Home Introduction Climate Variability Impacts on Ecosys. Degradation & Restoration in Chesapeake Bay Baseline Variability: Chesapeake Bay >Chesapeake Bay WQ & Climate Variability FL Everglades: Hydro. Changes & Degradation Everglades Climate Variability & Relevance Role of Time in Restoration Planning Acknowledgements References Figures

As a partially mixed estuary, density-driven circulation in Chesapeake Bay is strongly influenced by variability in regional rainfall and freshwater river inflow over seasonal, interannual, and longer timescales, and secondarily by tides, winds and bathymetry (Boicourt et al., 1999). The Susquehanna River contributes more than 50% of river discharge into the bay, and discharge is positively correlated with regional rainfall (Najjar, 1999) and salinity in the bay (Gibson and Najjar, 2000). Therefore, precipitation is a primary driver of salinity gradients, turbidity (due to sediment influx), in situ biological production, dissolved nutrient loadings, primary productivity, dissolved oxygen, and the distribution of salinity-sensitive species. Intervals of extreme drought or high precipitation therefore have significant impact on estuarine water quality, and sustainable restoration goals must incorporate the variability inherent to the system.

Likewise, periods characterized by unusually warm or cool temperatures influence the distribution of phytoplankton, fish, and other living resources in the bay and watershed. The late Holocene record of Chesapeake paleotemperature based on magnesium/calcium ratios in ostracode shells and precipitation based on oxygen isotope ratios (¹⁸O) in benthic foraminiferal shells from sediment cores are shown in Figure 4. It should be emphasized that geochemical proxies from Chesapeake calcareous microfossils (and most proxy methods used in paleoclimate research) undergo careful, quantitative calibration using modern environments and verification through comparison with the instrumental records. Oxygen isotopes are widely used in estuarine and marginal marine sediments to estimate past salinity.

Microfaunal analyses from Chesapeake Bay show large fluctuations in spring water temperatures throughout the last 2,000 years, with especially cool temperatures during parts of the Little Ice Age (LIA, 1500-1900 AD), and warmer temperatures during the early Medieval Warm Period (MWP, 800-1200 AD) and the 20^th century. In fact, short-term temperature extremes during the last 150 years are among the warmest of the past 2000 years (Fig. 5). Precipitation also varies greatly at multi-decadal timescales, and the 20^th century exhibits periods of extreme wet climate (Cronin et al., 2000; 2005). Extended droughts reconstructed from sediment cores confirm evidence for regional droughts based on tree ring records (i.e., Stahle et al., 1998). One important conclusion from paleoclimate studies is that Holocene precipitation patterns in the mid-Atlantic region are clearly linked to hemispheric climate patterns, particularly atmospheric changes in tropical regions (Cronin et al., 2005; Willard et al. 2005). Therefore, climate model simulations of future greenhouse gas-induced changes in ocean and atmospheric circulation have bearing on the potential range of regional climate changes that can be expected in the eastern United States.

These developments pertain to several management questions. Collectively, the proxy data indicate that, in addition to the overriding influence of human activities during the last 200 years seen in diatoms (Cooper and Brush, 1991), dinoflagellates (Willard et al., 2003), biogenic silica (Bratton et al., 2003), molybdenum (Zheng et al., 2003), benthic foraminifera (Karlsen et al., 2000) and ostracodes (Cronin and Vann, 2003), climate variability continues to exert a large influence on bay ecosystems even in its degraded state. High flow rates of the 1970's and 1980's exacerbated the impact of increasing fertilizer usage and nutrient input (Fig. 3), leading to unprecedented hypoxia and anoxia in the main channel and increased sediment flux. Therefore, it is important to realize that extreme climate events or decadal-scale shifts in precipitation have the potential to override management actions designed to improve water quality and other environmental parameters. Likewise, as we discuss below, land-use changes may alter regional patterns of precipitation and temperature variability. Consideration of the potential interactions of land-use and climate change should be an important component of ecosystem management decisions.

Taken together, the paleo- and instrumental records significantly improve our understanding of multiple causes of temporal patterns of Chesapeake anoxia and other environmental parameters and demonstrate the value of long-term records. They settle a contentious, decades-long debate on whether the bay has experienced long-term increases in hypoxia (reviewed in Hagy et al., 2004). The data also pertain directly to current management efforts to restore the bay. In 2003, the Chesapeake Bay Program was criticized for lack of progress in bay restoration in two popular books (Ernst, 2003; Horton, 2003). The criticisms focused mainly on political and societal aspects of restoration efforts, which did not attain desired goals in indicators such as seagrass coverage and volume of hypoxic water in the bay. If restoration goals and timetables, however, do not incorporate decadal extremes in climate, it is unrealistic to expect improvements in a climate-precipitation-driven system when wet conditions persist, even if nutrient reduction is accomplished. The notion that there are "normal" or average years is inconsistent with evidence about climate variability. Today, the CBP forecasting effort recognizes that summer winds, excessive precipitation, or extreme storm events can lead to inaccurate forecasts. The relative impacts of land management actions and uncontrollable climatic processes on restoration progress must continually be evaluated in light of past and future changes.