Combined water-column mixing and benthic boundary-layer flow in mesocosms: key for realistic benthic-pelagic coupling studies

Abstract

We developed 2 scaled linked mesocosms that realistically mimicked both water-col- umn mixing and benthic boundary-layer flow, enabling more realistic benthic-pelagic coupling experiments. The first was a 'large' 1000 l system linking a mesocosm with an annular flume; the sec- ond a 'small' 100 l system linking a mesocosm with a Gust microcosm. We compared bottom shear velocity, flow speeds, and internal mixing energies between linked and isolated mesocosms that were the same in volume and shape, and compared them to nature. In addition, we performed scaled 4 wk long comparative ecosystem experiments with oysters in the large and small mesocosms to determine if a realistically mimicked benthic boundary-layer flow and system shape could signifi- cantly affect ecosystem processes. We scaled all 4 systems to have the same realistic water-column turbulence levels and increased bottom shear velocity to moderate levels in the linked mesocosms. Bottom shear remained unrealistically low compared to nature in the isolated tanks. In addition, the water column and the sediment-water interface were more realistically connected in the linked than in the isolated mesocosms. The linked mesocosms had a similar scaling relationship of turbulence intensity and bottom shear velocity of 1.6, as found in nature. System shape and bottom shear signif- icantly affected ecosystem properties through changes in light, microphytobenthos biomass growth and erosion, sediment inorganic nutrient fluxes, oyster growth, and water column nutrient dynamics. In this study we show that a commonly used system shape in ecosystem studies and unrealistically low bottom shear in mesocosms both produce significant artifacts in benthic-pelagic coupling stud- ies. We also demonstrate improved systems without these artifacts. System shape, bottom shear, water-column turbulence levels, and their ratios should all be considered in designing mesocosms to mimic natural processes.