BENTHIC BOUNDARY LAYER CHARACTERISTICS IN CONTRASTING COLD-WATER CORAL ECOSYSTEMS

• B io lo g y : F ee d in g , G ro w th a nd R ep ro du c t io n Dynamics within the bottom boundary layer (BBL) are fundamental to the control of the overall benthic ecosystem functioning through processes of organic matter fluxes, turbulence, seabed frictional stresses, and re-suspension processes. Observations of BBL structure using high frequency HF-ADCPs have been made to estimate the frictional stresses within, and adjacent to, coral reef habitats. Measurements at the Tisler Reef, Skagerrak, both in and outside the coral reef have shown that the logarithmic layer approach to estimating bottom stresses can be used and is comparable to directly measured Reynolds stress measurements. Outside regions of live coral clumps, where the seabed is characterised by rubble material, friction velocities (u *, which determines the bottom stress, τ=ρ.u * 2 ) may reach values up to 2 cm s-1 with a mean of 1 cm s-1. A corresponding roughness length, characterising the boundary layer height generated by the seabed, is 2 cm. In contrast, within localized live coral stands, a higher BBL is generated with a roughness length scale of 50 cm and higher friction velocity – a mean of 2.5 cm s-1 and maximum of 5 cm s-1. Interestingly the friction velocities generated by the live reef are higher than those in a rubble area for any particular impinging flow speed, as a result of the greater turbulence generated by the larger bottom topography of the live reef system. This may result in an increased time period for particles to remain in suspension over a reef system compared to other seabed environments. The measurements have been contrasted to those made at a deep carbonate mound located west of Ireland. Here, two bottom landers equipped with HF-ADCPs were deployed near the summit of a mound and in the adjacent gully between two mounds. Boundary layer characteristics in the gully region are similar to those found in the rubble region for the Tisler reef measurements. Measurements near the mound summit indicated higher frictional velocities present compared to the gulley region, again similar to the shallow water observations. The values of frictional velocity generated per impinging flow speed, however, fall between those found for the Tisler observations in and outside the reef, reflecting the bottom topography, which consisted of smaller scale, and more widely dispersed, coral clumps than at the Tisler reef. The results have obvious implications for the fluxes of organic mater, material suspension (availability) and feeding behavior at a coral ecosystem and we suggest they require inclusion into micro habitat mapping and reef growth models.

Ämnesord

NATURVETENSKAP  -- Biologi -- Ekologi (hsv//swe)

NATURAL SCIENCES  -- Biological Sciences -- Ecology (hsv//eng)

NATURVETENSKAP  -- Geovetenskap och miljövetenskap -- Oceanografi, hydrologi och vattenresurser (hsv//swe)

NATURAL SCIENCES  -- Earth and Related Environmental Sciences -- Oceanography, Hydrology and Water Resources (hsv//eng)

Nyckelord

Cold-water corals

coral reef

Lophelia pertusa

marine ecology

BBL

frictional stress

Reynolds stress

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