*4.1. Flow Dynamics*

All five CWC reefs are healthy, but their limited spatial extent suggests that areas with suitable living conditions for CWCs are narrow both vertically and horizontally. CWC occurrences, both on the walls and banks coincide with the most dynamical parts of the fjord.

The flow regime in Langenuen is controlled by the Norwegian coastal current and modified by bathymetry, wind conditions, and tidal forcing [83–86]. In the upper 100 m, the flow is strong with peak speeds of >100 cm s<sup>−</sup>1. At *L. pertusa* living depths (100–220 m), water flow is predominantly southward with mean speeds of <20 cm s−<sup>1</sup> and peak speeds of ~60 cm s−<sup>1</sup> (Figure 9). This is comparable to other CWC sites in the NE Atlantic [27,31,87]. The flow down to ~120 m is driven by southerly winds and Norwegian coastal water entering the fjord above the sill depth from the south [88,89]. Flow in deeper water layers is driven by seasonal density differences outside the fjord system. For example, the flow is reversed to northward-dominated at 120–200 m depth in December and in January (Figure 9C), when prevailing northerly winds (Figure 4D) [90,91] create coastal upwelling that pushes warm and deep North Atlantic water over the sill and into the deep fjord basin [92]. Intrusions of high salinity and relatively warm North Atlantic water renew the basin water in the fjords [89,93] and maintain aerobic conditions at the bottom [94]. At Nakken, the tidal flow was small compared to the residual flow (~5 vs. ~20 cm s−1). This is common in these coastal waters [95,96]. Similar amplitudes have been recorded in Hardangerfjord, the main fjord to which Langenuen is a tributary [93].

In the Langenuen Fjord system, the distribution of CWC reefs, both on walls and banks, seems to be governed by small-scale hydrodynamics. At the wall reefs of HH and SN, *L. pertusa* corals live just beneath the seasonal thermocline and the fastest flow layer (>100 m). The upper vertical limit of the corals could be set by this fast flow layer as in strong currents, the polyps could bend backward, reducing the feeding surface [97], and prey could escape from the polyps [98], restricting energy uptake in the coral. The 100 m depth also coincides with the strongly stratified layer in late summer and autumn (N > 20 × <sup>10</sup>−<sup>3</sup> <sup>s</sup>−1, Figure 2), which could reduce the zooplankton migration to CWC living depths [99,100]. Below 100 m, flow is slower, and the framework of the CWCs further reduces flow speeds due to friction [28] toward the efficient prey capture speeds of the *L. pertusa* [34,98]. Moreover, the CWC framework acts as a natural sediment trap allowing the corals to capture and use the enhanced particle delivery. At these depths, thriving colonies of *L. pertusa* and *M. oculata* occur on the vertical walls. The lower vertical distribution limit of the CWC growth on vertical walls is created by the lack of hard bottom substrate when moving beneath 200 m depth at Straumsneset and Huglhammaren and below 240 m depth at Hornaneset.

At the bank reefs, the topography-flow interaction is suggested to directly influence the health and growth of CWCs [22]. At the Nakken bank reef, the flow supports periodic hydraulic jumps at high flow speeds. This will create a link between the surface and the deep reefs. During these turbulent events, resuspended organic particles that have settled to the sea bed would locally elevate food supply to the reefs compared to the surrounding deep sea-bed. These processes could be particularly important during periods of food limitation [101]. However, during the periods of periodic hydraulic jumps also mineral particles will be resuspended and settled. It is assumed that patchy reefs with similar flow regimes would have high vertical growth rates to prevent burial [22]. Maier et al. (2020) [101] estimated the linear growth at NK to be 13 mm year−<sup>1</sup> for new polyps. This is 30% to 100% higher than the linear growth measured for other Norwegian reefs located farther north [102,103]. These areas occur likely in a more dynamic flow state preventing the settling of particles and keeping the reef clear from heavy sedimentation. This would reduce the need for large vertical growth while increasing the need for horizontal growth and bridging between polyps, enforcing the coral skeleton to withstand high physical forcing without breaking [22]. At the Nakken bank reef, turbulent conditions created by hydraulic jumps are only present in winter, i.e., from October to December (Umean >20 cm s<sup>−</sup>1, and Umax >50 cm s<sup>−</sup>1, Figure 9). It is plausible that the emergence and growth of new polyps, taking place at this particular reef from December to March [101], is initiated by the increased sedimentation caused by the periodic hydraulic jumps. Growth rates 60% higher than those measured at NK have been documented for *L. pertusa* in the Gulf of Mexico, where corals form similar patches as observed in NK [104].
