ABSTRACT
Shoreline erosion supplies sediments to estuaries and coastal waters, influencing water clarity and primary production. Globally, shoreline erosion sediment inputs are changing with anthropogenic alteration of coastlines in populated regions. Chesapeake Bay, a prime example of such a system where shoreline erosion accounts for a large proportion of sediments entering the estuary, serves here as a case study for investigating the effects of changing sediment inputs on water clarity. Long-term increases in shoreline armoring have contributed to decreased erosional sediment inputs to the estuary, changing the composition of suspended particles in surface waters. This study examined the impact of shoreline erosion on water clarity using a coupled hydrodynamic-biogeochemical model. Experiments were conducted to simulate realistic shoreline conditions representative of the early 2000s, increased shoreline erosion, and highly armored shorelines. Together, reduced shoreline erosion and the corresponding reduced rates of resuspension result in decreased concentrations of inorganic particles, improving water clarity particularly in the lower Bay and in dry years where and when riverine sediment influence is low. This clarity improvement relaxed light limitation, which increased organic matter production. Differences between the two extreme experiments revealed that in the mid-estuary in February to April, surface inorganic suspended sediment concentrations decreased 3-7 mg L-1, while organic suspended solids increased 1-3 mg L-1. The resulting increase in the organic-to-inorganic ratio often had opposite effects on clarity according to different metrics, improving clarity in mid-Bay central channel waters in terms of light attenuation depth, but simultaneously degrading clarity in terms of Secchi depth because the resulting increase in organic suspended solids decreased the water's transparency. This incongruous water clarity effect, the spatial extent of which is defined here as an Organic Fog Zone, was present in February to April in all years studied, but occurred farther south in wet years.
ABSTRACT
As the oyster aquaculture industry grows and becomes incorporated into management practices, it is important to understand its effects on local environments. This study investigated how water quality and hydrodynamics varied among farms as well as inside versus outside the extent of caged grow-out areas located in southern Chesapeake Bay. Current speed and water quality variables (chlorophyll-a fluorescence, turbidity, and dissolved oxygen) were measured along multiple transects within and adjacent to four oyster farms during two seasons. At the scale of individual aquaculture sites, we were able to detect statistically significant differences in current speed and water quality variables between the areas inside and outside the farms. However, the magnitudes of the water quality differences were minor. Differences between sites and between seasons for water quality variables were typically an order of magnitude greater than those observed within each site (i.e. inside and outside the farm footprint). The relatively small effect of the presence of oysters on water quality is likely attributable to a combination of high background variability, relatively high flushing rates, relatively low oyster density, and small farm footprints. Minimal impacts overall suggest that low-density oyster farms located in adequately-flushed areas are unlikely to negatively impact local water quality.
