This data set was collected from incubations of sediment collected from the intertidal sandbank Janssand, behind the back barrier island Spiekeroog, in the German Wadden Sea. The rate of oxygen consumption (microsensor), hydrogen accumulation (GC), iron accumulation (ferrozine, chlorometric), and sulfate reduction (35S sulfate + acid-chromium distillation) were all measured in constantly mixed slurries, with and without the ROS-removing enzymes superoxide dismutase and catalase. It additionally includes depth profiles of oxygen and hydrogen peroxide in cores, determined with amperometric microsensors.

Corals are globally important calcifiers that exhibit complex responses to anthropogenic warming and acidification. Although coral calcification is supported by high seawater pH, photosynthesis by the algal symbionts of zooxanthellate corals can be promoted by elevated pCO2. To investigate the mechanisms underlying corals' complex responses to global change, three species of tropical zooxanthellate corals (Stylophora pistillata, Pocillopora damicornis, and Seriatopora hystrix) and one species of asymbiotic cold-water coral (Desmophyllum pertusum, syn. Lophelia pertusa) were cultured under a range of ocean acidification and warming scenarios. Under control temperatures, all tropical species exhibited increased calcification rates in response to increasing pCO2. However, the tropical species' response to increasing pCO2 flattened when they lost symbionts (i.e., bleached) under the high-temperature treatments—suggesting that the loss of symbionts neutralized the benefit of increased pCO2 on calcification rate. Notably, the cold-water species that lacks symbionts exhibited a negative calcification response to increasing pCO2, although this negative response was partially ameliorated under elevated temperature. All four species elevated their calcifying fluid pH relative to seawater pH under all pCO2 treatments, and the magnitude of this offset (Δ[H+]) increased with increasing pCO2. Furthermore, calcifying fluid pH decreased along with symbiont abundance under thermal stress for the one species in which calcifying fluid pH was measured under both temperature treatments. This observation suggests a mechanistic link between photosymbiont loss ('bleaching') and impairment of zooxanthellate corals' ability to elevate calcifying fluid pH in support of calcification under heat stress. This study supports the assertion that thermally induced loss of photosymbionts impairs tropical zooxanthellate corals' ability to cope with CO2-induced ocean acidification.

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Rhodolith beds built by free-living coralline algae are important ecosystems for marine biodiversity and carbonate production. Yet, our mechanistic understanding regarding rhodolith physiology and its drivers is still limited. Using three rhodolith species with different branching morphologies, we investigated the role of morphology in species' physiology and the implications for their susceptibility to ocean acidification (OA). For this, we determined the effects of thallus topography on diffusive boundary layer (DBL) thickness, the associated microscale oxygen and pH dynamics and their relationship with species' metabolic and light and dark calcification rates, as well as species' responses to short-term OA exposure. Our results show that rhodolith branching creates low-flow microenvironments that exhibit increasing DBL thickness with increasing branch length. This, together with species' metabolic rates, determined the light-dependent pH dynamics at the algal surface, which in turn dictated species' calcification rates. While these differences did not translate in species-specific responses to short-term OA exposure, the differences in the magnitude of diurnal pH fluctuations ( 0.1–1.2 pH units) between species suggest potential differences in phenotypic plasticity to OA that may result in different susceptibilities to long-term OA exposure, supporting the general view that species' ecomechanical characteristics must be considered for predicting OA responses.

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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.

Aquatic eddy covariance oxygen flux was determined over two seagrass (Posidonia oceanica) meadows at Elba, Italy. The first meadow (open-water) was located 300 m from the southwest corner of the island, and was studied over two continuous days from 15 to 18 May 2016. The second meadow (nearshore) was located 60 m from the north shore of the island, and was studied over two discontinuous days on 13 and 25 May 2017. Both meadows were located at 13 m depth. Eddy covaraince instruments were mounted to a lightweight frame and positioned over seagrass meadows such that the measurement volume was approximately 0.3 m above the top of the canopy. Eddy covariance velocity data were collected at 16 Hz with an acoustic Doppler velocimeter (Vector, Nortek-AS, Norway). Measurements of turbulent fluctuations in oxygen concentration were made 2 cm outside the measuring volume of the Vector using an optode minisensor (O2 Minisensor, Pyroscience GmbH, Germany). The 90% response time of the minisensor was less than 0.3 s. Stable oxygen measurements above and within the canopy were determined with galvanic oxygen sensors (OxyGuard, RBR Ltd., Canada). Eddy covariance fluxes were calculated from the product of turbulent fluctuations in vertical water velocity and oxygen concentration according to standard techniques (see Berg et al., 2003 for details on the aquatic eddy covariance technique, doi:10.3354/meps261075). Further details on calculations of flux, and their correction for the nighttime depletion of oxygen within the seagrass canopy, are presented in the linked manuscript (Koopmans et al., 2020, doi:10.3389/fmars.2020.00118).

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