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

Abstract

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.

Figure 1. Map and example habitats of the sites studied in the northeast Atlantic during the ‘Changing Oceans’ expedition: (a) map (produced using ODV) showing the location of the four main sites: MRC = Mingulay Reef Complex, HTS = Hebrides Terrace Seamount, Logachev, and Pisces; (b) bathymetric map of the Logachev area (produced using ArcGIS 9, ESRI), highlighting the location of the fine-scale study areas LS = Logachev South, LN = Logachev North, and Section 1; (c) example of the ‘Wilson ring’ coral patches at Pisces; (d) example of the large coral carbonate mounds topped with large Lophelia pertusa structures at Logachev; (e) example of the shallower L. pertusa reefs at the MRC; and (f) example of the ‘coral garden' habitats some of which are structured by the colonial scleractinian Solenosmilia variabilis at the HTS.

Photographic images taken during Changing Oceans Expedition 2012 (RRS James Cook cruise 073). Images c, d, e courtesy Heriot-Watt University. Image f courtesy Heriot-Watt University and the Joint Nature Conservation Committee.

Figure 2. -S plot with isopycnols, for the four main reef locations also showing the relevant water masses either as mixing lines (East North Atlantic Water (ENAW) and Wyville-Thomson Overflow Water (WTOW)) or boxes (Mediterranian Water (MW), Sub-Arctic Intermediate Water (SAIW) and Labrador Sea Water (LSW)), taken from McGrath et al.

(2012b). Surface water (SW) and coastal freshening (freshwater dilution) are also indicated. Isopycnols are drawn in light grey. In colour, data points are marked as: Mingulay Reef Complex (MRC) = purple, Logachev mounds = green, Pisces = dark orange, and Hebrides Terrace Seamount (HTS) = pale orange.

Figure 3. Average (mean ± standard deviation) profiles through the water column at three of the sites: (first row) Mingulay Reef Complex, (second row) Pisces, and (third row) Logachev, for (first column) nitrate (μM), (second column) phosphate (μM), (third column) silicate (μM), and (fourth column) ammonium (μM).

Also showing the depth ranges where CWC reefs were observed during ROV dives (colored areas on each graph). No nutrient data were collected at the Hebrides Terrace Seamount. Note the depth scale changes.

Figure 4. Average (mean ± standard deviation) profiles through the water column at all sites: (first row) Mingulay Reef Complex, (second row) Pisces, (third row) Logachev, and (fourth row) Hebrides Terrace Seamount, for (first column) total alkalinity (A_T), (second column) dissolved inorganic carbon (C_T), (third column) pH (total scale), and (fourth column) aragonite saturation state (Ω_aragonite).

Also showing the depth ranges where CWC reefs were observed during ROV dives (colored areas on each graph). Note the depth scale changes.

Figure 5. Relationships between (a) nitrate and phosphate, (b) nC_T and phosphate, (c) nC_T and potential temperature, and (d) nC_T and oxygen saturation (%).

Coloured symbols represent the different sites (as in Figure 2); dashed black lines in each plot represents the line of best fit to the data.

Figure 6. Profiles of density (a, c, e) and nC_T (μmol kg⁻¹) estimated from Oxygen % saturation (see text, equation 1) (b, d, f) across a transect ‘Section 1’ up onto the Rockall Bank (a and c), and through time, over one 24 hour period, for stations at Logachev North ‘LN’ (c and d) and Logachev South ‘LS’ (e and f).

See figure 1 for location of the transect and the two stations LS and LN.