Field and laboratory observations of bed stress and associated nutrient release in a tidal estuary

Nutrient release driven by sediment resuspension in a shallow coastal estuarine system is examined with field observations of bed stress and bed elevation, coupled with laboratory erosion experiments on sediment cores. Two field experiments were conducted over near-cohesive muddy-sand sediments in the Great Bay Estuary, New Hampshire. In the first deployment, boundary layer development during typical summer tidal forcing was observed, while the second deployment occurred under enhanced wind forcing of Tropical Storm Irene. In situ bed stress and erosion depths were estimated with a profiling acoustic Doppler velocimeter. Sediment cores were subjected to EROMES erosion chamber experiments to determine erosion depth and nutrient release as a function of applied shear stress. Results show erosion depths are consistent with in situ observations over shear stresses ranging from 0.10 N m−2 (incipient motion) to 0.35 N m−2 (resuspension events). Erosion chamber experiments showed that ammonium release (up to 2 mmol m−2) increased with bed stress in both spring and summer. However, phosphate release was more variable, with no phosphate release during resuspension in spring and a variable phosphate flux (ranging −0.5–2 mmol m−2) in summer. Increased hydrodynamic forcing during a storm event in the summer generated shear stresses (up to 0.58 N m−2) during flood tides that exceeded the threshold for sediment motion, and resulted in erosion of the seabed. EROMES results predict there was concomitant release of nutrients into the water column from the muddy sediments of the Bay, and the release of dissolved inorganic nitrogen and phosphate was up to 10% and 65%, respectively, of the summer monthly riverine input of these nutrients. Results indicate qualitatively that in shallow, tidally dominated estuaries, fine-grained sediment beds may be a source of nutrients that are particularly important during storms that enhance near bed shear stresses.