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. SIMILAR CONTENT BEING VIEWED BY OTHERS PROGRESSIVE SEAWATER ACIDIFICATION ON THE GREAT BARRIER REEF CONTINENTAL SHELF Article Open access 27

October 2020 “PINK POWER”—THE IMPORTANCE OF CORALLINE ALGAL BEDS IN THE OCEANIC CARBON CYCLE Article Open access 27 September 2024 DIFFERENCES IN CARBONATE CHEMISTRY UP-REGULATION OF

LONG-LIVED REEF-BUILDING CORALS Article Open access 18 July 2023 INTRODUCTION Specific dynamics such as temperature, salinity, oxygen availability, light and water currents, have been shown

to control reef and coral growth on tropical shallow-water zooxanthellate coral reefs [e.g.1,2], but also, more recently, on cold-water non-zooxanthellate coral reefs [e.g.3,4]. While the

physical dynamics are clearly important, the biogeochemical dynamics (interlinked with the physics), such as the carbonate ion concentration or nutrients5, are also recognized to contribute

to reef and coral growth. On tropical coral reefs, carbon and nutrient dynamics have been shown to be highly variably6,7,8, but the presence of the reefs themselves can also contribute to

the variability in carbon and nutrients over daily and seasonal time-scales9,10,11. Dullo _et al._4 suggested that cold-water coral (CWC) reefs are not randomly distributed but instead can

be found at certain density ranges, specifically from 27.35 to 27.65. This density pre-requisite has been confirmed in more recent observations of both deep and shallow CWC reefs

[e.g.12,13], however not all CWC habitats are found in such a narrow density range14 and it is highly relevant we consider all these types of habitats. While this identifier has been used to

locate living CWC reefs, Dullo _et al._4 also highlight the need for further research on related processes including nutrient inventories, carbonate chemistry and, critically, mechanisms by

which food is supplied to the benthos. At present, nutrient and carbonate system data are primarily only available from global studies such as the World Ocean Atlas (WOA)15 and the GLobal

Ocean Data Analysis Project (GLODAP)16 and while useful for global assessments, provide very limited fine-scale spatial and temporal coverage that would describe a detailed baseline of the

physicochemical environment that these CWC reef organisms experience. _McGrath et al._17 analyzed data from local and World Ocean Circulation Experiment (WOCE) cruises for the Rockall Trough

between 1991–2010, providing a longer-term temporal analysis of the carbonate system at a more local scale, however the main focus was on the water column structure and change through time,

as opposed to the CWC reef environmental conditions. The WOA and GLODAP datasets have been used to provide broad environmental envelopes for conditions that CWCs could exist on a global

scale [e.g.4], but they are more limited in their use for regional analysis. Lunden _et al._18 provided a comprehensive survey of carbonate chemistry measurements specifically surrounding

CWC reefs in the Gulf of Mexico and Flögel _et al._19 more recently provided a summary of carbonate chemistry measurements for CWC in the wider North Atlantic region. However, in general

fine-scale data is still hard to come by for these important organisms and their habitats, yet these CWC exist in highly dynamic environments and their local distribution may be associated

with this natural variability. In the present study, we aim to assess the variability of nutrient and carbonate system dynamics across a range of temporal and spatial scales at four sites of

known (but different) CWC reef habitats in the northeast Atlantic, representing a large diversity of CWC habitats surveyed in one research expedition. The Rockall Bank is situated in the

northeast Atlantic, approximately 400 km west of the Outer Hebrides (Fig. 1a). The depth ranges from over 1000 m at the base of the Bank, to 200 m across much of the top of the bank. Oceanic

banks such as the Rockall Bank are characteristic in deviating major ocean currents to run along their flanks20 and these hydrodynamic regimes facilitate colonisation of CWC reefs21,22. The

flanks of the Rockall Bank are known to contain extensive CWC reef habitats22,23. Two such sites are the Logachev coral carbonate mounds on the southwest flank24; and a site on the

