*4.3. Long-Term Changes*

The measured time period (from February 2016 to August 2017) is not long enough to fully capture the interannual variability nor the long-term trends of environmental conditions occurring in the Langenuen Fjord. However, because of the water exchange between Langenuen and the open ocean above sill depth, the mean hydrographic conditions at Langenuen CWC depths can be correlated to the fixed coastal station Utsira south of Langenuen (Figures 1A and 10) [110]. Sætre et al. (2003) [38] reported that both temperature and salinity decreased between 1950 and 1989 along the Norwegian coast. At Utsira, the winter temperature (JFM) decreased from 7.6 to 7.0 ◦C and salinity from 35 to 34.8 [38] at 150 m depth during this period. These trends are affected by basin-wide phenomena such as "great salinity anomalies" (GSAs) [111–113] and the North Atlantic oscillation (NAO) [114]. GSAs are low-salinity/low-temperature events that are formed by cold winters, freshwater outflow, strong northerly winds, and a large sea ice extent in the northwestern Atlantic [112]. A positive NAO index is associated with mild winters, an increase in westerly winds, higher winter precipitation over Scandinavia, and a deeper Norwegian coastal current. GSA salinity minima are reported at Utsira coastal station in 1977–1978, 1987–1989, and 1994–1996 [38]. High temperatures and salinities in between these periods are caused by an increase in the Atlantic inflow combined with the atmospheric conditions associated with periods of positive NAO and a general rise in

temperature over the past decades [115]. The decadal forcing acts together with climate change to determine the past, the present, and the future deep-water hydrography within the region and at Langenuen. The whole water column had warmed after the GSA in the 1980s when only salinity returned to previous levels (Figure 10), indicating ocean warming. At Utsira, the increase has been 0.45 and 0.25 ◦C per decade at 50 and 200 m depths, respectively, since 1975 (Figure 10). If we assume a similar warming trend in bottom temperature at Langenuen, the waters at CWC living depths were ~1 ◦C cooler in the 1970s compared to the study period 2016–2017. Furthermore, if these warming rates continue at Langenuen, temperatures could seasonally increase to >18 ◦C at 50 m depth and >10 ◦C at 200 m depth (Figure 10)**.** The summer thermocline with temperatures >12 ◦C would reach depths >100 m by the year 2100. The warming within the past 40 years has likely already increased the energy demand of CWCs compared to that in the 1970s, with possible effects on coral energy reserves and reproductive output if these are not met by increased food uptake rates [41].

**Figure 10.** Temperature (first row), salinity (second row) and density (third row) at Utsira coastal station (color) ©Havforskningsinstituttet, Korsfjorden-Langenuen-Bømlafjorden area (black) ©Havforskningsinstituttet, and at Langenuen/Nakken (gray). The first column shows Utsira annual mean ± 1σ and linear fit with 95% confidence for 1975–2020 at 50 m (orange line) and at 200 m (green line) depths and individual measurements inside the fjord system at 50 m (plus signs) and 200 m (cross signs) depths. The second column shows the annual cycle at 200 m depth, and the third column the annual cycle at 50 m depth. In the second and third columns, the dashed line shows the average year ± 1σ for Utsira for years 1976–1977, the solid line shows the average year ± 1σ for Utsira 2016–2017, and measurements from Nakken and dotted gray line is the linear estimation at Nakken for the year 2100.

The warming of coastal waters caused by climate change has likely led to a decreased frequency of intrusions of dense, oxygen-rich North Atlantic water with relatively high ΩAr levels and has caused a general oxygen decline in the basin waters of some western Norwegian fjords with shallow sills (i.e., <100 m) [45,116]. Furthermore, warmer water contains less oxygen. In the Masfjorden, north of Bergen, this multidecadal decline corresponds to a loss of 2.0 mL L−<sup>1</sup> over 42 years and an associated 1 ◦C rise in temperature [45]. A similar decline in oxygen has been reported in Byfjorden, off the city of Bergen, with an accompanying shift in benthic communities toward domination of opportunistic benthic species [116]. Given the observed warming in Utsira, oxygen concentrations have likely decreased in Langenuen as well and are likely to decrease even further in the future. Hebbeln et al. (2020) [35] suggested that *L. pertusa* populations are highly sensitive to low oxygen conditions of 40%–50% lower than the ambient oxygen values. From observed

conditions, a decline of 40% from present values would set the lower oxygen limit in Langenuen to ~2.5 mL L−<sup>1</sup> compared to the global limit of <1.5 mL L−<sup>1</sup> [36,117,118]. If a decline of 0.5 mL L−<sup>1</sup> per decade continued linearly in Langenuen, corals would be exposed to hypoxic conditions by the end of the century and to conditions <2.5 mL L−<sup>1</sup> by the year 2070.

