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Dissolved Oxygen Information

Dissolved Oxygen Depletion in Lake Erie

The overall goal for the Great Lakes Water Quality Agreement (GLWQA) “ . . . is to restore and maintain the chemical, physical and biological integrity of the waters of the Great Lakes Basin Ecosystem.” More specific goals for the control of phosphorus include, “Restoration of year-round aerobic conditions in the bottom waters of the Central Basin of Lake Erie,” and “Substantial reduction in the present [1978] level of algal biomass to a level below that of nuisance condition in Lake Erie.”  The United States and Canada have spent billions of dollars toward the reduction of phosphorus loadings into Lake Erie in order to reduce the amount of algal growth, and subsequently, to restore aerobic conditions to the Central Basin during the summer season.

The EPA Great Lakes National Program Office (GLNPO) monitors the status of dissolved oxygen in the water column of Lake Erie at a fixed network of stations several times each summer. These data are then used to assess the timing, extent and severity of reduced oxygen conditions. Over several years, trends in the rate of oxygen depletion over the summer should reflect the impact of management programs to limit both point sources and non-point sources of phosphorus loadings.

Why Dissolved Oxygen Important and How Much is Enough?

Oxygen is essential for all plants and animals to survive, whether they live on the land or in the water. Aquatic organisms rely on oxygen that is dissolved in the water. In most lakes and streams, the amount of oxygen in the water is continually being replenished by oxygen from the air. Sometimes, however, conditions exist in which the dissolved oxygen in the water is used up by organisms faster than it can be replaced from the air.

In recent decades, the bottom waters in the Central Basin of Lake Erie become anoxic (without oxygen) in the late summer. Aquatic creatures, including fish and bottom-dwelling animals, need oxygen in the water to live or they suffocate. The fish may be able to swim to better waters, but most of the other animals cannot.

How much oxygen do aquatice creatures need

There is no absolute value for how much oxygen in the water is enough. Depending on the water temperature, oxygen concentrations in equilibrium with the air can range from about 8 mg/liter (summer warm water) to over 14 mg/l (winter cold water). Some organisms are much more sensitive to low oxygen levels than others. Generally, the less oxygen available, the more stressed the organisms will be. Few species tolerate oxygen concentrations less than 1 or 2 mg/liter for very long. 

How Does the Dissolved Oxygen Problem Develop?

The configuration of Lake Erieis largely responsible for the problem. However, too many nutrients, especially phosphorus, make the problem much worse. Most of the excess phosphorus comes from human activities, including sewage treatment plants and agriculture.

Cross section of lake erie

In the summer, the water in the Great Lakes separates into two layers. The top layer is warmer than the bottom one, it receives the sunlight, and it mixes with oxygen from the air. The bottom layer is cooler than the top layer, it is usually dark, and it is cut off from the air so it cannot re-supply its oxygen

The Western Basin of Lake Erie is shallower than the typical thickness of the upper layer of water. Winds are able to keep the whole water column stirring, so there is plenty of opportunity for oxygen to dissolve into the water from the air. The bottom of the CentralBasin is a broad, shallow plain that is slightly deeper than the typical thickness of the upper layer. Therefore, the bottom layer is relatively thin and contains a relatively small volume of water. In the deeper Eastern Basin, the bottom layer is much thicker and contains more water.

If there is more phosphorus in the water, it acts like fertilizer, and more algae will grow in the warm, sunlit top layer. Eventually these algae will sink into the dark bottom layer, where they stop growing and begin dying. Bacteria and fungi then decompose the plant organic matter. Bacteria and fungi also need oxygen to live, and they use what is available in the water. Because the bottom layer is cut off from the air, over the summer, less and less oxygen remains in the water. If there is a small volume of water and a lot of decomposition going on, as exists in the Central Basin, the oxygen will be used up faster than if there is much water and/or little decomposition. This is not a problem in the Eastern Basin largely because of the much greater volume of water in the bottom layer.

Each autumn, the top layer cools, and the wind mixes it deeper and deeper into the bottom layer. Eventually the whole water column is the same temperature, and the wind can again mix it from top to bottom and the oxygen can be restored from the air. In the Central Basin, this phenomenon typically occurs in early to mid-September, when oxygen is restored from top to bottom.

GLNPO's Dissolved Oxygen Monitoring Program

Lake Erie sampling stations

 

Ten sampling stations for the dissolved oxygen monitoring program were selected to cover most of the affected area. In most years, each station is visited in early June, late June, mid-July, early August, late August and mid-September. The most severe oxygen depletion is observed during the late August and/or mid-September surveys.