northwest flank (Fig. 1a, b), the Pisces site, first examined by Wilson21 using the _Pisces III_ submersible in 1973 (Fig. 1a), which includes a benthic ecosystem incorporating ‘Wilson ring’

patches of _Lophelia pertusa_ reef habitat (Fig. 1c). Coral carbonate mounds, such as those at the Logachev site (Fig. 1d), are formed from successive periods of interglacial coral growth,

whereas CWC patches (e.g. Pisces) and reefs (e.g. Mingulay Reef Complex) are ‘single generation’ Holocene accumulations25,26. Living reefs have been found at depths from 400 to 800 m at

Logachev and from 200 to 400 m at Pisces. To contrast these deeper reefs, the third site is the Mingulay Reef Complex (MRC), a shallow CWC reef area consisting of a number of individual

reefs (Fig. 1e), including Mingulay Area 01 (MA01) and Banana reef. The MRC is located on the European continental shelf, east of the Outer Hebrides, Scotland (Fig. 1a), where living reefs

are found at depths from 120 to 190 m27,28. The final site was the Hebrides Terrace Seamount (HTS), which is found in approximately 2000 m water depth near the continental slope (Fig. 1a).

Novel visual surveys of the seamount benthos have revealed previously unknown ‘coral garden’ habitats some of which are structured by the colonial scleractinian _Solenosmilia variabilis_

(Fig. 1f). In the present study, additional focused sampling was carried out over a tidal cycle at three stations (MA01, ‘Logachev South (LS)’ and ‘Logachev North (LN)’, Fig. 1) to

investigate the influence of the fine-scale ocean hydrodynamics on the environmental conditions at the depth of the CWC reefs. At the MRC, there is a known tidally-driven downwelling and the

detailed dynamics of the biogeochemical conditions of that site are described in Findlay _et al._29 therefore, only the relative variability is summarised here. In the region of the

southwest Rockall Trough, containing the Logachev sites, there are known diurnal internal waves which cause vertical displacement of water masses resulting in relatively large fluctuations

in temperature and salinity30. Carbonate and nutrient dynamics were therefore measured over a tidal period for both north (LN) and south (LS) of one of the coral carbonate mounds at

Logachev. RESULTS TEMPERATURE, SALINITY & WATER MASSES The northeast Atlantic is a complex region with numerous water masses. The MRC showed distinctly different temperature and salinity

properties from all the other sites, which was primarily due to the coastal influence and addition of freshwater run-off (Fig. 2). All the other sites (Logachev, Pisces and HTS) surrounding

the Rockall Bank and Trough area, showed relatively similar properties throughout the water column (Fig. 2). The surface water (SW, Fig. 2) was indicated by an increase in temperature, but

with little change in salinity. Below the surface water layer was found predominantly East North Atlantic Water (ENAW, Fig. 2), at depths between about 200 m to 700 m. Only Logachev and HTS

were deeper than 700 m. At Logachev, below 700 m, Wyville-Thomson Overflow Water (WTOW, Fig. 2) was identified20 and at HTS, the WTOW was present at depths down to around 1200 m. Below 1200 

m the water was influenced by colder fresher Labrador Sea Water (LSW, Fig. 2). The water mass properties used here are taken from McGrath _et al._20 specifically for the Rockall Trough

region. FLUORESCENCE (CHLOROPHYLL), OXYGEN & PARTICLE ATTENUATION COEFFICIENT (_CP_) Chlorophyll fluorescence and _Cp_ were correlated at all sites, such that as fluorescence increased,

_Cp_ increased (overall slope β = 0.67, _r_ = 0.509, _p_ < 0.0001, supplementary Fig. S1 online). The slope of the fluorescence-_Cp_ relationship was significantly different for each site

