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 cm 2 s -3 ), and tidal cycles (4 h mixing-on and 2 h mixing- off). 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 composi- tion 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.