Gorgonian corals occur extensively at continental slope depths > 200 m off the southwestudGrand Banks of Newfoundland. Among these corals, Keratoisis grayi formsudgorgonian coral thickets on cobbles and boulders in otherwise muddy sand habitats.udThese thickets are believed to form a critical benthic habitat, in particular for juvenileudfish, and as such are an integral part of the ecosystem. These coral thickets areudimpacted by bottom trawling activity which therefore could have far reaching consequencesudfor the larger ecosystem. This thesis reports on a study of how the oceanudbottom boundary layer is affected by the presence of coral thickets. This informationudis important both to establish the characteristics of coral habitat but also to demonstrateudhow the removal of corals modifies the boundary layer which would in turnudmodify the benthic environment.udBottom boundary layer currents in coral habitat in Haddock Channel were characterizedudusing two 2-MHz acoustic Doppler current profilers.The profilers were deployedudon the seafloor at a depth of 700 m, looking upward, for 85 hours, beginningudJuly 17th, 2007. The effective vertical profiling range was 4 meters, with 1 meteruddepth resolution, sampling every 2.7 minutes. One instrument was placed in an areaudwhere bamboo corals (Keratoisis grayi ) extend approximately to 1 meter in heightudand occur with a density on the order of 1 colony per square meter (Coral Site). Theudsecond instrument was deployed 100 meters away in an area with visually similar sea floor characteristics, but from which the corals had been removed by a researchudbottom trawl (Mud Site). Mean flow speeds at both the Mud and Coral Site areudon the order of 10 cm s⁻¹, which is consistent with previous current data from theudgeneral area. Observed currents showed some evidence of tidal forcing but other nonlinearudprocesses clearly influence the current regime. Speed profiles were fitted to theudlogarithmic law of the wall to obtain bottom roughness zₒ, and friction velocity u∗udestimates. Both the Mud and Coral Site appear to conform to the logarithmic law ofudthe wall for turbulent boundary layers.udFriction velocity (u∗) estimates at flow speeds less than 5 cm s⁻¹, were consistentlyudhigher at the Coral Site, relative to the Mud Site; Mud Site u∗ values were 30%udto 80% of Coral Site estimates, indicating increased turbulence due to the presenceudof corals. However, friction velocity increased faster with flow speed at the MududSite, suggesting that at higher flow speeds coral induced bottom roughness is lessudimportant to friction velocity. There was significant uncertainty in bed roughnessudestimates, however bed roughness values at the Coral Site (mean zₒ = 0.51 ± 0.28udcm), were found to be generally higher at flow speeds below 5 cm s⁻¹, compared toudthe Mud Site (mean zₒ = 0.27 ± 0.40 cm), again possibly indicating that the coraludare affecting the hydrodynamic roughness at low speeds.udBackscatter intensity was also examined as an indication of suspended organicudmaterial. It was impossible to make relative absolute comparisons between the twoudsites but relative changes in backscatter intensity could be compared. Backscatterudlevels from both sites increased as flow speeds increased, up to 7 cm s⁻¹. Backscatterudincreased faster with flow speed at the Mud Site, relative to the Coral Site, for speedsudbetween 2.5 and 7 cm s⁻¹, which is broadly consistent with the suggestion of greaterudincrease in friction velocity values seen at the Mud Site.udAbove 7 cm s⁻¹, Coral Site backscatter intensity increased substantially, while the corresponding Mud Site backscatter intensity declined. The reason for this change inudtrend is unclear, a possible explanation being a transition into a different flow regimeudwhereby increased flow speeds interact favourably with the rough topography createdudby the corals. However, the large uncertainty estimates for backscatter levels at theseudhigher speeds were such that it was very difficult to draw firm conclusions.udWe conclude that higher u∗ and zₒ estimates at low flow speeds at the Coral Site,udrelative to the Mud Site, are consistent with the hypothesis that the enhancement ofudturbulence due to the coral behaving as roughness elements is significant only at lowudflow speeds, enabling coral polyps greater opportunities to extract organic materialudfrom the water column due to resuspension of organic material from the sea floor.
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