(MRC β = 0.644, Logachev β = 0.3724, HTS β = 0.8143 and Pisces β = 1.946; supplementary Fig. S2 online). At MA01 there was a second increase in _Cp_ towards the reef, which did not have a

concomitant increase in chlorophyll (labeled as ‘A’ in Supplementary Fig. S1 online). This increase at depth was also evident at Logachev and at the HTS and is likely to be due to

resuspension of particulate matter from the seabed. There was also a deviation from the general trend in the surface waters at Pisces and the HTS, where fluorescence decreased without _Cp_

decreasing (labeled as ‘B’ in Supplementary Fig. S1 online). At these sites there was clear evidence of a coccolithophore bloom in the upper water column at this time (late May-Early June);

therefore this signal may represent particulates produced from both coccoliths and organic particles during the blooms at these sites. Dissolved oxygen ranged from 206.1 to 288.8 μmol L−1

across all sites and in all cases was seen to decrease with depth. Oxygen saturation ranged between 70 and 90% at the depths where the CWC reefs were located. The shallower sites did not

reach as low oxygen concentrations as the deeper sites (dissolved oxygen was 250.3–286.9 μmol L−1 at the MRC vs. 206.1–268.2 μmol L−1 at Logachev), although there was still a removal of

oxygen at the depths of the reefs. At the HTS, an oxygen minimum (203.6 μmol L−1) was observed at about 1025 m, which corresponded approximately to the depth at the top of the seamount.

Below this oxygen minimum, DO increased again to 257.5 μmol L−1 at about 1900 m. NUTRIENTS AND THE CARBONATE SYSTEM Nitrate, silicate and phosphate concentrations increased with depth at all

sites (Fig. 3). Surface nitrate and phosphate concentrations were lowest at MA01 (2.2 μM and 0.3 μM, respectively) whereas at Logachev and Pisces, the concentrations were still relatively

high in the surface layers (>4 μM and >0.4 μM, respectively) (Table 1). Dissolved inorganic carbon (CT) increased with depth at all sites, even when normalized to salinity (nCT) (Fig.

4). AT was generally more stable through the water column, although there was greater variation through time. When normalized to salinity (nAT), each site showed a slightly different pattern

in alkalinity with depth (Fig. 4). At MA01 and Banana reef there was, on average, a small decrease in nAT with depth (ΔnAT (from surface to the depth of the reef) = ~8 μmol kg−1 and ~5 μmol

kg−1, respectively). At Logachev, the water column was generally well mixed with respect to nAT above 450 m, below which it began to increase from about 2285 μmol kg−1 to a maximum of 2320 

μmol kg−1 at 700 m. The HTS showed a slight decrease from the surface to 250 m (~2310 μmol kg−1 to ~2290 μmol kg−1) and remained stable until below 1000 m when it began to increase again to

~2310 μmol kg−1 at 1500 m. At Pisces there was an increase in nAT with depth, however, this appears to be driven by a decrease in alkalinity in the surface waters as the result of

calcification from a coccolithophore bloom that was occurring at the time of sampling. Formation and remineralisation of organic matter is generally assumed to occur at a C:N:P ratio (the

Redfield ratio) of 106:16:131. Across all sites the nCT:P ratio was lower than the Redfield ratio, at approximately 75:1 (Fig. 5), while the N:P ratio was much closer to the Redfield ratio

at 15.7:1 (Fig. 5). Therefore again indicating an overall signal of carbon addition, although this signal is influenced by the greater sampling effort that occurred at Logachev (Fig. 5). CT

also increased with decreasing temperature by approximately 15 μmol kg−1 °C−1 across all sites except for Mingulay, which showed a shift in CT without a significant change in temperature

(Fig. 5). Statistically, CT and AT profiles were significantly affected by the sites themselves (ANOSIM, _Global R_ = 0.326, _p_ = 0.01). However, pairwise tests revealed MA01 and Banana

reef were not statistically different (_R_ = −0.02, _p_ = 0.57), but MA01 was most distinct from all the other sites (_p_ = 0.01 in all cases). Furthermore, when depth included as a factor,

