Beschreibung: Two coral reef sediments have been subjected to OA scenarios in the laboratory. Sediments from Magnetic Island (year 2014, 3 pCO2 treatments) were investigated under diffusive conditions in flumes. Parameters studied were: oxygen fluxes, porewater pH and oxygen concentrations. Sulfate reduction rates and pigment concentrations as a measure for microphytobenthos abundance were measured at the end of the experiment. Sediments from Davies Reef (year 2015, 2 pCO2 treatments) were studied under advective conditions using stirred chambers. Microphytobenthos growth over the experimental period was studied using hyperspectral imaging. Porewater pH profiles were measured in different regions of the chambers. Fluxes of oxygen, total alkalinity (as a measure of CaCO3 dissolution), dissolved organic carbon and nutrients were measured using incubations. Pigment concentrations were measured at the end of the experiment as a measure for microphytobenthos abundance and to calibrate the hyperspectral imaging results. In both experiments, elevated pCO2 did not affect biotic processes. Elevated pCO2 caused an increase in dissolution of the Davies Reef sediments. The porewater pH measurements indicated that this is likely caused by the dissolution of high-magnesium calcites.

Beschreibung: This dataset contains underwater hyperspectral imagery that can be used by researchers in the domains of computer vision, machine learning, remote sensing and coral reef ecology. A diver-operated hyperspectral imaging system (HyperDiver) was used to survey 147 transects at 8 coral reef sites around the Caribbean island of Curaçao. The proximal sensing approach produced cm-scale images of more than 2.2 billion points of detailed optical spectra. Of these, more than 10 million data points have been annotated for benthic taxonomic identity with hierarchical labels. In addition to HyperDiver survey data, we also include images and annotations from traditional (color photo) quadrat surveys conducted along 23 of the 147 transects, which enables comparative reef description between two types of reef survey methods.

Beschreibung: 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.