Tidal downwelling and implications for the carbon biogeochemistry of cold-water corals in relation to future ocean acidification and warming HELEN S. FINDLAY*, YURI ARTIOLI*, JUAN MORENO NAVAS † , SEBASTIAN J. HENNIGE † , LAURA C. WICKS † , VEERLE A. I. HUVENNE ‡ , E. MALCOLM S. WOODWARD* andJ. MURRAY ROBERTS † §¶ *Plymouth Marine Laboratory, Prospect Place, West Hoe, Plymouth PL1 3DH, UK, †Centre for Marine Biodiversity & Biotechnology, School of Life Sciences, Heriot-Watt University, Edinburgh EH14 4AS, UK, ‡National Oceanography Centre Southampton, European Way, Southampton SO14 3ZH, UK, §Scottish Association for Marine Science, Oban PA37 1QA, UK, ¶Center for Marine Science, University of North Carolina Wilmington, 601 S. College Road, Wilmington, NC 28403-5928, USA Abstract Cold-water coral (CWC) reefs are recognized as ecologically and biologically significant areas that generate habitats and diversity. The interaction between hydrodynamics and CWCs has been well studied at the Mingulay Reef Com- plex, a relatively shallow area of reefs found on the continental shelf off Scotland, UK. Within ‘Mingulay Area 01’ a rapid tidal downwelling of surface waters, brought about as an internal wave, is known to supply warmer, phyto- plankton-rich waters to corals growing on the northern flank of an east-west trending seabed ridge. This study shows that this tidal downwelling also causes short-term perturbations in the inorganic carbon (C T ) and nutrient dynamics through the water column and immediately above the reef. Over a 14 h period, corresponding to one semi-diurnal tidal cycle, seawater pH overlying the reef varied by ca. 0.1 pH unit, while pCO 2 shifted by >60 latm, a shift equiva- lent to a ca. 25 year jump into the future, with respect to atmospheric pCO 2 . During the summer stratified period, these downwelling events result in the reef being washed over with surface water that has higher pH, is warmer, nutrient depleted, but rich in phytoplankton-derived particles compared to the deeper waters in which the corals sit. Empirical observations, together with outputs from the European Regional Shelf Sea Ecosystem Model, demonstrate that the variability that the CWC reefs experience changes through the seasons and into the future. Hence, as ocean acidification and warming increase into the future, the downwelling event specific to this site could provide short- term amelioration of corrosive conditions at certain times of the year; however, it could additionally result in enhanced detrimental impacts of warming on CWCs. Natural variability in the C T and nutrient conditions, as well as local hydrodynamic regimes, must be accounted for in any future predictions concerning the responses of marine ecosystems to climate change. Keywords: biogeochemistry, climate change, cold-water corals, ecologically and biologically significant Areas (EBSAs), hydrog- raphy, ocean acidification, Vulnerable Marine Ecosystems (VMEs) Received 12 February 2013; revised version received 12 April 2013 and accepted 1 May 2013 Introduction Cold-water coral (CWC) reefs grow in relatively spe- cific environmental niches with regards to temperature and salinity, but also with respect to other parameters such as water currents (for food supply) and available oxygen (Dullo et al., 2008; Roberts et al., 2008). CWC reefs in the North Atlantic have been reported from depths of >600 m to shallower regions of 100–200 m, for example, on the continental slope, shelf and within fjords (Roberts et al., 2006, 2009), where they provide highly complex habitats supporting rich associated communities (Henry & Roberts, 2007; Roberts et al., 2008). These reefs cross a variety of environmental gra- dients but are usually found on sloping topography, such as offshore banks, seamounts, or coral carbonate mounds (Wilson, 1979; Mortensen et al., 2001; Roberts et al., 2006). Interesting hydrodynamic features are often associated with these slope and reef structures; such features are suggested to be important for supply- ing a sufficient flux of food particles to support the reef and its associated communities (Davies et al., 2009). The interaction between hydrodynamics and CWCs has been well studied at the Mingulay reef complex (MRC) (Dullo et al., 2008; Duineveld et al., 2012), a rela- tively shallow CWC reef found on the continental shelf off Scotland, UK (Fig. 1). The Scottish western Correspondence: Helen S. Findlay, tel: +44(0)1752 633450, fax: +4401752 633101, e-mail: hefi@pml.ac.uk © 2013 John Wiley & Sons Ltd 2708 Global Change Biology (2013) 19, 2708–2719, doi: 10.1111/gcb.12256