J. Murray Roberts

Monitoring environmental variability around cold-water coral reefs: the use of a benthic photolander and the potential of seafloor observatories

The environmental sensitivies of cold-water corals and their associated biota are likely to be determined by the natural variability of the cold-water coral reef environment. The sensitivity of reef biota to sedimentation and resuspension events is largely unknown and the influence of seasonal phytodetrital deposition is poorly understood. Here we describe the use of a benthic photolander to monitor this variability by the Sula Ridge reef complex on the mid-Norwegian continental shelf and from the Galway carbonate mound in the Porcupine Seabight. The photolander provides a platform for time-lapse digital and film cameras to image the seabed while recording the current regime and optical characteristics (light transmission, backscatter and fluorescence) of the seawater. In its first two deployments carried out in 2001 and 2002 by the Sula Ridge the lander recorded a dynamic environment around the reef site with a tidal current regime and periods of sediment resuspension. Current speeds by the Sula Ridge reef complex reached a maximum of 28 cm s−1 and 70 cm s−1 on the Galway carbonate mound, reinforcing much speculation about the dependence of these communities on current-swept conditions. Seabed photographs show intense feeding activity of echiuran worms (Bonellia viridis) near the Sula Ridge reef complex pointing to rapid bioturbation of the sediment. Fish were recorded sheltering near sponges that had colonised glacial dropstones. Longer term monitoring in situ is needed for study of seasonal change, to identify functional roles of associated fauna and to monitor potential coral spawning events. Benthic landers and seafloor observatories have great potential in these areas. Only with a better understanding of the natural variability of the cold-water coral environment can informed decisions about the environmental sensitivity of cold-water coral reefs and their management be made.

Cold Water Coral Reefs

Scleractinian hard corals in deep, cold waters have been known since the eighteenth century. However, advances in deep-ocean exploration are now revealing the true scale and distribution of cold-water coral reefs. This prompted a resurgence of interest with dramatic discoveries of deep-water reef and coral carbonate mound provinces rapidly followed by improved understanding of coral reproduction, feeding, and molecular genetics. Hundreds of tropical coral species build shallow reefs, but less than 10 cold-water species form deep reef frameworks. Of these, the best characterized is Lophelia pertusa , which dominates in the Northeast Atlantic. Assemblages of octocorals and hydrocorals are found in other parts of the worlds oceans, such as the North Pacific. Cold-water coral skeletons provide well-preserved, high-resolution paleoclimatic archives and recent advances have been made in interpreting geochemical proxies for seawater temperature and ocean ventilation history. The reefs form structurally complex habitats supporting many other species. This complexity makes them vulnerable to mechanical damage from deep-water bottom trawling and modeled scenarios suggest that cold-water coral reefs may be threatened by ocean acidification. Integrated basin-scale studies are needed to understand linkages between reef provinces as a critical step toward developing meaningful systems of protected areas for their conservation.

Role of cold-water Lophelia pertusa coral reefs as fish habitat in the NE Atlantic

The rate of discovery of reefs of the cold-water coral Lophelia pertusa (Linnaeus, 1758) has been remarkable, and attributable to the increased use of underwater video. These reefs form a major three-dimensional habitat in deeper waters where little other ‘cover’ for fish is available. They are common in the eastern North Atlantic, and occur at least in the western North Atlantic and off central Africa. There are also other non-reef records of Lophelia in the Atlantic, and in Indian and Pacific oceans. Thus, not only are these reefs a significant habitat on a local scale, but they may also provide an important habitat over a very wide geographic scale.

Lipids and nitrogen isotopes of two deep-water corals from the North-East Atlantic: initial results and implications for their nutrition

The lipid and organic nitrogen isotopic (δ15N) compositions of two common deep-water corals (Lophelia pertusa and Madrepora oculata) collected from selected locations of the NE Atlantic are compared to the composition of suspended particulate organic matter, in order to determine their principle food source. Initial results suggest that they may feed primarily on zooplankton. This is based on the increased abundances of mono-unsaturated fatty acids and alcohols and the different ratios of the polyunsaturated fatty acids, 22:6/20:5 of the corals when compared to those of the suspended particulate organic matter. There is enrichment in L. pertusa of mono-unsaturated fatty acids and of δ15N relative to M. oculata. It is unclear whether this reflects different feeding strategies or assimilation/storage efficiencies of zooplankton tissue or different metabolism in the two coral species.

Northeastern Atlantic cold-water coral reefs and climate

U-series age patterns obtained on reef framework-forming cold-water corals collected over a nearly 6000-km-long continental margin sector, extending from off Mauritania (17 degrees N; northwest Africa) to the southwestern Barents Sea (70 degrees N; northeastern Europe), reveal strong climate influences on the geographical distribution and sustained development of these ecosystems. Over the past three glacial-interglacial cycles, framework-forming cold-water corals (Lophelia pertusa and Madrepora oculata) seem to have predominantly populated reefs, canyons, and patches in the temperate East Atlantic and the Mediterranean Sea. Above 50 degrees N corals colonize reefs in the northern East Atlantic primarily during warm climate periods with the biogeographic limit advancing from similar to 50 degrees N to similar to 70 degrees N. We propose that north-south oscillations of the biogeographic limit of reef developments are paced by ice ages and may occur synchronously with north-south displacement of cold nutrient-rich intermediate waters and surface productivity related to changes of the polar front.

