Browsing by Subject "Experimental ecosystem"
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Item Combined water-column mixing and benthic boundary-layer flow in mesocosms: key for realistic benthic–pelagic coupling studies(Inter-Research, 2004) Porter, Elka T.; Sanford, Lawrence T.; Gust, Giselher; Porter, F. ScottWe developed 2 scaled linked mesocosms that realistically mimicked both water-column 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 second 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 significantly 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 significantly 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 studies. 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.Item Effects of shear stress and hard clams on seston, microphytobenthos, and nitrogen dynamics in mesocosms with tidal resuspension(Inter-Research, 2013) Porter, Elka T.; Mason, Robert P.; Sanford, Lawrence P.To test the interacting effects of hard clams Mercenaria mercenaria and bottom shear stress on nutrient- and ecosystem dynamics, we performed a 4 wk experiment in six 1000 l shear turbulence resuspension mesocosms (STURM). Three tanks each contained 50 hard clams (RC set-up), and 3 tanks had no clams (R set-up). All tanks contained defaunated muddy sediment and estuarine water and had similar water column turbulence intensities (~1 cm s−1), energy dissipation rates (~0.08 cm2 s−3), and tidal cycles (4 h mixing on and 2 h off). The same high instantaneous bottom stress (0.35 to 0.4 Pa) was applied to all tanks during the mixing-on cycles. Hard clams in interaction with high bottom shear stress initially destabilized the sediments and increased seston levels to ~200 mg l−1 in the RC tanks during the mixing-on cycles. Over time, seston concentrations declined in the RC tanks until they reached levels similar to the R tanks of ~60 mg l−1. Bivalve feeding in the RC tanks significantly reduced phytoplankton biomass and shifted the phytoplankton community structure to Chlorophyceae/Prasinophytes. Nutrient (particulate phosphorus, nitrogen, and carbon, dissolved inorganic nitrogen, nitrate + nitrite, phosphate) concentrations were significantly enhanced in the RC tanks, mediated by high sediment resuspension and bivalve excretion. A brown tide organism, Aureococcus anophagefferens, bloomed in 2 of 3 RC tanks. Bivalve feeding and light limitation reduced microphytobenthos biomass in the RC tanks. Microphytobenthos biomass was low overall but significantly higher in the R tanks. Phytoplankton abundance, microphytobenthos biomass, seston concentrations, and nitrogen dynamics were significantly affected by interactions between hard clams and bottom shear stress.