Spatial and temporal variation in cohesive sediment erodibility in the York River estuary, eastern USA: A biologically influenced equilibrium modified by seasonal deposition

摘要

Sediment erodibility was measured at three sites in the York River, a sub-estuary of the Chesapeake Bay, monthly to bimonthly from April 2006 through October 2007. Erodibility at the three sites was similar during the summer and fall. A site near the estuary mouth maintained this level of erodibility greater than 90 percent of the time while two sites in the more physically dominated mid-estuary region exhibited a consistent and pronounced increase in erodibility in the late winter and spring. Weak to non-existent correlations between bed erodibility, solids volume fraction, and surficial concentrations of organic matter, colloidal carbohydrate, and extracellular polymeric substances, were not sufficient to explain the observed seasonal pattern in bed erodibility. Digital X-radiographs revealed thick sequences (10-20+ cm) of laminated sediments at the surface in the middle estuary coincident with the period of highest erodibility and more biologically reworked sediment during the rest of the year, suggesting that periodic rapid deposition introduced new sediment that was seasonally easy to erode. The finding that seasonal deposition influenced erodibility at the mid-estuary sites is consistent with previous results indicating the occasional presence of a secondary turbidity maximum. Comparison of the biologically reworked, but still "low" erodibility condition in the York to other published Chesapeake Bay erodibility data revealed a consistent critical shear stress range and profile, suggesting that this equilibrium critical stress profile may be representative of other similar estuarine environments in the absence of rapid deposition. At relatively low stresses and in the absence of rapid deposition, we speculate that burrowing and/or pelletization may play a role in maintaining high equilibrium bulk water content without reducing the strength of the surface of the seabed.