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Dissolved Oxygen Trends
Dissolved oxygen (DO) levels vary naturally in lakes, estuaries and oceans over different temporal and spatial scales due to many biological, chemical, and physical processes. In estuaries like Chesapeake Bay, freshwater inflow influencing water column stratification, nutrient input and cycling, physical processes such as density-driven circulation, tides, winds, and water temperature, and bacterial activity are among the most important factors. These processes lead to large natural seasonal and interannual variability in oxygen levels in many parts of Chesapeake Bay.
Superimposed on this natural DO variability has been a progressive increase in the intensity and frequency of hypoxia and anoxia over the past 100-150 years, most notably since the 1960s, due to anthropogenic nutrient influx. This human-induced eutrophication is evident from both instrumental data and geochemical and faunal/floral "proxies" of dissolved oxygen obtained from the sedimentary record.
The instrumental record, although incomplete before the inception of the large monitoring program in the 1980's, suggests that as early as the 1930's and especially since the 1960's, summer oxygen depletion has been recorded in the bay. At issue, is whether or not - and to what degree - oxygen depletion is naturally-occurring phenomenon in the bay. Long sediment core (17 to > 21 m length) records indicate the bay formed about 7500 years ago (Cronin et al., 2000, Colman et al., 2002) when rising sea level following the final stage of Pleistocene deglaciation flooded the channel. The modern estuarine circulation and salinity regime probably began during the mid-late Holocene, about 4000-5000 years ago based on the appearance of "pre-colonial" benthic foraminiferal, ostracode and dinoflagellate assemblages. It is against this mid-late Holocene baseline that we can view the post-colonial and modern oxygen regime of the bay.
During the past decade, we have recontructed the bay's late Holocene record of dissolved oxygen using several proxies from sediment cores that have been dated using the most advanced geochronological methods. These "paleo-DO proxies" place the monitoring record of the modern bay into a long-term perspective and permit an evaluation of natural variability of DO levels in the context of restoration targets. Several major themes emerge which can be found in the papers cited on this website and references.
First, the twentieth century sedimentary record confirms what limited monitoring records of DO have shown: there has been a progressive decrease in DO levels, including periods of complete anoxia that have been prominent during the last 40 years. In terms of the timing of the development of hypoxia, most studies provide strong evidence that there was a greater frequency and/or duration of seasonal anoxia beginning in the late 1930's/40's and again around 1970, reaching unprecedented severity in the past few decades in the bay and several tributaries. This eutrophication is shown in all geochemical and paleoecological proxies yet studied and it has had a large impact on benthos and phytoplankton (both diatom and dinoflagellate) communities.
Second, extensive late 18th and 19th century land clearance also led to oxygen depletion and hypoxia, which exceeded that characteristic of the prior 2000 years. Best estimates for deep channel mid-bay seasonal oxygen minima from 1750 to ~ 1950 are 0.2 to 1-2 ml l-1 and are based on shifting to dinoflagellate cyst assemblages with species having low DO tolerances, a 4-5-fold increase in the flux of biogenic silica, a > 2-fold (5-10 per mil) increase in nitrogen isotope ratios (15N), and periods of common (though not dominant) Ammonia parkinsoniana, a facultative anaerobic foraminifer. These patterns are likely due to increased sediment influx and nitrogen and phosphorous runoff due to extensive land clearance and agriculture.
Third, prior to the 17th century, DO proxy data suggest interannual and decadal oxygen levels in the deep channel of Chesapeake Bay varied over decadal and interannual timescales. Although it is difficult to quantify the extremes, DO probably fell to 2-4 ml l-1 but rarely if ever fell below 1-2 ml l-1. These paleo-DO reconstructions are consistent with the bay's natural tendency to experience seasonal oxygen depletion due to its bathymetry, fresh-water driven salinity stratification, high primary productivity and organic matter, and nutrient regeneration
In summary, the main channel of Chesapeake Bay most likely experienced oxygen depletion prior to large-scale post-colonial land clearance due to natural factors such as climate-driven variability in fresh-water inflow. However this progressive decline in summer oxygen minima, beginning in the 18th century and accelerating during the second half of the 20th century, is superimposed on past and present interannual and decadal patterns of DO variability. Human activity during the post-colonial period has very likely contributed to the trend towards hypoxia and most recently (especially post-1960's) anoxia in the main channel of the bay and some of its larger tributaries. The impact of these patterns has been observed in large-scale changes in benthos and phytoplankton communities, which are manifestations of habitat loss and degradation.
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