Turbulent, stratified flow through a suspended shellfish canopy: implications for mussel farm design

Abstract

Observations were used to quantify the influences of turbulent flow and mixing through the suspended canopy formed by a shellfish aquaculture farm in the microtidal Danish Limfjorden. Observations included current meter/profiler timeseries data and turbulence microstructure profiling. The influence was 2-way in that the turbulence is partially driven by the canopy and in turn the canopy is affected by the turbulence. The canopy reduced the flow speeds within its interior, which was sufficient to reduce levels of turbulent kinetic energy so that withincanopy rates of turbulent kinetic energy dissipation ε were between 10 −8 and 5 × 10 −6 m 2 s −3. Stratification and turbulence were generally inter-related. It was difficult to link the canopy effect to changes in stratification. This was partly due to a high degree of natural variability and partly because the canopy did not appear to generate that much mixing relative to background variability. Vertical diffusivities enabled estimates of the effect of mixing on nutrient depletion and suggest that in the investigated farm set-up vertical diffusivities are secondary in terms of contribution to this relationship but that they could play a dominant role in farms with a more spacious or compressed set-up of canopies. However, vertical flux estimates imply that there must be transverse fluxes of material. Results suggest several avenues for enhanced sustainable shellfish production. For example, canopy flushing can be enhanced with suitable arrangement of crop within a farmed area (heterogeneity). In addition, stratification persists within the canopy and so exposure to nutrient-deplete water can be minimized through staggered crop heights. However, to benefit from this knowledge, improved understanding of long-term variability in the background environment is required.

Magnitude, spatial scale and optimization of ecosystem services from a nutrient extraction mussel farm in the eutrophic Skive Fjord, Denmark

“…Current velocities measured inside the farm were used to account for the effect of mussel structure drag on currents (Cranford et al 2014). Current speeds within the farm averaged 0.027 m s −1 and are in accordance with previously measured current speeds in and around mussel farms in Skive Fjord (Stevens & Petersen 2011). Published CR values for individual M. edulis of standard size vary over a wide range, owing largely to responses to ambient food concentration and composition; in situ CR measurements can differ greatly from the theoretical maximum (e.g.…”

“…Increasing the mussel biomass within the mussel farm would, on the other hand, cause additional flow reduction (e.g. Petersen et al 2008, Stevens & Petersen 2011, Cranford et al 2014, which would increase water re-filtration by the mussels and reduce realized CRs, growth rates and nutrient extraction. Increasing mussel density may therefore be expected to reach a self-limited threshold for the extraction of excess phytoplankton that is specific to a given area.…”

Modelling the cultivation and bioremediation potential of the kelp Saccharina latissima in close proximity to an exposed salmon farm in Norway

“…By comparison, production of Laminaria hyperborea has been reported to be as high as 3000 g C m −2 (Abdullah & Fredriksen 2004). We have not taken into consideration the effects of the fish and kelp farms themselves on the water currents, which may reduce the fluxes of nutrients within the IMTA system (Grant & Bacher 2001, Plew 2011, Stevens & Petersen 2011. Neither have we considered small-scale reduction in DIN concentrations around single kelp ropes due to crowding of the plants.…”

The use of shellfish for eutrophication control

“…The flux of particles into the culture will depend not only on particle concentration but also on current velocity, which is affected by the culture unit structures and the mussels themselves by creating turbulence (e.g. Strohmeier et al 2005;Plew 2011;Stevens and Petersen 2011). The effect of the farm structure on the water column is mainly a modification of the flow pattern, stratification and turbulence in and around the shellfish canopy, but it has been shown that in Danish waters, culture units were not able to mix the water column and disturb stratification (Stevens and Petersen 2011).…”