Seasonal Dissolved Oxygen Levels

 

DO Concentrations 1991-2000

DO concentrations 2001.2005

DO concentrations 2006-2010

Over the course of the summer, dissolved oxygen levels in the bottom waters steadily decline; however, this does not always occur at the same rate.  In some instances, oxygen concentrations can change very little over a two week period (0.1mg/L between mid-July and early August surveys) and then rapidly decline more than 6mg/L between the next two surveys, as took place in 1997.This varied rate can also affects the time it takes for oxygen concentrations to reach critical levels. For instance, during 1993, 1997 and 1999 oxygen concentrations did not fall below 6mg/L until the beginning of August, while this same level was reached in late June or early July during 1991, 2001, 2006 and 2009.The few data that are available for 2000 indicate that conditions were among the poorest in recent years (reaching oxygen concentrations around 1.5mg/L by the beginning of August; about 10-15 days before this level was reach in any other year).In most years, the Central Basin beginnings to turn over between late August and early September, restoring oxygen to the entire water column. If there is sufficient mixing, however, of the upper water layer into the lower one, as in 2004, average oxygen concentration can remain higher; as they stayed above 2mg/L from mid-August through mid-September.  However, if the two water layers remain intact into September, the dissolved oxygen levels can continue to decline to less than 1mg/L, as they did in 1992, 1997, 1998, 2001, 2003, 2005, and 2007 2010.The fact that oxygen levels have continued to reach below 1mg/L so often within recent years is of concern to many scientists and will continue to be watched closely.

Progression of Dissolved Oxygen Depletion over Time

Grape diagram of Lake Erie DO Concentrations

The reduction in dissolved oxygen concentrations from early June through mid-September is not uniform across all sampling stations, and averaging the data together from all 10 stations can be deceiving.  While looking at the data from each station can offer a better depiction of what is occurring, using computer programs can provide an even more accurate interpretation by filling in the gaps between stations. 

The three most westerly stations (left side of the diagram) exhibited a pattern of more rapid oxygen depletion than the other stations, and at these stations the two water layers were less likely to have mixed together by mid-September.  For all years except 2000 and 2004-2006, low oxygen conditions (less than 1 mg/l, blacken area) were observed at least part of the season at all three stations.  Also, this low oxygen condition persisted from late August into mid-September at one or more of these stations every year except 2002, 2004 and 2005.

The three most easterly stations (right side of the diagram) generally exhibited a pattern of less rapid oxygen depletion than at the other stations, and the two water layers were more likely to have mixed together by mid-September. Although low oxygen conditions were observed at one or more of these stations in all years except 1991, 1992, 1993, 1997, and 2004 the condition usually did not persist over two sampling periods except for 2003, 2007, 2008 and 2010.

Missing station data makes the interpolation process less accurate and therefore, the program was not run if data from all stations were not available.  Instead, circles were used to indicate the oxygen concentrations for the station where data were obtained. 

Thickness and Temperature of Bottom Water Layer

Thickness and Temperature of Bottom Water Layer 2001-2005

Thickness and Temp of bottom water layer 2006-2010

 

Variability in the rate of dissolved oxygen depletion, its severity, and its duration are related to year-to-year differences in the thickness and temperature of the bottom water layer in the Central Basin of Lake Erie. These differences are determined by the climate over Lake Erie in the spring, i.e., average air temperature and wind velocity. Rapidly climbing air temperature with calm winds will result in a thinner, warmer top water layer and a thicker, cooler bottom layer. A cooler, windy spring will permit the entire water column to warm to some extent before the top layer separates at a deeper depth, resulting in a warmer, thin bottom layer.

More oxygen will be retained longer into the season if the bottom layer is thicker and/or cooler than average, as in 1997, 2005 and 2006. In years that the bottom water layer is thinner and/or warmer than usual, as in 1992, 2001, 2002 and 2009 oxygen will be lost more quickly and the reduction may be more severe. 

The influence of nutrients and algal growth on the rate and severity of oxygen depletion are superimposed on the natural variability due to springtime weather.

Calculated Oxygen Depletion Rate over Time

Oxygen depletion rate graph

 

To reduce the amount of variability in the year-to-year data about dissolved oxygen in Lake Erie, the data can be statistically adjusted to approximate the oxygen concentrations and the rate of depletion as if the bottom layer were always one thickness (4.6 meters) and one temperature (10 degrees Celsius). The resultant rate of dissolved oxygen depletion (mg/liter/month) is artificial for any given year, but the rates can be fairly compared between years.

From 1970 through 1989, the average oxygen depletion rate showed a relatively steady declined. The lowest rates were observed during 1988 and 1989 for this time period.  However, from 1990 through 2010, the average adjusted oxygen depletion rate has been declining much more slowly. The calculated depletion rate for 2001 was encouraging, being the lowest in this time series and consistent with the long term trend. However, the depletion rates for 2002 – 2010 returned to an elevated level and remain inconsistent with the trend observed from 1970-1989. 

At least two factors may be influencing the rate of oxygen depletion in recent years. The concentration of total phosphorus in the springtime waters of the Central Basin of Lake Erie has been increasing slightly since 1990. Steady or increasing phosphorus levels could support continued algal growth which would generate enough organic matter to lead to oxygen depletion in the bottom waters.

The influence of the non-native, invasive zebra mussels and quagga mussels (close relatives) on nutrient cycling and subsequently dissolved oxygen depletion in the Central Basin of Lake Erie is not well understood. They could have an ecological effect that obscures the rate of oxygen depletion from decomposition of algal organic matter.

 


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