MA01 is distinct from all sites (_p_ < 0.012 in all cases), except Banana reef (_p_ = 0.598). Between the other three sites (Logachev, Pisces and HTS), only Logachev and Pisces were

statistically distinct (_R_ = 0.255, _p_ = 0.045). pHT ranged from 8.19 to 7.92, decreasing with depth from an average pH of >8.10 in the upper mixed layer. ΩAragonite ranged from 1.1 to

2.6 across all sites and depths; again, generally showing a decrease in saturation state with depth at all sites (Fig. 4). nCT and ΩAragonite both had strong significant correlations with

oxygen % saturation. The relationships are provided here for Logachev site only; these relationships are explored in more detail in supplementary information (S2) online: LOGACHEV FINE-SCALE

DYNAMICS Data from a transect (‘Section 1’ Fig. 1a) onto the Rockall Bank in the Logachev region shows the fine-scale spatial variability and indications of potential upwelling of deeper

waters by internal waves onto the carbonate mounds, where CWC are located between 800 and 600 m (Fig. 6). At both LS and LN, internal waves influence temperature and salinity most

significantly below 500 m30, as evidenced in our dataset: At LS, the daily Δtemperature and Δsalinity at 500 m was 0.71°C and 0.06 psu; at 600 m was 0.90°C and 0.07 psu; and at 800 m was

2.05°C and 0.14 psu. At LN, the daily Δtemperature and Δsalinity at 500 m was 0.78°C and 0.06 psu; at 600 m was 0.88°C and 0.06 psu; and at 800 m was 1.77°C and 0.11 psu. Nitrate, phosphate,

silicate and nCT all showed corresponding changes through time in the water column below 500 m, while nAT showed very little variability through a daily cycle, although fewer discrete

samples for CT, AT and nutrients were taken compared to the CTD data. For example, at both LS and LN, at the depth of 600 m, nCT varied by ~40 μmol kg−1 through the 12 h period, while nAT

varied by only 14 μmol kg−1. The change in CT caused a change in the other carbonate system parameters, for example aragonite saturation state varied by ~0.2 over the period. Because of the

strong relationships with oxygen saturation (%) and the more frequent data available from the oxygen sensor mounted on the CTD, this relationship (Eq. 1) was used to calculate full profiles

for nCT over the 12 h periods at the two Logachev sites (Fig. 6). DISCUSSION Empirical observations for nutrient and carbonate system dynamics are shown for a number of CWC habitats: one

coral carbonate mound, two cold-water coral (CWC) reefs and one Seamount found in the northeast Atlantic region (Fig. 1). The dissolved inorganic carbon (CT) and alkalinity (AT)

concentrations are within 1.5% and 1% respectively of data extracted from the GLODAP database (comparing GLODAP gridded data at 55.5 N, 15.5 W16 with mean values from Logachev at 55.5 N,

15.7 W). The environmental conditions at the study sites predominantly fall within the global envelope for CWCs described in Davies _et al._5. The nutrient concentrations fall within the

range prescribed for the NE Atlantic5; however, here we provide data for the carbonate system parameters that were not previously available at a high resolution of sampling. Flögel _et

al._19 recently published a comparison of available carbonate system data for CWC in the North Atlantic, including the Mediterranean, which fits well with our data collected at similar

locations and supports earlier data from this region [e.g.11,32]. Flögel _et al._19 suggest coral “quality” is associated with a seawater density range similar to that originally described

by Dullo et al.4 and highlighted in niche width models by Davies _et al._5, but also that low quality CWCs are exposed to CT concentrations <2170 μmol kg−1. A number of recent

publications provide a wider comparison of carbonate and nutrient conditions across sites of known CWC reefs, between the NE Atlantic [our data5,19], the Mediterranean, the Gulf of Cadiz and

