments of the reference site (water column pHT = 8.1) in-situ microprofiles showed a buffered porewater pH of 7.7 to 7.9. In contrast, sites with diffuse CO2 seeping (water column pHT 8.0 to 7.7) experienced porewater pH of less than 6 to 7. At the seep sites, the sediments were almost free of carbonates and were dominated by silicates. We found that this resulted in reduced grain sizes leading to decreased permeability and oxygen penetration into the sediment. Areal oxygen consumption and sulfate reduction rates declined at the seep sites. The pattern in oxygen consumption could be explained by oxygen limitation due to lower permeability. However, sulfate reduction was never limited by electron acceptor, indicating that the seep site sediments were limited in electron donors. In line with lower process rates, abundances of microorganisms and meiofauna declined at the seep sites. Our findings suggest that an enhanced dissolution of carbonate sediments due to OA could impact their biocatalytical filtration function. This could slow down the intense element cycling in coral reefs and other coastal carbonate environments, with consequences for ecosystem productivity and functioning.