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Marine Ecology Progress Series

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MEPS 479:25-45 (2013)  -  DOI:

Effects of shear stress and hard clams on seston, microphytobenthos, and nitrogen dynamics in mesocosms with tidal resuspension

Elka T. Porter1,*, Robert P. Mason2, Lawrence P. Sanford3

1Biology Department, Washington College, 300 Washington Ave, Chestertown, Maryland 21620, USA
2Department of Marine Sciences, University of Connecticut, 1080 Shennecosset Road, Groton, Connecticut 06340, USA
3Horn Point Laboratory, University of Maryland Center for Environmental Science, 2020 Horns Point Road, Cambridge, Maryland 21613, USA

ABSTRACT: 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.

KEY WORDS: Mercenaria mercenaria · Bivalve · Bottom shear · Experimental ecosystem · STURM · Destabilization · Benthic flux · Scaling

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Cite this article as: Porter ET, Mason RP, Sanford LP (2013) Effects of shear stress and hard clams on seston, microphytobenthos, and nitrogen dynamics in mesocosms with tidal resuspension. Mar Ecol Prog Ser 479:25-45.

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