Experimental assessment of the effects of cold-water coral patches on water flow

Despite their importance for ecosystem functioning, little is known about flow patterns at the cold-water coral-patch scale (i.e. cm to m). In situ flow speed measurements using moorings or landers only provide general near-reef flow patterns, and experimental studies so far have focussed on single coral branches for feeding and behavioural studies. We addressed this knowledge gap by means of a flume study. The effect of different coral patches (4, 10 and 25 cm height), created from dead coral branches, on the ambient flow field and the refreshment rate of water within a patch was assessed using 3 realistic current speeds (0.05, 0.15 and 0.3 m s ⁻¹ ). High bottom roughness in combination with strong currents increased current velocity and turbulence in the wake of all patches, even with very low relief. The formation of 2 dynamically different environments was observed. The framework−water interface was characterised by high turbulence and enhanced vertical turbulent transport of momentum, while in the wake, reduced turbulence and vertical mixing activity was observed, characterised by near stagnant flows. Subsequently, water refreshment rates within a patch drastically decreased at current speeds <0.2 m s ⁻¹ , while near to unobstructed stream conditions were observed at current speeds >0.2 m s ⁻¹ . Combining flume observations with available in situ data suggests that heterogeneity and patchiness of cold-water coral growth is likely induced by flow patterns at the coral-patch scale, influencing the fate of particulate and dissolved matter as well as oxygen exchange rates in and around the reef.