USGS Circular 1316
Synthesis of U.S. Geological Survey Science for the Chesapeake Bay Ecosystem and Implications for Environmental Management
Chapter 10: Long-Term Changes in Climate and Water Quality
The CBP has restoration goals to increase dissolved oxygen and water clarity in the Bay to improve water-quality conditions for fisheries and SAV. Because land-use practices in the Chesapeake Bay watershed have a great influence on estuarine water quality and its biota, most of the restoration actions focus on changing land-use practices to reduce nutrient and sediment loads. Regional climate variability also has a significant impact on water quality. Precipitation and river flow into the Bay directly affect salinity stratification within the estuary, which in turn influences the timing and extent of seasonal hypoxia, independent of nutrient loads. Likewise, a climatically induced fluctuation in river flow to the Bay affects the amount of suspended sediment in the water column. Therefore, the proposed management strategies to improve estuarine water quality need to consider the impacts of natural climatic fluctuations on nutrient and sediment loads. The USGS has summarized results from a series of integrated studies designed to document the long-term variability of Chesapeake Bay water quality (salinity, temperature, and dissolved oxygen).
The Chesapeake Bay is underlain by a thick sequence of sediments that provide an archive of past ecosystem response to a series of climatic and land-use changes. These sediments have been deposited continuously throughout the approximately 7,000-year history of the modern Bay, and previously when the paleo-Susquehanna River flowed through the valley that ultimately was flooded by sea-level rise to form the modern Chesapeake Bay. Biological and geochemical indicators are analyzed from sediment cores, which serve as proxies for environmental parameters (temperature, salinity, and dissolved oxygen), to assess water-quality changes during the past several thousand years. Age models of the cores are developed using radiogenic isotope methods (carbon 14, lead 210, cesium 137) and pollen biostratigraphy (see Willard and others, 2003, for a complete discussion). Reconstruction of the history of temperature, salinity, and dissolved oxygen in the Bay is based on quantitative analysis of microfossils of pollen, ostracodes, foraminifers, mollusks, dinoflagellates, diatoms, and sediment geochemistry.
An understanding of the natural variability in river flow, which is strongly influenced by precipitation, is important for developing sustainable management plans to limit nutrient and sediment loads in the Bay. The relation among rainfall, river flow, and Chesapeake Bay salinity over the past 175 years was quantified by USGS researchers using instrumental records, and established foraminiferal and ostracode indicators for salinity made it possible to reconstruct past variability in salinity and river flow during the last 7,000 years (Cronin and others, 2000). Examination of the sediment records reveals a significant difference between Chesapeake salinities of the early Holocene (7,200 to 5,000 years before present, or yrBP), when mean was 28 ppt (parts per thousand) and the last 2,000 years, when salinity averaged 20 ppt (Cronin and others, 2005). The persistent occurrence of multidecadal salinity and temperature oscillations (every 20–40 years) during the entire history of the Bay indicates that climate variability is an inherent component of the North Atlantic climate system (Cronin and others, 2005). Over a shorter time scale, detailed records spanning the last 1,500 years document both extended periods of drier than average conditions (during the Medieval Warm Period ranging from 1200–600 yrBP), and wetter than average conditions during the Little Ice Age (from 500–100 yrBP). The 20th century is characterized by a series of precipitation extremes that indicate anomalous behavior of the climate system. The occurrence of such extreme variability in river flow over annual to decadal periods can have a much greater influence on delivery of nutrient and sediment loads to the estuary than the management actions designed to reduce these loads. The results imply that managers need to better account for natural variability when assessing progress in reducing nutrient and sediment loads to the estuary and assessing attainment of water-quality standards.
Seasonal and interannual temperatures of Chesapeake Bay surface waters are influenced both by inflowing waters from the continental shelf and regional atmospheric temperatures. The potential for 21st century warming related to greenhouse gas concentrations also is likely to affect estuarine temperatures. Using magnesium/ calcium ratios from ostracode shells from sediment cores, USGS researchers, in collaboration with colleagues at Duke University, reconstructed long-term, estuarine surface-water temperatures over the past 2,000 years. These records indicate that surface-water temperature maxima occurred approximately every 70 years during the interval between 2200 yrBP and 250 yr (fig. 10.1A). This pattern indicates the long-term persistence of multi-decadal processes such as the North Atlantic Oscillation (Cronin and others, 2000; Cronin and Vann, 2003). Temperatures during the late 19th and 20th centuries exhibited greater extremes (fig. 10.1B) than those observed during the previous 2,000 years, including the relatively warm Medieval Warm Period (MWP) and cooler Little Ice Age (LIA) (fig. 10.1A). These results are consistent with other studies in the North Atlantic region that indicate anomalous 20th century climate variability when compared to the past 2,000 years (fig. 10.1C). The implications of these findings are that long-term changes in climate, due to both natural variability and increasing greenhouse gases from human sources, and changes in land-use practices have to be addressed to improve water-quality conditions in the Bay.
Seasonal oxygen depletion in waters of the Chesapeake Bay has been documented for much of the 20th century by a number of research efforts. Research by USGS scientists has focused on reconstruction of dissolved oxygen trends in Chesapeake Bay during the past 2,500 years (Bratton and others, 2003; Cronin and Vann, 2003; Karlsen and others, 2000; Willard and others, 2003) and indicates that the deep channel of the Bay may have been briefly hypoxic (concentrations less than 2 mg/L) during relatively wet periods prior to European colonization (prior to 1600 AD). Seasonal anoxia (a lack of dissolved oxygen lasting weeks to months) probably occurred periodically during the relatively wet periods between 1600 AD and 1960 AD, and became more frequent after 1970 (fig. 10.2). These findings, together with earlier research, clearly indicate that hypoxia and anoxia were much more severe and extensive in Chesapeake Bay and its tributaries during the past four decades than at any time in the past 500–2,500 years.
Bratton, J.F., Colman, S.M., and Seal, R.R., II, 2003, Eutrophication and carbon sources in Chesapeake Bay over the last 2,700 years: Human impacts in context: Geochimica et Cosmochimica Acta, v. 67, p. 3,385–3,402.
Cronin, T.M., Thunell, R., Dwyer, G.S., Saenger, C., Mann, M.E., Vann, C., and Seal, R.R., II, 2005, Multiproxy evidence of Holocene climate variability from estuarine sediments, eastern North America: Paleoceanography, v. 20, PA4006, doi: 10.1029/2005PA001145.
Cronin, T., Willard, D., Karlsen, A., Ishman, S., Verardo, S., McGeehin, J., Kerhin, R., Holmes, C., Colman, S., and Zimmerman, A., 2000, Climatic variability in the eastern United States over the past millennium from Chesapeake Bay sediments: Geology, v. 28, no. 1, p. 3–6.
Karlsen, A.W., Cronin, T.M., Ishman, S.E., Willard, D.A., Holmes, C.W., Marot, M., and Kerhin, R., 2000, Historical trends in Chesapeake Bay dissolved oxygen based on benthic foraminifera from sediment cores: Estuaries, v. 23, no. 4, p. 488–508.