*4.2. Hydrographic and Biogeochemical Conditions*

Seasonal signals in hydrography and biogeochemistry in the Langenuen Fjord are strong, similar to the general area [92]. All CWC reefs in Langenuen occur in welloxygenated (O2 > 4.9 mL L−1) and inorganic carbon-rich (CT > 2135 μmol kg−1) waters. The hydrographic (Θ and SA), alkalinity (AT), and inorganic nutrient (NO3 −, PO4 <sup>3</sup>−, and SiO4 <sup>4</sup>−) ranges are within previously reported values from NE Atlantic CWC sites (Table 5) [11,13,29,39,53].

Due to surface warming in spring and summer, the seasonal isopycnal reaches ~100 m. The uppermost CWCs on the wall reef setting (at 80–100 m, HH) experience temperatures >12 ◦C in late summer before the water is mixed in late autumn. This is ~4 ◦C warmer than the mean temperature at these shallow reefs and likely enhances the metabolism of *L. pertusa* and increases their energetic demand [36,41]. Together with strong flow speeds (limiting prey capture rates in *L. pertusa*), seasonal high temperatures could restrict the

upper limit of the CWCs on the fjord wall. Beneath the seasonal warming layer of 100 m, annual ranges of hydrographic variables were small (ΔΘ = 1.92 ◦C, ΔSA = 0.96 g kg−1) particularly when compared to the largest measured temperature fluctuation at any CWC site, namely the ~9 ◦C (5.8–15.2 ◦C) temperature fluctuation within a day registered in the Cape Lookout area, NW Atlantic [105]. Bottom temperatures decrease in late winter due to dissipation from the surface and remain low throughout summer due to the pronounced water column stratification established by the spring freshwater flood [106]. Due to relatively warm winter temperatures in the deeper layer and the freshwater influence, reefs occur in less dense waters than suggested for healthy CWC sites in NE Atlantic (i.e., σ<sup>Θ</sup> = 27.35–27.65 kg m<sup>−</sup>3, [12]).

At Langenuen, the dissolved inorganic carbon was at times high, and a maximum value of 2192 μmol kg−<sup>1</sup> was observed. This is higher compared to what is previously linked to thriving NE Atlantic CWC occurrences (CT < 2170 μmol kg<sup>−</sup>1, [15]). On the other hand, total alkalinity (2300–2343 μmol kg−1) was similar to other NE Atlantic CWC sites (2287–2377 μmol kg<sup>−</sup>1, Table 5). For comparison, the typical carbonate chemistry properties in Atlantic core water in the Norwegian Sea is about 2160 μmol kg−<sup>1</sup> and 2310 μmol kg−<sup>1</sup> for CT and AT, respectively [107]. Regional studies of carbonate chemistry surrounding CWC ecosystems in the Gulf of Mexico [14], Gulf of Cadiz [15], Mauretania [15], Mediterranean [10,15], and Marmara Sea [10] have shown that CT > 2170 μmol kg−<sup>1</sup> is common for CWC sites outside the NE Atlantic (Table 5) while significantly higher (AT > 2500 μmol kg−1) alkalinity levels are measured only at the Mediterranean and the Marmara Sea CWC sites. Together with regional hydrographic (Θ, SA) conditions, the relatively high AT result that the aragonite saturation is also relatively high (ΩAr > 2.5) in the Mediterranean CWC sites compared to other basins, where values <2.0 are common (Table 5). Consequently, this study emphasizes the importance of estimating both CT and AT since it is the relationship between them that determines the CaCO3 saturation. These regional ranges are obtained from single time point measurements that do not include the variability on the diurnal level.

**Table 5.** The range (published min–max) of environmental conditions at different regions, specifically at the depth and site of known *Lophelia pertusa, Madreora oculata, or Desmophyllum dianthus* occurrences, comparing data from this study with data available in the literature. Abbreviations: GoC: Gulf of Cadiz, Ma: Mauretania, Me: Mediterranean, GoM: Gulf of Mexico, CF: Chilean Fjords, MS: Marmara Sea, TS: Tassman Seamount, C-Refs.: References for carbon system data, N-Refs.: References for nutrient data, nd: no data reported, \* Only mean salinity reported in [108].


The short-term variability in carbonate chemistry in Langenuen is large, and the highest values are linked to the tidal cycle. Similarly, high CT concentrations have been observed in dynamical NE Atlantic CWC mounds in the southern Rockall Bank [53]. There, values up to 2186 μmol kg−<sup>1</sup> were measured over a tidal cycle, indicating that *L. pertusa* is resilient to CT levels exceeding 2170 μmol kg<sup>−</sup>1, at least over short time scales. At Rockall Bank, the tidal range of CT was 58 μmol kg−1. This is similar to short-term (within 16-h) variability measured at NK reef (ΔCT ≈ <sup>57</sup> <sup>μ</sup>mol kg−1) in January 2017 and to the annual ranges of CT at CWC living depths at other Langenuen reefs (ΔCT ≈ <sup>51</sup> <sup>μ</sup>mol kg−<sup>1</sup> at HH, and <sup>Δ</sup>CT ≈ <sup>39</sup> <sup>μ</sup>mol kg−<sup>1</sup> at SN). The short-term changes in carbonate chemistry at Langenuen reefs cannot be explained by salinity and temperature changes alone. For example, the pCO2 change is about 4.2% per 1 ◦C, implying that the temperature effect on pCO2 could explain ~5 μatm of the 124 μatm difference recorded at Nakken reef during 16 h in January 2017. This large variability is most likely caused by rapid change in the dominant water mass from Norwegian coastal water to North Atlantic water. NAW has high pCO2, thus large CT, but also relatively high AT. Therefore, pHT and ΩAr are generally higher in NAW than in waters containing freshwater (such as NCW), and large changes in the whole carbonate chemistry are possible during short time periods. It is also plausible that the presence of North Atlantic water with higher ΩAr compared to Norwegian coastal water is buffering ocean acidification in Langenuen, and strengthening of the coastal stratification outside the fjord system caused by warming could reduce its presence at the Langenuen coral reefs in the future.

Observed differences between the five reefs were not consistent between the different time periods or seasons sampled. This variability is at least partly a consequence of the short-term variability that is not captured with single time point measurements or replicates taken close to each other in time. The observed variability over the 16 h sampling period in January 2017 was larger both between the sites and at NK compared to other months. The high fluctuation in winter could be caused by a combination of the storm event and dynamical winter conditions (i.e., weaker stratification, stronger flow, water source variation between the north (Korsfjorden) and south (~Utsira) more than during other seasons), see Figures 2, 4, 6 and 9. This kind of extreme event is likely to occur each year, and based on meteorological reports [90,91], it is likely that even more energetic events occurred in winter 2016–2017. Since the tidal component causes large variability in the carbonate chemistry, the full seasonal and diurnal variability of the carbonate and nutrient chemistry should be measured in further ocean acidification studies [48].
