Abstract
In coral reefs, sediments play a crucial role in element cycling by contributing to primary production and the remineralization of organic matter. We studied how future ocean acidification (OA) will affect biotic and abiotic processes in sediments from two coral reefs of the Great Barrier Reef, Australia. This was investigated in the laboratory under conditions where water-sediment exchange was dominated by molecular diffusion (Magnetic Island) or by porewater advection (Davies Reef). OA conditions (+ΔpCO2: 170–900 µatm, -ΔpH: 0.1–0.4) did not affect photosynthesis, aerobic and anaerobic organic matter remineralization and growth of microphytobenthos. However, microsensor measurements showed that OA conditions reduced the porewater pH. Under diffusive conditions these changes were limited to the upper sediment layers. In contrast, advective conditions caused a deeper penetration of low pH water into the sediment resulting in an earlier pH buffering by dissolution of calcium carbonate (CaCO3). This increased the dissolution of Davis Reef sediments turning them from net precipitating (-0.8 g CaCO3 m-2 d-1) under ambient to net dissolving (1 g CaCO3 m-2 d-1) under OA conditions. Comparisons with in-situ studies on other reef sediments show that our dissolution rates are reasonable estimates for field settings. We estimate that enhanced dissolution due to OA will only have a minor effect on net ecosystem calcification of the Davies Reef flat (< 4%). However, it could decrease recent sediment accumulation rates in the lagoon by up to 31% (by 0.2–0.4 mm year-1), reducing valuable reef space. Furthermore, our results indicate that high-magnesium calcite is predominantly dissolving in the studied sediments and a drastic reduction in this mineral can be expected on Davis Reef lagoon in the near future, leaving sediments of an altered mineral composition. This study demonstrates that biotic sediment processes will likely not directly be affected by OA. Ensuing indirect effects of OA-induced sediment dissolution on biotic processes are discussed.
Highlights
The world’s oceans have taken up approximately one third of the anthropogenic carbon dioxide (CO2) produced since the industrial revolution (Khatiwala et al, 2013)
Our study demonstrates in two independent experiments that medium-term exposure to elevated pressure of CO2 (pCO2) does not significantly influence biotic processes associated with photosynthesis and organic matter (OM) remineralization in coral reef sediments
The results of our study indicate that ocean acidification (OA)-induced sediment dissolution may have dramatic consequences for the carbonate budget of Davies Reef
Summary
The world’s oceans have taken up approximately one third of the anthropogenic carbon dioxide (CO2) produced since the industrial revolution (Khatiwala et al, 2013). This leads to ocean acidification (OA), a phenomenon already well measurable (Caldeira and Wickett, 2003). As pH = −log10 [H+], it is expected that the surface ocean pH will decrease by up to 0.4 units by 2100 (Orr et al, 2005). This leads to a concentration decrease of carbonate ions (CO23−): CO2 + H2O + CO23− = 2HCO−3 (2). The reduction in CO23− concentrations lowers the saturation state ( x) with respect to calcium carbonate (calcite, aragonite) and of.
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