This review provides an integrated synthesis with timelines and evaluations of ecological responses to eutrophication in Chesapeake Bay, the largest estuary in the USA. Analyses of dated sediment cores reveal initial evidence of organic enrichment in ~200 yr old strata, while signs of increased phytoplankton and decreased water clarity first appeared ~100 yr ago. Severe, recurring deep-water hypoxia and loss of diverse submersed vascular plants were first evident in the 1950s and 1960s, respectively. The degradation of these benthic habitats has contributed to declines in benthic macroinfauna in deep mesohaline regions of the Bay and blue crabs in shallow polyhaline areas. In contrast, copepods, which are heavily consumed in pelagic food chains, are relatively unaffected by nutrient-induced changes in phytoplankton. Intense mortality associated with fisheries and disease have caused a dramatic decline in eastern oyster stocks and associated Bay water filtration, which may have exacerbated eutrophication effects on phytoplankton and water clarity. Extensive tidal marshes, which have served as effective nutrient buffers along the Bay margins, are now being lost with rising sea level. Although the Bay's overall fisheries production has probably not been affected by eutrophication, decreases in the relative contribution of demersal fish and in the efficiency with which primary production is transferred to harvest suggest fundamental shifts in trophic and habitat structures. Bay ecosystem responses to changes in nutrient loading are complicated by non-linear feedback mechanisms, including particle trapping and binding by benthic plants that increase water clarity, and by oxygen effects on benthic nutrient recycling efficiency. Observations in Bay tributaries undergoing recent reductions in nutrient input indicate relatively rapid recovery of some ecosystem functions but lags in the response of others. KEY WORDS: Eutrophication · Nutrients · Chesapeake Bay Resale or republication not permitted without written consent of the publisherChesapeake Bay is a large estuary which has undergone many changes in its ecological properties and processes in response to nutrient enrichment over the last 2 centuries. Susceptibility of the Bay to eutrophication arises in part from the long dendritic shoreline that intimately connects it to its large watershed (covering an area 15 times that of the Bay) which contains expanding human population centers and extensive agricultural activities. (Satellite image from MODIS,
Abstract. A comprehensive study of the strongly wind driven midlatitude buoyant plume from the Columbia River, located on the U.S. west coast, demonstrates that the plume has two basic structures during the fall/winter season, namely, a thin (---5-15 m), strongly stratified plume tending west to northwestward during periods of southward or light northward wind stress and a thicker (---10-40 m), weakly stratified plume tending northward and hugging the coast during periods of stronger northward stress. The plume and its velocity field respond nearly instantaneously to changes in wind speed or direction, and the wind fluctuations have timescales of 2-10 days. Frictional wind-driven currents cause the primarily unidirectional flow down the plume axis to veer to the right or left of the axis for northward or southward winds, respectively. Farther downstream, currents turn to parallel rather than cross salinity contours, consistent with a geostrophic balance. In particular, during periods when the plume is separated from the coast, currents tend to flow around the mound of fresher water. At distances exceeding about 20 km from the river mouth, the along-shelf depth-averaged flow over the inner to midshelf is linear, and depth-averaged acceleration is governed to lowest order by the difference between surface and bottom stress alone. In this region, along-shelf geostrophic buoyancy-driven currents at ---5 m (calculated from surface density) and along-shelf geostrophic wind-driven currents (computed from a depth-averaged linear model) are comparable in magnitude (---10-25 cm s-•).
[1] Regional Ocean Modeling System (ROMS) is used to develop a new three-dimensional hydrodynamic model for the Chesapeake Bay estuary. Hindcast simulations are conducted for 2 years with markedly different annual river discharges and are compared with time series measurements and high-resolution hydrographic data. The model shows skill in reproducing observed temporal variability in sea level height, salinity, and subtidal current. The agreement with observations is better in the normal runoff year 1997 than in the high runoff year 1996. The model qualitatively reproduces the along-channel and cross-channel salinity distributions during low-to-medium runoff periods. However, during high runoff periods it predicts weaker stratification and a more diffuse halocline than shown by observations. This model/data discrepancy is related to the deficiency of turbulent mixing parameterizations in strong stratification. We have experimented with four turbulence closure schemes (Mellor-Yamada/k-kl, k-e, k-w, and KPP models) in ROMS but found little difference in the model results. However, vertical stratification shows a strong sensitivity to the background diffusivity. The vertical diffusivity inferred from the model is found to be set by the background diffusivity except in the surface and bottom boundary layers where the turbulence schemes produce similar diffusivity distributions. Among the schemes explored, KPP and k-kl scheme with a background diffusivity of 10 À5 or 10 À6 m 2 s À1 provide the best simulations of the Chesapeake Bay estuary. Both the model sensitivity study and model/data comparison highlight the importance of obtaining a more realistic parameterization for turbulence mixing in a strong pycnocline.
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