Mauritania11,19,32, the Gulf of Mexico (GoM)18, Chilean fjords32,33, the Marmara Sea and the Tasman Seamount32 (Table 2). CWC reefs in the NE Atlantic (excluding the Mediterranean) appear

to experience slightly lower CT concentrations than CWC reefs in the GoM, but similar levels of AT, which results in the GoM experiencing, on average, lower ΩAragonite than in the NE

Atlantic (Table 2). Although no assessment of CWC quality is provided by Lunden _et al._18, CT is higher than the proposed limit of 2170 μmol kg−119. Furthermore, Lunden _et al._18 found no

significant relationship between skeletal density and ΩAragonite, which suggests these conditions were not limiting for calcification. The Chilean fjords have similar and slightly lower

levels of CT but significantly lower salinity and consequently significantly lower AT and again therefore experience lower ΩAragonite than the NE Atlantic (Table 2). However, different coral

species (_Desmophyllum dianthus_) are abundant at the Chilean sites and this also needs to be taken into account when assessing CWC sensitivities. While these datasets provide just one or

two data points for each location (for an average or “spot” condition), we highlight here that measuring through time at one site provides as much variability as is found across the sites

(Table 1 and Table 2). For example, at MA01, the localised downwelling caused CT to decrease below the lowest reported value from Flögel _et al._19, to <2090 μmol kg−1, with a total CT

range at MA01 of 40 μmol kg−1. At the Logachev site, upwelling and internal waves caused CT to increase above the highest reported value from Flögel _et al._19, to >2185 μmol kg−1. Across

the Logachev site we found a total CT range of 58 μmol kg−1, which is nearly double the reported range for the Western Rockall Bank (24 μmol kg−119,) for CWC given a quality category of

CI-CII19. Interestingly, the majority of CWC reefs attributed to quality category CIII19 were found predominantly in the Mediterranean, which naturally has high temperatures and salinities,

but additionally in Mauretania and in the Gulf of Cadiz, both of which also have higher temperatures than the majority of CI and CII sites19. pHT and ΩAragonite varied by approximately 0.27

and 1.51 respectively (Table 1), over the spatial (and depth) region investigated here, although the hydrodynamics clearly increased the variability for pHT and ΩAragonite at the sites where

finer time-scale measurements were carried out (MA01, LS and LN; Table 1). Indeed, using _in situ_ lander measurements, Mienis _et al._30 described the fine-scale dynamics at the southwest

Rockall Trough margin (Logachev region) over a longer sampling period (about 1 year) and found that the water current speeds peaked just before or at the same time as the peaks in

temperature and salinity. The physical conditions (temperature and salinity) in our dataset correspond to the observations of Mienis _et al._30, where internal waves cause a diurnal shift in

the physical conditions. The transport of CT and nutrients, along with sediments and particles _via_ these internal waves could prevent a build-up of high CO2, low oxygen conditions around

the CWC reefs, flushing them with fresh material, nutrients and lower CO2 conditions over a diurnal cycle. Therefore on a fine-scale these hydrodynamics create additional variability not

accounted for by cruises that only sample once per station. Indeed Flögel _et al._19 suggest a narrower range of conditions could be considered if Mingulay Reef is excluded from their

assessment because of its downwelling impacts, yet the very nature of this downwelling contributes to a higher variability in chemical components such as CT and nutrients and consequently

impacts the local environment for CWCs and associated biodiversity29. Kenyon _et al._24, suggested that CWC reefs in the North Atlantic were found in the Oxygen Minimum Zone (OMZ). An OMZ

usually corresponds with high CT and pCO2 because OMZs arise from remineralisation of organic matter and therefore consumption of oxygen and release of CO2. Our dataset shows a strong

negative relationship between oxygen % saturation and nCT, such that at the lowest oxygen levels there was highest nCT concentration (e.g. nCT ≈ 2160 μmol kg−1 at 70% oxygen saturation).

