Abstract
Cold-Water Corals (CWCs), and most marine calcifiers, are especially threatened by ocean acidification (OA) and the decrease in the carbonate saturation state of seawater. The vulnerability of these organisms, however, also involves other global stressors like warming, deoxygenation or changes in sea surface productivity and, hence, food supply via the downward transport of organic matter to the deep ocean. This study examined the response of the CWC Desmophyllum dianthus to low pH under different feeding regimes through a long-term incubation experiment. For this experiment, 152 polyps were incubated at pH 8.1, 7.8, 7.5 and 7.2 and two feeding regimes for 14 months. Mean calcification rates over the entire duration of the experiment ranged between −0.3 and 0.3 mg CaCO3 g−1d−1. Polyps incubated at pH 7.2 were the most affected and 30% mortality was observed in this treatment. In addition, many of the surviving polyps at pH 7.2 showed negative calcification rates indicating that, in the long term, CWCs may have difficulty thriving in such aragonite undersaturated waters. The feeding regime had a significant effect on skeletal growth of corals, with high feeding frequency resulting in more positive and variable calcification rates. This was especially evident in corals reared at pH 7.5 (ΩA = 0.8) compared to the low frequency feeding treatment. Early life-stages, which are essential for the recruitment and maintenance of coral communities and their associated biodiversity, were revealed to be at highest risk. Overall, this study demonstrates the vulnerability of D. dianthus corals to low pH and low food availability. Future projected pH decreases and related changes in zooplankton communities may potentially compromise the viability of CWC populations.
Highlights
Over the last two decades, and in parallel with the quantification of the oceanic absorption of anthropogenic CO2 (Gruber et al, 2019 and references therein), large efforts have been devoted to the study of past, present and future trends in ocean acidification (OA, Pelejero, Calvo & Hoegh-Guldberg, 2010; Gattuso & Hansson, 2011; Bopp et al, 2013)
The findings of this research provide insight into the sensitivity of Cold-Water Corals (CWCs), and especially the solitary species D. dianthus, to the combination of OA and changes in food supply. Our data from this long-term incubation experiment show the vulnerability of D. dianthus coral populations to low pH conditions over long periods of time, starting at pH 7.5
High frequency feeding had a positive impact on net calcification rates of polyps regardless of the seawater pH
Summary
Over the last two decades, and in parallel with the quantification of the oceanic absorption of anthropogenic CO2 (Gruber et al, 2019 and references therein), large efforts have been devoted to the study of past, present and future trends in ocean acidification (OA, Pelejero, Calvo & Hoegh-Guldberg, 2010; Gattuso & Hansson, 2011; Bopp et al, 2013). Instrumental time-series, which cover up to three decades, corroborate that the average surface ocean pH has decreased ~0.1 units since pre-industrial times (Bates et al, 2014; Kapsenberg et al, 2017) This rate of change is about 100 times faster than during glacial to interglacial transitions (Pelejero, Calvo & Hoegh-Guldberg, 2010 and references therein). Further declines in seawater pH are projected in the future, accompanied by major changes in marine ecosystems and the organisms that constitute them (Hurd et al, 2018) Not surprisingly, both tropical and Cold-Water Corals (CWCs), as marine calcifiers, were the focus of most of the initial research, since the lowering in pH and the associated decrease in carbonate ion (CO23−) concentration, together with the shoaling of the aragonite saturation state (ΩA) horizon, could potentially compromise the calcification of their carbonate exoskeletons (Cohen & Holcomb, 2009). The future shoaling of the aragonite saturation horizon projected for the global ocean could soon threaten coral provinces in these basins by increasing their exposure to corrosive waters (Guinotte et al, 2006; Touratier & Goyet, 2009; Tittensor et al, 2010a; Ciais et al, 2013; Hassoun et al, 2015; Perez et al, 2018)
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