Beta diversity of cold-water coral reef communities off western Scotland

Spatial heterogeneity in coral reef communities is well documented. This “species turnover” (beta diversity) on shallow warm-water reefs strongly conforms to spatial gradients in the environment as well as spatially autocorrelated biotic processes such as dispersal and competition. But the extent to which the environment and spatial autocorrelation create beta diversity on deep cold-water coral reefs such as those formed by Lophelia pertusa (Scleractinia) is unknown. The effects of remotely sensed and ground-truthed data were tested on the community composition of sessile suspension-feeding communities from the Mingulay Reef Complex, a landscape of inshore Lophelia reefs off the Scottish west coast. Canonical correspondence analysis determined that a statistically significant proportion (68%) of the variance in community composition could be explained by remotely sensed environmental variables (northerly and easterly aspect, seabed rugosity, depth), ground-truthed environmental variables (species richness and reef macrohabitat) and geospatial location. This variation was further partitioned into fractions explained by pure effects of the environment (51%), spatially structured environmental variables (12%) and spatial autocorrelation (5%). Beta diversity in these communities reflected the effects of both measured and unmeasured and spatially dependent environmental variables that vary across the reef complex, i.e., hydrography. Future work will quantify the significance and relative contributions of these variables in creating beta diversity in these rich communities.

Fine-scale nutrient and carbonate system dynamics around cold-water coral reefs in the northeast Atlantic

Ocean acidification has been suggested as a serious threat to the future existence of cold-water corals (CWC). However, there are few fine-scale temporal and spatial datasets of carbonate and nutrients conditions available for these reefs, which can provide a baseline definition of extant conditions. Here we provide observational data from four different sites in the northeast Atlantic that are known habitats for CWC. These habitats differ by depth and by the nature of the coral habitat. At depths where CWC are known to occur across these sites the dissolved inorganic carbon ranged from 2088 to 2186 μmol kg−1, alkalinity ranged from 2299 to 2346 μmol kg−1, and aragonite Ω ranged from 1.35 to 2.44. At two sites fine-scale hydrodynamics caused increased variability in the carbonate and nutrient conditions over daily time-scales. The observed high level of variability must be taken into account when assessing CWC sensitivities to future environmental change.

Environmental variability and biodiversity of megabenthos on the Hebrides Terrace Seamount (Northeast Atlantic)

We present the first remotely operated vehicle investigation of megabenthic communities (1004–1695 m water depth) on the Hebrides Terrace Seamount (Northeast Atlantic). Conductivity-temperature-depth casts showed rapid light attenuation below the summit and an oceanographic regime on the flanks consistent with an internal tide, and high short-term variability in water temperature, salinity, light attenuation, aragonite and oxygen down to 1500 m deep. Minor changes in species composition (3–14%) were explained by changes in depth, substratum and oceanographic stability, whereas environmental variability explained substantially more variation in species richness (40–56%). Two peaks in species richness occurred, the first at 1300–1400 m where cooler Wyville Thomson Overflow Water (WTOW) mixes with subtropical gyre waters and the second at 1500–1600 m where WTOW mixes with subpolar mode waters. Our results suggest that internal tides, substrate heterogeneity and oceanographic interfaces may enhance biological diversity on this and adjacent seamounts in the Rockall Trough.

Ecohydrodynamics of Cold-Water Coral Reefs: A Case Study of the Mingulay Reef Complex (Western Scotland)

Ecohydrodynamics investigates the hydrodynamic constraints on ecosystems across different temporal and spatial scales. Ecohydrodynamics play a pivotal role in the structure and functioning of marine ecosystems, however the lack of integrated complex flow models for deep-water ecosystems beyond the coastal zone prevents further synthesis in these settings. We present a hydrodynamic model for one of Earths most biologically diverse deep-water ecosystems, cold-water coral reefs. The Mingulay Reef Complex (western Scotland) is an inshore seascape of cold-water coral reefs formed by the scleractinian coral Lophelia pertusa. We applied single-image edge detection and composite front maps using satellite remote sensing, to detect oceanographic fronts and peaks of chlorophyll a values that likely affect food supply to corals and other suspension-feeding fauna. We also present a high resolution 3D ocean model to incorporate salient aspects of the regional and local oceanography. Model validation using in situ current speed, direction and sea elevation data confirmed the models realistic representation of spatial and temporal aspects of circulation at the reef complex including a tidally driven current regime, eddies, and downwelling phenomena. This novel combination of 3D hydrodynamic modelling and remote sensing in deep-water ecosystems improves our understanding of the temporal and spatial scales of ecological processes occurring in marine systems. The modelled information has been integrated into a 3D GIS, providing a user interface for visualization and interrogation of results that allows wider ecological application of the model and that can provide valuable input for marine biodiversity and conservation applications.

Physiological response of the cold-water coral Desmophyllum dianthus to thermal stress and ocean acidification

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.