Ocean Acidification Changes Abiotic Processes but Not Biotic Processes in Coral Reef Sediments

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 (+ pCO 2 : 170-900 µatm, − pH: 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 (CaCO 3 ). This increased the dissolution of Davis Reef sediments turning them from net precipitating (−0.8 g CaCO 3 m −2 d −1 ) under ambient to net dissolving (1 g CaCO 3 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.

Seasonal variability of calcium carbonate precipitation and dissolution in shallow coral reef sediments

“…Interestingly, both pH and Ω arag were significantly lower in December, and could have driven the lower than expected NCC NIGHT . We found a significant relationship with Ω arag ( p = 0.03) and pH ( p = 0.004) similar to previous studies (Cyronak et al ; Fink et al ; Eyre et al ). However, the incubations in this study were conducted over a much smaller range in pH and Ω arag than studies that manipulated CO 2 levels.…”

“…The structural maintenance of reefs requires the sum of constructive processes and import to be greater than destructive processes and export (Chave et al ; Milliman ). Constructive processes are dominated by biogenic CaCO 3 production by calcifying organisms such as coral and coralline algae and other organisms like foraminifera, bryozoans, and echinoderms, although, nonbiogenic CaCO 3 precipitation may also play a role in the sediments (Vecsei , ; Courtney et al ; Wurgaft et al ; Fink et al ). Destructive processes are defined as any process that erodes and/or removes calcium carbonate, and includes bio‐erosion through mechanical and chemical activity (e.g., parrotfish, eroding sponges), dissolution of calcium carbonate material through the production of metabolic CO 2 (which reduces the aragonite saturation state—Ω arag ), and mechanical or physical erosion (e.g., the breakdown of coral framework by the action of waves, currents, and storms) (Neumann ; Eyre et al ).…”

“…The contribution of carbonate sediment dissolution/precipitation to net ecosystem calcification (NEC) will most likely vary between coral reefs with different characteristics such as coral to sediment areas, depth, and sediment solubilities. There are few studies that have quantified the contribution of sediment dissolution/precipitation to reef calcification with values ranging between 1% and 4%, for sediment cover between 5% and 30% (Courtney et al ; Fink et al ). On other reefs with higher sediment cover, the contribution of sediment dissolution/precipitation to NEC is expected to be much higher (Eyre et al ).…”

“…The overlying seawater chemistry and benthic metabolism of reefs vary over diel and seasonal cycles, which can drive temporal variability in CaCO 3 sediment dissolution (Cyronak et al ). Metabolic activity by BMA and heterotrophs in the surface sediments and pore‐water microenvironments changes seawater chemistry, which results in large diel swings of carbonate chemistry in the overlying water column and pore water (Schoon et al ; Santos et al ; Shaw et al , ; McMahon et al ; Fink et al ). The microalgal community within the sediments can contribute up to 20% of ecosystem productivity and is dominated by diatoms, dinoflagellates, and cyanobacteria (Roelfsema et al ; Werner et al ).…”

The response of seagrass (&lt;i&gt;Posidonia oceanica&lt;/i&gt;) meadow metabolism to CO&lt;sub&gt;2&lt;/sub&gt; levels and hydrodynamic exchange determined with aquatic eddy covariance

“…However, Johnson et al, (2013) found changes in benthic diatom assemblages and increases in chlorophyll a concentrations at a CO2 vent. Nevertheless, the negligible enhancement of 280 primary production at the CO2 vent is consistent with the small net effects of acidification on benthic diatoms and benthic primary production in other studies (Alsterberg et al, 2013;Fink et al, 2017). Nutrient limitation in oligotrophic environments may limit the response of phytoplankton to elevated CO2 (Gazeau et al, 2017).…”