Correspondingly, these low DO areas also exhibit the lowest pH and low ΩAragonite (e.g. at 70% oxygen saturation, pH ≈ 7.94 and ΩAragonite ≈ 1.3, although ΩAragonite continued to decrease

with depth after the OMZ because of the influence of low saturation state in the LSW). CWC reefs therefore appear to be thriving in conditions that are conventionally considered unsuitable

for many organisms, particularly calcifiers and this confirms that the variable physicochemical environment found here is not limiting CWC reef growth. The natural fluctuations in the

physicochemical conditions (including oxygen, carbon, nutrients and food supply), driven by large hydrodynamics, may provide flexibility for these organisms, giving them increased adaptation

potential for surviving a range of conditions, as has been found with warm-water corals and temperature34. This naturally fluctuating environment could help to explain the high levels of

variability found in CWCs ocean acidification experiments, especially as several recent laboratory experiments CWCs are able to continue calcifying under high CO2, low ΩAragonite conditions

[e.g.35,36,37]. Interestingly, the carbonate mounds (between 400 and 700 m water depth) at Logachev appear to provide an additional, albeit relatively small, carbon (alkalinity) source to

the water column, compared to the other sites that consist of coral habitats but do not have carbonate mounds present. On average, nCT steadily increased over this depth range at Logachev

yet normalized alkalinity remained the same, hence the seawater pH was somewhat buffered and therefore also remained relatively uniform across the depth range of the mound. The caveat to

this interpretation is the different end-members of CT and AT associated with different water masses; given the complex physical oceanography and diversity of water masses and potential

mixing in this area, further work is required to elucidate if this apparent alkalinity increase is really from the carbonate mounds or if it is from high alkalinity water masses, for example

flowing south from the Arctic, which has a different set of implications in the context for global climate change and ocean acidification. Carbonate system measurements from the deepest

samples taken at Logachev and at the Hebrides Terrace Seamount, show that the aragonite saturation state approaches 1.2–1.3 across the Rockall Trough at approximately 1000 m, decreasing to

near 1.0 at 1500 m. The present day depth of the aragonite saturation horizon (ASH) in the NE Atlantic is therefore similar, or in localised areas shallower, than in model projections for

the North Atlantic (e.g. the 1994 depth was approximately 2500 m38). Future model projections for the ASH show it shoaling to 600–800 m by mid-century (2046–2065) and then further

surface-ward to around 200 m by the end of century (2080–2099)38. Indeed, McGrath _et al._17, show that CT has increased, resulting in an equivalent decrease in pH of 0.040 ± 0.003 units in

the sub-surface waters, over the 19 years that were investigated for the Rockall Trough region. This was concomitant with a decrease in saturation state of both aragonite and calcite and a

shoaling of the ASH. While it appears CWC reefs are naturally exposed and thereby appear tolerant to, a wide range of conditions, the deeper reefs could be at risk of dissolution or be

affected by a shift in their physiology as a result of hypercapnia39,40. Even if the corals themselves have a degree of acclimation in response to high CO2 conditions32,35,36,37,39,40,

dissolution of the coral carbonate mounds, such as those found at Logachev and any dead coral structure, which provides the hard substrate on which corals and abundant epifauna can grow41,

could become unstable as the ASH shoals. Coupled with predicted increased efficiency of bio-eroding sponges42 and a multitude of additional threats43, these deeper reefs could face

significant threat of degradation. Furthermore, the differing rates of acidification occurring in the different water masses that are associated with these reefs (e.g. increased

acidification rate in LSW compared with surface waters17) will also alter the regional threat to these important communities. METHODS A total of 30 CTD and rosette sampling casts were

carried out across the sites (MRC, Logachev, Pisces and HTS (Fig. 1)) between 21st May and 9th June 201244,45 during the ‘Changing Oceans Expedition 2012’, on board the RRS _James Cook_,

cruise JC07346. A Sea-Bird 911 plus CTD system (9_plus_ underwater unit and Sea-Bird 11_plus_ deck unit) was deployed with a Rosette water sampling unit, fitted with 10 L Niskin water

