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Great Lakes Lakewide Management Plans (LaMPS)

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Lakewide Management Plans

Tracking Depletion of Dissolved Oxygen

Background: The Phosphorus-Oxygen Link

Of all the Great Lakes, Erie has been the most severely impacted by many years of heavy loadings of phosphorus from human activity. This phosphorus is released through urban and agricultural activities, including fertilizer application and farmland erosion and sewage treatment plant discharges. Whether it arrives through direct discharge or is delivered via the Lake's tributaries, once in the Lake, it enables the heavy growth of algae. When the algae become very concentrated in early summer so that light penetration is diminished, and as the cells age, algal dieoff begins. The dead cells fall to the bottom and are decomposed by bacteria, which use a considerable amount of oxygen in the process

The bottom layer of water (the hypolimnion), because it is cold, tends to not mix with warmer surface water (the epilimnion), and therefore has no opportunity for oxygen replenishment from the atmosphere. This oxygen depletion was so severe in the central basin of Lake Erie by the end of the 1960's that large numbers of fish, starved for oxygen, were dying and washing ashore along with heavy mats of decaying, floating algae. The central basin of Lake Erie is the primary problem area because the western basin is sufficiently shallow that the deepest waters warm enough to mix into the epilimnion in early summer. In contrast, the deeper eastern basin is not threatened by oxygen depletion because decomposition of algae and sediments cannot deplete the much greater water volume in its hypolimnion over the span of the spring and summer. In the fall, the distinction between epilimnion and hypolimnion disappears when surface waters cool and complete mixing resumes.

Diagram showing dissolved oxygen concentration in the Lake Erie Central Basin
Diagram showing dissolved oxygen concentration in the Lake Erie Central Basin

Since the early 1970's, major steps have been taken by both the U.S. and Canada to reduce the loading of phosphorus along with the resultant algal bloom and hypolimnetic oxygen depletion. This has reduced ongoing loads of phosphorus to levels that should prevent significant fish kills due to inadequate oxygen in the waters of the hypolimnion. Incompletely degraded algal materials accumulated in the sediments from the many years of bloom, have, however, continued to cause depletion of hypolimnetic oxygen in the summer. Over time, that effect should diminish as the proportion of more completely degraded materials in the sediments come to predominate.

Diagram showing oxygen depletion rate in the Lake Erie Central Basin
Diagram showing oxygen depletion rate in the Lake Erie Central Basin

Tracking Progress

Since 1973 (excepting 1994-1996), the Great Lakes National Program Office of USEPA and its cooperators have conducted an annual series of surveys to track the occurrence, degree, timing and trend of oxygen depletion in the hypolimnion of the Lake Erie central basin. These data are collected to monitor the efficacy of the binational phosphorus reduction program and to verify its target levels. Year-to-year comparisons are difficult because weather variability, including duration of ice cover, summer heating, and the timing, severity, and duration of wind storm events, profoundly affects thickness and duration of the hypolimnion. These, of course, are major factors affecting oxygen depletion.

To collect the dissolved oxygen data, the Great Lakes National Program office utilizes the EPA research Vessel, Lake Guardian. On occasion, when the R/V Lake Guardian is unavailable, another vessel, such as the U.S. Geological Survey - Biological Research Division's R/V Musky II is used. The survey is conducted six times, once every three weeks, over the spring and summer seasons. Ten specific locations, or stations, are sampled in the central basin of the Lake over two days. The thermal and oxygen profiles are determined using a side-cast instrument array, and samples are taken to verify the instrument readings. These samples are immediately analyzed on board the vessel to measure dissolved oxygen. On some surveys, samples are also taken for measurement of phosphorus to monitor that trend as well so that it may be related to the dissolved oxygen response.

While the trend of oxygen depletion has lagged that of phosphorus reduction, this was anticipated. The oxygen depletion rate has trended downward since monitoring began. When spring phosphorus levels reached their targets at the beginning of the decade, summer hypolimnetic oxygen depletion was also reduced. It is evident that, currently, under some weather conditions, the hypolimnion is capable of sustaining aquatic life over the summer season. This trend is expected to continue and improve. Monitoring for summer hypolimnetic dissolved oxygen was suspended in years 1994-1996 so little more recent data are available. Preliminary results from the 1997 season indicate, however, that depletion still is occurring at some sites in late summer and is, therefore, still a significant concern.

Further Reading

A thorough discussion of the story of phosphorus and dissolved oxygen in Lake Erie can be found in Total Phosphorus and Dissolved Oxygen Trends in the Central basin of Lake Erie, 1970-1991 by Paul Bertram, Ph.D, in the Journal of Great Lakes Research, 1993, 19(2):224-236.


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