Seasonal and spatial variation in cohesive sediment erodibility in the York River estuary: physical deposition versus biological reworking

Introduction and Methods Cohesive sediments in marine and coastal environments are responsible for degrading water quality, influencing the cycling and availability of particle bound contaminants, and infilling navigable waterways. In spite of their significance to coastal environments, there are still major gaps in our understanding of many of the fundamental processes governing cohesive sediment transport and deposition. In particular, seabed erodibility, which facilitates sediment exchange between the bed and water column, is under-resolved in field studies and is often treated as a tuning parameter in numerical models. The goal of this work is to evaluate variation in seabed erodibility over several seasons in a cohesive estuarine environment and gain insight into the dominant physical and biological processes influencing these variations. This study was conducted on the York River estuary, a sub-estuary of the Chesapeake Bay, USA (Fig. 1). The York River is tidally energetic with tidal currents reaching magnitudes of ~ 1m s at the surface during spring tide. A persistent estuarine turbidity maximum (ETM) has been reported just upstream of West Point in the Pamunkey and Mattaponi Rivers, the York’s main tributaries. Additionally, an ephemeral secondary turbidity maximum (STM) has been reported in the middle estuary near Clay Bank, ~ 25 km from the York River mouth (Lin and Kuo, 2001). Schaffner et al. (2001) reported a strong estuarine gradient in ecological diversity from the ETM region of the York into the main stem of the Chesapeake Bay. Both the ETM and STM regions were characterized by high suspended sediment concentrations resulting in unfavorable conditions for benthic biota. In contrast, the main stem of the Chesapeake Bay had lower suspended sediment concentrations and a more diverse benthic community. In this study, sediment erodibility was measured at three sites on the York River estuary, monthly to bimonthly over a 19-month period. Two sites were established in the more physically dominated middle estuary near Clay Bank, and one site was established in the more biologically influenced lower estuary near Gloucester Point. Each time a site was visited, cores were collected for erodibility measurement, Xradiography, and analysis of grain size, water content, and organic components. Within a few hours of core collection, seabed erodibility over the range of 0.01 to 0.6 Pa was measured with a dual core Gust erosion microcosm. Results and Discussion The two sites located in the Clay Bank region of the estuary (CS and CC) exhibited a pronounced seasonal cycle in seabed erodibility (Fig. 2). Both CS and CC had consistent and relatively low erodibility in the summer and fall of 2006 and 2007 and elevated erodibility in the late winter and spring of 2007. CS and CC also exhibited what appeared to be transitional periods of moderate erodibility prior to and after periods of highest erodibility. In contrast, erodibility at the Gloucester Point (GP) site (Fig. 2) was generally low and did not exhibit the pronounced seasonal pattern found at the two Clay Bank sites. Interestingly, the range in eroded mass and mean eroded mass found at GP year-round (except for May 2007) was quite similar to that measured at the Clay Bank sites in the summer and fall of 2006 and 2007. Weak to nonexistent correlations between bed erodibility, solids volume fraction, and components of organic matter were not sufficient to explain the observed seasonal pattern in erodibility at Clay Bank. X-radiographs from the GP site (Fig. 3a-c) revealed little temporal variability in the structure of the seabed. Similar to the GP site, X-radiographs from the CS site often appear mixed with few laminations (Fig. 3d). However, a more uniform surface layer, occasionally containing thin laminations and ranging in thickness from 1 to 15 cm, was occasionally observed (Fig. 3e-f). This layer was most distinct in March to May of 2007. Digital X-radiographs from the CC site revealed the most dramatic seasonal variability in bed structure. From May to September of 2006 the bed appeared mottled (Fig. 3g). In November, a surface layer, 2 cm thick and containing many fine laminations appeared (Fig. 3h). Similar laminations were then observed down to 10 cm in January of 2007 (Fig. 3i) and down to 20+ cm in March and April. In May 2007 only traces of laminations remained, and by June cores from the CC site appeared mottled once again (Figure 3l). The presence at Clay Bank of (i) thick sequences of laminated sediments coincident with the period of highest erodibility and (ii) more biologically reworked sediment during the rest of the year suggests that periodic rapid deposition introduced new sediment that was seasonally easy to erode. The presence of laminated bedding in the X-radiographs likely highlighted times when physical processes dominated bioturbation due to rapid deposition overwhelming the ability of benthic biota to mix the seabed. In contrast, sediments appearing mottled were indicative of the times with more active bioturbation and lower rates of deposition. The finding that seasonal deposition influenced erodibility in the Clay Bank region is consistent with previous results indicating the occasional presence of a secondary turbidity maximum. Comparison of the biologically reworked, but still “low” erodibility condition found at all three York River sites to other published Chesapeake Bay erodibility data revealed a remarkably consistent eroded mass versus critical shear stress profile. In the absence of rapid recent deposition (i.e., outside of turbidity maxima zones), it appears that muddy areas of moderate depth exhibit a notably consistent level of bed erodibility in both space and time. These common erodibilities suggest an equilibrium critical stress profile may exist which may be broadly representative of other similar estuarine environments. At relatively low stresses and in the absence of rapid deposition, we further speculate that burrowing and/or sediment pelletization may play a role in maintaining high equilibrium bulk water content without reducing the strength of the surface of the seabed. Thus the presence of biologically-induced heterogeneity may confound the otherwise expected relationship between muddy seabed water content and erodibility. Fig. 4 presents a conceptual model of processes in the York River estuary influencing bed erodibility. Following periods of high river discharge, a secondary turbidity maximum (STM) forms near Clay Bank, resulting in high suspended sediment concentrations dominated by fines and flocs, rapid deposition of an ephemeral layer 10’s of centimeters thick, a physically dominated seabed, and high bed erodibility. The lower estuary, including the GP site, lies outside of the region typically occupied by the STM. The result at Gloucester Point is lower suspended sediment concentrations, highly pelletized surficial sediment, a more actively burrowed seabed, and low erodibility. After an extended period of low river flow, stratification throughout the estuary breaks down, and the STM either moves up-estuary or dissipates, resulting in a period of divergent sediment transport and/or sediment bypassing. During this period, the conditions near Clay Bank more closely resemble Gloucester Point, with lower suspended sediment concentrations, increasingly pelletized sediment, more intense bioturbation, and lower erodibility.