bottles. Water samples were taken at discrete depths throughout the water column with the deepest samples being taken approximately 2 m above the reef (using altimeter information on-board

the CTD). These samples are referred to as “immediately above the reef”. Pre-cruise laboratory calibrations were performed on the conductivity, temperature and pressure sensors, all giving

coefficients for linear fit. Also attached to the Rosette was a Sea-Bird 43 dissolved oxygen sensor, a Chelsea Aquatracka MKIII fluorometer (set to detect Chlorophyll α: excitation

wavelength of 430 nm and emission wavelength of 685 nm) and a Chelsea Aquatracka MKIII Transmissometer, which were used to measure dissolved oxygen, chlorophyll fluorescence and particle

attenuation coefficient (_Cp_; measured at wavelength of 660 nm; e.g. Behrenfeld and Boss47 and references therein), respectively. Dissolved oxygen was calibrated against Winkler

titrations48 made on discrete water samples collected from a range of depths and produced an offset between the titrations and sensor measurements of <1%. Because of the fine-scale of the

hydrodynamics being assessed here, the up-cast raw data were used for the analysis and interpretation of the discrete measurements to match water column state at the time of bottle firing

(see supplementary information S3 online for up-cast vs down-cast variability assessment). Further data processing was performed using the software SBE Data Processing (V7.21g) and for data

visualization Ocean Data View (V4.3.7)49 was used. Seawater was collected from the Niskin bottles for the dissolved inorganic carbon (CT) and total alkalinity (AT) analysis. Samples were

collected in borosilicate glass bottles with ground glass stoppers (50 mL), which were rinsed and filled according to standard procedures detailed in50. Samples were poisoned with 10 μL

mercuric chloride (HgCl2) and duplicate samples were taken from the same Niskin bottle. Samples were returned to the chemical laboratory onboard the RRS _James Cook_, where they were

normalized to room temperature (approx. 24°C) and analyzed for CT and AT within 24 hours of collection. CT was measured using a Dissolved Inorganic Carbon Analyzer (Apollo SciTech, Model

AS-C3) calibrated using CO2 Certified Reference Materials (Dickson, Batch 113). Duplicate measurements provided an estimate of measurement error, which was 0.2% across the entire dataset. An

assessment of errors for each site is provided in supplementary information S4 online. CT was corrected for the addition of HgCl2. AT was measured using the open-cell potentiometric

titration method using an automated titrator (Apollo SciTech Alkalinity Titrator Model AS-ALK2). Calibration was made using CO2 Certified Reference Materials (Dickson, Batch 113). Duplicate

measurements were made for each sample and the estimate of measurement error was 0.4% across the entire dataset. An assessment of errors for each site is provided in supplementary

information S4 online. AT was corrected for the addition of HgCl2. Seawater was collected for nutrient analysis from the CTD Niskin bottles directly after samples were taken for the carbon

analysis. 50 mL of collected seawater was filtered (0.45 μm acid-washed Millipore Fluoropore) into acid-cleaned, aged, 60 mL Nalgene bottles, duplicate samples were collected from each

Niskin. Bottles were stored and shipped back frozen (−20°C) to Plymouth Marine Laboratory, where analysis was carried out51 using a Bran and Luebbe AAIII segmented flow autoanalyzer for the

colorimetric determination of inorganic nutrients: combined nitrate and nitrite52, nitrite48, phosphate53 and silicate54. Nitrate concentrations were calculated by subtracting nitrite

concentration from the combined nitrate + nitrite concentration. The remaining carbonate system parameters (pHT, pCO2, Ωaragonite) were calculated from measured AT and CT, together with

depth, temperature, salinity, silicate and phosphate (when available), using the programme CO2sys55, with dissociation constants from56 refit by57 and for KSO4 from58. In addition to

temperature and salinity, three other major processes influence carbonate chemistry: organic matter formation and remineralisation, calcium carbonate calcification and dissolution and

air-sea gas exchange. At the depth of the CWC reefs described here, air-sea gas exchange is assumed to be negligible; therefore we assume that air-sea gas exchange only contributes to carbon

dynamics in the surface waters. To remove the effects of salinity, CT and AT were normalized to a reference salinity (_S__ref_ = 35) using standard methods (see59 for discussion of

normalization options) and are denoted as nCT and nAT, respectively. For example, for measured AT (_A__T__meas_) and measured salinity (_S__meas_): At the two Logachev stations LS and LN,

