No abstract

Rising temperatures and ocean acidification driven by anthropogenic carbon emissions threaten both tropical and temperate corals. However, the synergistic effect of these stressors on coral physiology is still poorly understood, in particular for cold-water corals. This study assessed changes in key physiological parameters (calcification, respiration and ammonium excretion) of the widespread cold-water coral Desmophyllum dianthus maintained for ∼8 months at two temperatures (ambient 12 °C and elevated 15 °C) and two pCO2 conditions (ambient 390 ppm and elevated 750 ppm). At ambient temperatures no change in instantaneous calcification, respiration or ammonium excretion rates was observed at either pCO2 levels. Conversely, elevated temperature (15 °C) significantly reduced calcification rates, and combined elevated temperature and pCO2 significantly reduced respiration rates. Changes in the ratio of respired oxygen to excreted nitrogen (O:N), which provides information on the main sources of energy being metabolized, indicated a shift from mixed use of protein and carbohydrate/lipid as metabolic substrates under control conditions, to less efficient protein-dominated catabolism under both stressors. Overall, this study shows that the physiology of D. dianthus is more sensitive to thermal than pCO2 stress, and that the predicted combination of rising temperatures and ocean acidification in the coming decades may severely impact this cold-water coral species.

Recent studies considering the contribution of biodiversity to ecosystem functioning have emphasised the functional importance of individual species and, in so doing, have rekindled the use of categorical descriptors that group species according to their relative contribution to ecosystem processes or functioning. Such functional effect groupings, however, tend to be based on specific traits or contributory roles that are assumed to adequately characterise the functional importance of a species, rather than being based on direct measures of ecosystem processes and functions. This decoupling of organism−environment interaction is difficult to reconcile and, when applied widely, distorts understanding of the mediating role that species play in natural ecosystems. In this study, we begin to address this problem by characterising the functional contributions of 7 benthic invertebrate species for 2 ecosystem processes (particle reworking and bioirrigation) linked to 4 ecosystem functions (changing concentrations of NH 4 -N, NO x -N, PO 4 -P and SiO 2 -Si) and use these data to derive functional effect groupings. We show that whilst it is possible to categorise species according to how they influence ecosystem properties, the membership and number of functional effect groups depends on which ecosystem property is considered. Furthermore, we demonstrate that categorisations based on functional effects are not synonymous with species taxonomy and that they cannot be applied generically even when considering closely linked biogeochemical processes. Collectively, our findings call for a rethink of how functional effect groups are defined and emphasise the need to interrogate presumed links between species and ecosystem properties across a range of biodiversity−environment contexts.