Great Lakes National Program Office Environmental Indicators

Trophic State of the Great Lakes

The EPA Great Lakes National Program Office is charged with monitoring the water quality of the Great Lakes. Every year, research teams collect an enormous amount of information: nutrient levels, water clarity, water temperature, dissolved oxygen levels, biological information including counts of phytoplankton, zooplankton and bottom dwelling organisms. Looking at each parameter individually provides valuable insight into the Lake ecosystem, but it is important to recognize how these factors interact and influence one another. Scientists have worked hard to develop appropriate multi-parameter indices that can help describe the health of a water body. Trophic state indices are multi-parameter indices that were developed to describe the surface water quality of lakes.

The trophic classification of lakes results from the division of a trophic continuum into a series of categories called trophic states. The trophic state of lakes is indicative of their biological productivity, that is, the amount of living material supported within them, primarily in the form of algae. The least productive lakes are called ‘oligotrophic’. These are typically cool and clear, and have relatively low nutrient concentrations. The most productive lakes are called ‘eutrophic’ and are characterized by high nutrient concentrations which result in algal growth, cloudy water, and low dissolved oxygen levels. Those lakes with a trophic status that falls along the continuum somewhere between oligotrophy and eutrophy are termed ‘mesotrophic’.

In the past the classification of lakes as either oligotrophic or eutrophic was somewhat arbitrary and based primarily on the abundance, or lack of, phytoplankton populations (algae). Trophic State Indices were developed as a result of scientists working to develop mathematical scales by which the trophic state of lakes could be determined. As scientists began to look at factors that were characteristic of certain types of lakes it became clear that the trophic concept was multi-dimensional, with several interrelated factors contributing to trophic status.

Scientists have developed many different methods and scales for determining the trophic status of water bodies. Indices have been developed using nutrient loading, nutrient concentration, productivity, indicator species, water clarity and hypolimnetic oxygen depletion. Because trophic status is a continuum, and because methods for determining trophic status use different scales and rely on different assumptions, different indices for a given water body are not always in exact agreement. These differences can give us even more information about the water body in question, as they force us to closely examine our original assumptions.

The results presented here are based on work published by Chapra and Dobson (1981). They developed a surface water quality index based on the underlying assumption that there is a direct relationship between phosphorus concentrations, chlorophyll a (algal biomass), and secchi depth (clarity); phosphorus drives algal growth which then affects water clarity. It is from this series of relationships that Chapra and Dobson derived the equations for their indices and the following quantitative scale: oligotrophy, 0-5; mesotrophy, 5-10; eutrophy, 10-15.

Two examples of when these relationships do not hold true are: 1) Lakes or areas of lakes that are nitrogen limited thereby preventing algal growth in the presence of excess phosphorus, 2) Lakes with naturally occurring color or turbidity which could be mistaken for decreased clarity due to algal growth.

Except in shallow bays and shoreline marshes, the Great Lakes were oligotrophic before European settlement and industrialization. Their size, depth and the climate kept them continuously cool and clear. The lakes received small amounts of fertilizers such as phosphorus and nitrogen from decomposing organic material in runoff from forested lands. Small amounts of nitrogen and phosphorus also came in from the atmosphere. Following the arrival of European settlers, the Great Lakes ecosystem experienced drastic changes, as waves of immigrants logged, farmed and fished commercially in the region. As agriculture intensified after the turn of the 20th century and more people moved into urban areas around the lakes, the Lakes started showing signs of distress. In the mid 1900s, the combination of synthetic fertilizers, existing sources of nutrient-rich organic pollutants, such as untreated human wastes from cities, and phosphate detergents caused an acceleration of biological production (eutrophication) in the lakes. In the 1950s, Lake Erie showed the first evidence of lake-wide eutrophic imbalance with massive algal blooms and the depletion of oxygen.

Phosphorus is an essential element for all organisms and is often the limiting factor for aquatic plant growth in the Great Lakes. Although phosphorus is found naturally in tributaries and run-off waters, the historical problems caused by elevated levels have predominately originated from man made sources. Sewage treatment plant effluent, agricultural runoff and industrial processes have released large amounts of phosphorus into the Lakes.

Strong efforts begun in the 1970’s to reduce phosphorous loadings have been successful in also reducing nutrient concentrations in the Lakes, although high concentrations still occur locally in some embayments and harbors. Phosphorus loads have decreased in part due to the removal of phosphorus from detergents, changes in agricultural practices (e.g., conservation tillage and integrated crop management), and improvements made to sewage treatment plants and sewer systems.

The Great Lakes National Program Office has been monitoring the Great Lakes for total phosphorus, chlorophyll a, and secchi depth, since the early 1980s. The measurements can be used to calculate a yearly trophic state index for each of the Great Lakes (Chapra and Dobson, 1981). Tracking this index gives us insight into the current health of the Great Lakes and can warn us about problems when they first appear. We can then take appropriate actions to mediate problems before they become severe. Tracking the trophic status for each of the lakes also tells us if we have been successful in the past with our efforts to improve the surface water quality. It is important to remember that this index describes the surface water quality of the open waters of the Great Lakes. It does not address nearshore or local conditions described in the Lakewide Management Plans.

Lake Superior is the deepest and the coldest of the five Great Lakes. According to a 1980 IJC Report on Phosphorus Management, the trophic state goal for Lake Superior is oligotrophy. Data for the trophic state index has been collected for Lake Superior only in the 1990s. The index indicates that surface water quality for Lake Superior has been “excellent” in the 1990s, and trends do not indicate any potential problems.

