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

One of the most severe environmental problems in Chesapeake Bay is seasonal development of hypoxia and anoxia in the deep channel that often extends to shallower waters by winds and tides (Boynton et al., 1995). Hypoxia/anoxia have long been attributed to enhanced nutrient flux from the watershed, and paleoecological evidence from multiple proxies substantiate the hypothesis that late 20^th century hypoxia was unprecedented compared to the colonial and pre-colonial periods (Brush, 1984; Cooper and Brush, 1991; Cornwell et al., 1996; Karlsen et al., 2000; Adelson et al., 2001; Zimmerman and Canuel, 2002; Bratton et al., 2003; Colman and Bratton, 2003).

The process of defining acceptable target levels of dissolved oxygen (DO) is an illustrative case study highlighting the challenges facing environmental managers and the advantage of paleoecological data to support decisions. In 2003, the Chesapeake Bay Program convened a group of experts, including ecologists, hydrologists, and paleoecologists, to define TMDLs for dissolved oxygen levels in designated use areas of the estuary. They focused particularly on the deep channel of the estuary because seasonal hypoxia first develops there, and because it was considered an important potential habitat for the anadromous short-nosed (Acipenser brevirostrum, an endangered species) and the Atlantic (Acipenser oxyrhynchus) sturgeon. Both species are native to Chesapeake Bay and were commercially valuable until population declines in the 20^th century. At issue was whether or not anoxia (or hypoxia) was a "natural" phenomenon in the deep channel of the bay and the appropriate restoration targets for DO in light of concerns about sturgeon, which had recently been reported from the deep channel.

With input from researchers who had reconstructed long-term DO proxy records from sediment cores (Fig. 3), the panel reached the consensus that Chesapeake Bay had probably been seasonally anoxic during some years between 1900 and 1960, before major fertilizer application in the watershed, especially in the deep channel where hypoxic/anoxic conditions may have persisted for several months. Anoxia during this period was probably geographically less extensive and less frequent than after the 1960's. These conclusions were supported by very limited observational data from the early 20^th century. Seasonally anoxic conditions (lasting weeks to months) probably occurred periodically in the deep channel between 1600 and 1900 AD. Before European colonization ~1600 AD, the deep channel of the bay may have been briefly hypoxic (< 2 mg L^-1) during relatively wet periods (which were common based on the paleoclimate record) (Fig. 4) and anoxic only during exceptionally wet conditions. Because the late 16th and much of the 17^th century was an extremely dry period, it was not conducive to oxygen depletion.

Based on the understanding of nutrient influx, oxygen depletion, and hydrodynamics of the bay, it was concluded that it was unlikely that the deep channel of the bay could be restored to mid-20^th century conditions under reasonable nutrient reduction targets. Additional factors making restoration difficult were remnant nutrients locked in sediment in the bay and behind dams, expected increases in precipitation due to climate change, and rapid population growth. Most researchers believed that restoring the bay to conditions prior to 1900 was not realistic, because the temporal variability (year to year and decadal) in "naturally occurring" hypoxia, the irreversible effects of land use, nutrient cycling, and sedimentary processes all render a single target DO level impossible to define.

These results have obvious implications about whether, or when the deep channel had been suitable habitat for sturgeon in particular, and for what baseline period might be chosen for DO restoration targets in general. Because of such climatic complexities and because paleoecological proxies are not yet precise enough to specify the duration of annual anoxic/hypoxic events, an intermediate target value was selected (U.S. EPA, 2003). These issues are emblematic of just some of the challenges, uncertainties and assumptions of ecosystem restoration and the value of retrospective paleoecological data.

Figure 3. Natural extremes in precipitation influence the effectiveness of management efforts to improve estuarine water quality. Instrumental (red and blue lines) and proxy-based (black lines) records of: a) fluvial discharge at Harrisburg, Pennsylvania; b) fertilizer use in the Chesapeake Bay watershed (blue line) and nitrate loading to Chesapeake Bay (red line); c) dissolved oxygen based on abundance of Ammonia, a hypoxia-tolerant benthic foraminifer (a-c from Karlsen et al., 2000); d) North Atlantic Oscillation Index reconstructed from tree rings (from Cook et al., 2002). Although fertilizer use increased steadily throughout the late 20th century, nitrate loading decreased during low-discharge years of the 1970's. [larger image]

Figure 4. Large, mutidecadal-scale fluctuations in temperature and precipitation are an inherent part of the eastern U.S. climate system, as demonstrated by proxy-based reconstructions of: a) spring temperature (°C) and b) monthly precipitation (cm month-1) for last 2,200 years in Chesapeake Bay. Temperature reconstructions are based on Mg/Ca analyses of ostracode shells (Loxoconcha) (from Cronin et al., 2005), and precipitation reconstructions are based on 18O values from benthic foraminifers (from Saenger et al., 2006). The initial Ambrosia rise at this site occurred ~1750 AD. [larger image]