Effect of tidal resuspension on benthic–pelagic coupling in an experimental ecosystem study

Abstract

To test the effect of sediment resuspension on the nutrient and ecosystem dynamics we performed a 4 wk experiment in three 1000 l shear-turbulence-resuspension-mesocosm (STURM) resuspension (R) tanks and three 1000 l non-resuspension (NR) tanks. All tanks contained defaunated muddy sediment and brackish 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 mixingoff). However, while bottom shear velocity (stress) was low in the NR tanks, high instantaneous bottom shear produced resuspension in the R tanks during the mixing-on cycles. Tidal resuspension in the R tanks resulted in concentrations of 120 to 220 mg l–1 total suspended solids when mixing was on, decreasing to between 10 and 20 mg l–1 when mixing was off. Particulate nitrogen, phosphorus, and carbon concentrations, as well as dissolved inorganic nitrogen, nitrate + nitrite, and phosphate levels were higher in the R tanks. Phytoplankton biomass was also higher in the R tanks, though light was limiting. Tidal resuspension affected water-column algal and zooplankton community composition and induced significantly higher concentrations of brown tide Aureococcus anophagefferens. Microphytobenthos biomass was significantly higher in the NR tanks. Dissolved inorganic nitrogen sediment effluxes were similar in both tanks; however, polychaetes and amphipods developed in the NR, but not in the R tank sediments. Tidal resuspension shifted processes from the benthos to the water column. Regular tidal resuspension profoundly affected ecosystem structure and function, often through indirect pathways and linkages.