**5. Conclusions**

The distribution of cold-water corals in the narrow Langenuen Fjord in southwestern Norway (health category II) is limited by physical environmental boundaries and the hydrodynamical setting driven by seasonal and short-term forcings. Summer thermocline with temperatures of over 12 ◦C reached the uppermost populations on the vertical wall reefs (~80 m). This is >4 ◦C warmer than the mean temperature (7.8 ◦C) at the coral living depths and may be over their tolerance limit, thus limiting the vertical extent of coral growth. Variability of chemical parameters during a recorded winter storm (ΔAT = 26 μmol kg<sup>−</sup>1, ΔCT = 57 μmol kg<sup>−</sup>1) was comparable to the measured annual variability (ΔAT = 42 μmol kg<sup>−</sup>1, ΔCT = 57 μmol kg<sup>−</sup>1). This short-term variability was driven by rapid changes in flow conditions and water masses. For all reef settings, but in particular the bank reefs, our findings highlight the importance of sampling at different phases within the tidal cycle when collecting samples for environmental monitoring programs.

Norwegian coastal waters outside Langenuen have warmed since the 1970s on average at a rate of 0.25 ◦C per decade at depths >100 m and up to 0.44 ◦C per decade at 50 m depth. Waters have warmed more during summer and autumn compared to the remaining seasons. This warming has reduced water mass density and its oxygen concentration and strengthened summer and autumn stratification, changing the physical boundaries toward "non-optimal" for coral growth in Langenuen. 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, and corals cannot physically move to adjust to that. Clearly, this aspect needs more attention and should be tackled in future research.

If warming continues at a similar rate in the future, the summer thermocline with temperatures >12 ◦C would reach depths >100 m, and corals would be exposed to hypoxic and corrosive events by 2100. In conclusion, more frequent short-term stress events and the gradual change of mean conditions may restrict the depth zonation of corals and change the benthic species composition in Langenuen, as well as other CWC sites, dramatically.

**Author Contributions:** Conceptualization, S.F., T.K., M.C. and K.J.; software, K.J.; validation, K.J., T.K., M.C. and S.F.; formal analysis, K.J.; investigation, T.K., M.C., K.J. and S.F.; resources, T.K. and S.F.; data curation, K.J., T.K. and S.F.; writing—original draft preparation, K.J.; writing—review and editing, K.J., T.K., M.C., W.-C.D. and S.F.; visualization, K.J.; project administration, T.K. and S.F. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research is part of FATE—Fate of cold-water coral reefs—identifying drivers of ecosystem change project funded by the Research Council of Norway (project no. 244604/E40). We are grateful for additional financial support from the Osk. Huttunen foundation doctorate research grant and the Norwegian Environment Agency—Ocean Acidification Monitoring Program.

**Institutional Review Board Statement:** Not applicable.

**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** The data presented in this study are available on request from the corresponding author.

**Acknowledgments:** We would like to thank captains, crew, ROV pilots, chief scientists, and scientific teams during cruises on RV Håkon Mosby and on RV Kristine Bonnevie (2016–2017) as well as GEOMAR technicians Thorben Berghäuser and Asmus Petersen for their help with the benthic landers. We are also grateful for the valuable feedback provided by the three reviewers and final comments by the academic editors.

**Conflicts of Interest:** The authors declare no conflict of interest.