Lake Huron is the third largest of the lakes by volume. According to a 1980 IJC Report on Phosphorus Management, the trophic state goal for Lake Huron is oligotrophy. The trophic state index indicates that surface water quality for Lake Huron is currently “excellent.” Trends do not indicate any potential problems, but rather show improved surface water quality since the 1980s.

Lake Michigan, the second largest Great Lake by volume, is the only Great Lake entirely within the United States. According to a 1980 IJC Report on Phosphorus Management, the trophic state goal for Lake Michigan is oligotrophy. The trophic state index indicates that surface water quality for Lake Michigan is currently “excellent.”  Total phosphorus levels are declining and other measurements such as dissolved reactive silica and chlorophyll "a" indicate improving conditions in the offshore waters.

Lake Erie is the smallest of the lakes in volume and is exposed to the greatest effects from urbanization and agriculture. As mentioned before, Lake Erie was the first Great Lake to show evidence of lake-wide eutrophic imbalance with massive algal blooms and depletion of oxygen.

Lake Erie can be separated into three distinct basins. According to a 1980 IJC Report on Phosphorus Management, the trophic state goal for the Eastern Basin, the deepest of the three, is oligotrophic.

The trophic goal for the Central Basin, which is not as deep as the Eastern Basin, is oligomesotrophy. Although this goal is currently being met for surface water quality, because of its shape and depth the basin often experiences depletion of oxygen at its lower depths during the summer months. This depletion of oxygen is unacceptable for aquatic creatures which need oxygen in the water to live. In addition, phosphorus concentrations have been elevated in the Central Basin during the 1990's and cannot be solely explained by external total phosphorus loadings to the lake. Although the surface water quality goal is currently being met, the depletion of oxygen at its lower depths and potentially rising phosphorus concentrations may result in problems for the Central Basin of Lake Erie.

According to a 1980 IJC Report on Phosphorus Management, the trophic goal for the Western Basin, the most shallow of the basins, is mesotrophy. This goal is currently being met. Large yearly fluctuations in the index obscure any underlying trends.

Lake Ontario is the fourth largest lake in volume. According to a 1980 IJC Report on Phosphorus Management, the trophic state goal for Lake Ontario is oligomesotrophy. Currently this goal is being met. Long term trends indicate a large improvement in surface water quality since the late 1980s. Total phosphorus levels have fallen and other measurements such as dissolved reactive silica and chlorophyll "a" indicate improving conditions in the offshore waters.

Currently all of the Great Lakes are meeting their trophic state goals. Efforts to reduce phosphorus concentrations in the Lakes have been successful and have resulted in fewer nuisance algal blooms and increased water clarity in the open waters. Scientists are concerned, however, that current control measures are no longer sufficient. Phosphorus concentrations appear to be increasing in the Central Basin of Lake Erie, and may be an early warning to future problems. Additional loadings to the Great Lakes can be expected in the future because of increasing populations exerting greater demands on existing sewage treatment facilities. Also, increasing numbers of second homes or vacation dwellings may also lead to increased non-point sources of phosphorus loadings.

Disruptions of the food web by nuisance species can also affect surface water quality in the Great Lakes even without changes in external loadings of phosphorus. For example, the relationship between zebra mussels and phosphorus cycling is not fully understood, but in the vicinity of zebra mussel infestations, both food web dynamics and nutrient cycling have been greatly altered. Nuisance infestations of cladophora (coincident with the occurence of zebra mussels) occur in Lake Michigan and Lake Erie. Severe blue-green algae blooms in Lake Erie in recent summers may be related to zebra mussels interrupting normal biological and chemical processes.

Acknowledgements

Chapra, S.C. and Dobson, H.F.H. 1981. “Quantification of the Lake Trophic Typologies of Naumann (Surface Quality) and Thienemann (Oxygen) with Special Reference to the Great Lakes,” Internat. Assoc. Great Lakes Res. 7(2): 182-193.

International Joint Commission. Indicators for Evaluation Task Force. Indicators to Evaluate Progress under the Great Lakes Water Quality Agreement. Windsor, Ontario, April 1996, 82 pp.

International Joint Commission. Phosphorus Management Strategies Task Force. Phosphorus Management for the Great Lakes. Final Report to the Great Lakes Water Quality Board and Great Lakes Science Advisory Board. Windsor, Ontario, July 1980, 129 pp.

Lake Erie Lakewide Management Plan, 2000
Lake Huron Initiative Action Plan Updates, 2002
Lake Michigan Lakewide Management Plan, 2000
Lake Ontario Lakewide Management Plan, 2000
Lake Superior Lakewide Management Plan, 2000

The trophic state goals outlined in this presentation draw from the Great Lakes Water Quality Agreement of 1972, International Joint Commission reports and desired outcomes and the State of the Lakes Ecosystem Conference ecosystem objectives (see references). As one of the IJC’s five main stresses on the Great Lakes ecosystem, cultural eutrophication or undesirable algae caused by larger than normal total phosphorus concentrations in the lakes can result in loss of beneficial uses such as swimmability and undesirable biotic changes. In its most recent report, the IJC recommends that the governments, in consultation with the public, investigate a desired outcome for a “balanced nutrient regime” for the Great Lakes ecosystem (1996: 43).