Besides warming waters and oxygen decline, salinity in the surface layer has been decreasing along the Norwegian coast [38]. At Utsira, the decline at 50 m depth has been up to −0.05 g kg−<sup>1</sup> per decade between May and September since 1975 (Figure 10), but salinity has increased slightly during other months and at other depths. This longterm decrease in salinity at the surface is partly caused by increased precipitation and retreat/melting of glaciers, substantially increasing freshwater run-off. Førland and Hanssen-Bauer (2000) [119] demonstrated that precipitation in northern Norway has increased by 1.7% per decade during 1961–1990. Hanssen-Bauer et al. (2003) [120] projected an annual increase of 15% in precipitation from 1980–1999 to 2030–2049 in western Norway. During the last decade, the glaciers of Norway have continuously retreated ([121] and references therein). This could further increase the surface stratification and reduce the surface-bottom interaction during the melt water period.

Driven by warming, the water column has become less dense within the past 40 years in Utsira at a rate of 0.037 kg m−<sup>3</sup> per decade at 50 m and ~0.02 kg m−<sup>3</sup> per decade at 200 m depth. If density has changed similarly in Langenuen, waters at CWC living depths would have been within the suggested sigma-theta range for thriving CWC sites in the NE Atlantic (>27.35 kg m<sup>−</sup>3) [12] during the 1970s but are lower than that now. Such changes may be critical because the depth zonation of CWCs on the vertical walls may be related to physical boundary conditions at specific depths that act to concentrate food particles [122], and corals cannot physically move to adjust to that.

Freshwater from rivers, rain, and melting glaciers have been observed to decrease AT and ΩAr substantially and play a large role in increasing ocean acidification in surface waters [123,124]. These waters could reach the coral depths during autumn and winter mixing even though the summer stratification is likely getting stronger. Therefore, there is a weak positive indication that the influence of fresh coastal water into the deeper water masses of the fjord may locally accelerate ocean acidification at CWC living depths.

Our hindcast suggests that ocean warming and acidification may already today affect the functioning of Langenuen CWCs. As oceanic uptake of atmospheric CO2 continues [115], warming and ocean acidification continue in the western Norwegian fjords. We, therefore, stipulate that these shallow fjord reefs could serve as windows into the future and forewarn about the likely effects of ocean acidification and warming on CWC reef biodiversity and productivity. Studies from tropical coral sites suggest that the rate of calcification is lower under recent conditions (~400 ppm) compared to pre-industrial (280 ppm) times [125,126], and enhanced reef growth has been observed in situ in lagoons where pCO2 levels have been manipulated to pre-industrial levels. McGrath et al. (2012) [127] showed that CT has increased in subsurface waters in the NE Atlantic between 1991 and 2010. This was concomitant with a decrease in ΩAr and a shoaling of the ASH. Within this century, ~70% of the known CWC habitats are predicted to be in corrosive waters [49]. The ocean acidification monitoring program of Norwegian waters [48] has shown an annual decrease in ΩAr and pH of 0.007 and 0.02 in Korsfjorden at depths of ~670 m between 2007 and 2017, respectively. The decrease in ΩAr and pH has been reported to be greater in coastal areas than in deeper waters (~2000 m) offshore [48]. If the carbonate system changed at similar rates at ~200 m depth in Langenuen, CWCs would be exposed to corrosive conditions (ΩAr < 1) by 2090. Given their shallower location in a narrow fjord, it is likely that the rate of change is even larger, and ΩAr < 1 conditions would occur even sooner [51]. The living coral can withstand some corrosive conditions with pH up-regulation [10,128], and in the Mediterranean, ΩAr < 0.92 was found to be the calcifying limit for *L. pertusa* [129]. However, the exposed dead coral skeleton, which frequently forms the largest portion of the CWC reef, starts to dissolve in corrosive conditions [130].
