*Article* **Cold-Water Coral Reefs in the Langenuen Fjord, Southwestern Norway—A Window into Future Environmental Change**

**Katriina Juva 1,\*, Tina Kutti 2, Melissa Chierici 3, Wolf-Christian Dullo <sup>1</sup> and Sascha Flögel <sup>1</sup>**


<sup>3</sup> Institute of Marine Research in Norway, Fram Centre, 9007 Tromsø, Norway; melissa.chierici@hi.no

**\*** Correspondence: katriina.juva@iki.fi

**Abstract:** Ocean warming and acidification pose serious threats to cold-water corals (CWCs) and the surrounding habitat. Yet, little is known about the role of natural short-term and seasonal environmental variability, which could be pivotal to determine the resilience of CWCs in a changing environment. Here, we provide continuous observational data of the hydrodynamic regime (recorded using two benthic landers) and point measurements of the carbonate and nutrient systems from five *Lophelia pertusa* reefs in the Langenuen Fjord, southwestern Norway, from 2016 to 2017. In this fjord setting, we found that over a tidal (<24 h) cycle during winter storms, the variability of measured parameters at CWC depths was comparable to the intra-annual variability, demonstrating that single point measurements are not sufficient for documenting (and monitoring) the biogeochemical conditions at CWC sites. Due to seasonal and diurnal forcing, parts of the reefs experienced temperatures up to 4 ◦C warmer (i.e., >12 ◦C) than the mean conditions and high CT concentrations of 20 μmol kg−<sup>1</sup> over the suggested threshold for healthy CWC reefs (i.e., >2170 μmol kg<sup>−</sup>1). Combined with hindcast measurements, our findings indicate that these shallow fjord reefs may act as an early hotspot for ocean warming and acidification. We predict that corals in Langenuen will face seasonally high temperatures (>18 ◦C) and hypoxic and corrosive conditions within this century. Therefore, these fjord coral communities could forewarn us of the coming consequences of climate change on CWC diversity and function.

**Keywords:** ocean acidification; ocean warming; carbonate chemistry dynamics; biogeochemical dynamics; in situ monitoring; natural variability of environmental conditions; *Lophelia pertusa*
