Effects of shear stress and hard clams on seston, microphytobenthos, and nitrogen dynamics in mesocosms with tidal resuspension
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Type of Work22 pages
Citation of Original PublicationPorter, Elka T.; Mason, Robert P.; Sanford, Lawrence P. (2013) Effects of shear stress and hard clams on seston, microphytobenthos and nitrogen dynamics in mesocosms with tidal resuspension. Marine Ecology Progress Series 479:25-45.
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.