CTD profiling was conducted continuously for a period of just over 12 hours at each site (first at LS and then at LN); with additional discrete samples taken at 500 m and 600 m: four times

at (LS) and six times at (LN). At this site the deepest samples were taken immediately above the coral carbonate mounds (see supplementary Fig. S7 online). An ROV survey of the top of the

carbonate mound revealed _Lophelia_ reefs on the top of the mound. An Analysis of Similarities (ANOSIM) test was conducted to assess similarities in the carbonate system and nutrients across

the different sites, using Primer v660. Correlations between variables was analyzed for statistical significance using Pearsons correlation coefficients and in some cases the slopes were

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references ACKNOWLEDGEMENTS This work was carried out as part of the UK Ocean Acidification Research Programme's Benthic Consortium project (awards NE/H01747X/1 and NE/H017305/1)

funded by NERC, Defra and DECC. S.J.H., J.M.N., J.M.R. and L.W. acknowledge additional support from Heriot-Watt University's Environment & Climate Change theme. The authors would

also like to thank the Captain, crew and technicians aboard the RRS _James Cook_ (cruise JC073); special thanks go to Lissette Victorero Gonzalez for laboratory assistance and Karl Attard

for carrying out oxygen Winkler Titrations. All data from cruise JC073 have been deposited at the British Oceanographic Data Centre (BODC). AUTHOR INFORMATION AUTHORS AND AFFILIATIONS *

Plymouth Marine Laboratory, Prospect Place, West Hoe, Plymouth, PL1 3DH, England, UK Helen S. Findlay & E. Malcolm S. Woodward * Centre for Marine Biodiversity & Biotechnology,

School of Life Sciences, Heriot-Watt University, Edinburgh, EH14 4AS, Scotland, UK Sebastian J. Hennige, Laura C. Wicks, Juan Moreno Navas & J. Murray Roberts * Scottish Association for

Marine Science, PA37 1QA, Oban, UK J. Murray Roberts * Center for Marine Science, University of North Carolina, Wilmington, 601 S. College Road, Wilmington, 28403-5928, North Carolina, USA

J. Murray Roberts Authors * Helen S. Findlay View author publications You can also search for this author inPubMed Google Scholar * Sebastian J. Hennige View author publications You can also

search for this author inPubMed Google Scholar * Laura C. Wicks View author publications You can also search for this author inPubMed Google Scholar * Juan Moreno Navas View author

publications You can also search for this author inPubMed Google Scholar * E. Malcolm S. Woodward View author publications You can also search for this author inPubMed Google Scholar * J.

Murray Roberts View author publications You can also search for this author inPubMed Google Scholar CONTRIBUTIONS H.S.F., S.J.H., L.C.W. and J.M.N. collected the data. H.S.F. and E.M.S.W.

analyzed the data. H.S.F. wrote the paper and all authors (H.S.F., S.J.H., J.M.N., L.C.W., E.M.S.W. and J.M.R.) contributed to the final text and figures. ETHICS DECLARATIONS COMPETING

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ARTICLE CITE THIS ARTICLE Findlay, H., Hennige, S., Wicks, L. _et al._ Fine-scale nutrient and carbonate system dynamics around cold-water coral reefs in the northeast Atlantic. _Sci Rep_ 4,

3671 (2014). https://doi.org/10.1038/srep03671 Download citation * Received: 06 September 2013 * Accepted: 10 December 2013 * Published: 20 January 2014 * DOI:

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