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What is the Desired Outcome?

Eliminating algae entirely is not the goal. Too little algal growth would not support a healthy aquatic ecosystem. The goal is to identify and achieve the right level and type of algal growth to support a healthy and productive Lake Erie ecosystem.

The Agreement provides guidance in relation to what constitutes a healthy and productive ecosystem from an algal production perspective. It does this in the form of the following six Lake Ecosystem Objectives:

  1. Minimize the extent of hypoxic zones associated with excessive phosphorus.
  2. Maintain the levels of algae below the level constituting a nuisance condition.
  3. Maintain algal species consistent with healthy aquatic ecosystems in the nearshore waters of the Great Lakes.
  4. Maintain cyanobacteria at levels that do not produce concentrations of toxins that pose a threat to human or ecosystem health in the waters of the Great Lakes.
  5. Maintain an oligotrophic state, relative algal biomass, and algal species consistent with healthy aquatic ecosystems, in the open waters of Lakes Superior, Michigan, Huron and Ontario.
  6. Maintain mesotrophic conditions in the open waters of the western and central basins of Lake Erie, and oligotrophic conditions in the eastern basin of Lake Erie.

How Were the Recommended Targets Determined?

The GLWQA Nutrients Annex Subcommittee studied information on algal patterns and species, lake circulation, and sources and loadings of phosphorus.

First, the group decided to align the relevant Lake Ecosystem Objectives to each of the three main basins of Lake Erie.

In the Western Basin, where blue-green algae (cyanobacteria) is the problem, the GLWQA Nutrients Annex Subcommittee determined that the focus should be on Lake Ecosystem Objective #4 (Maintain cyanobacteria at levels that do not produce concentrations of toxins that pose a threat to human or ecosystem health in the waters of the Great Lakes”). Having reviewed the available data, the GLWQA Nutrients Annex Subcommittee proposed that in order to reasonably achieve this Objective in the Western Basin, efforts should be directed toward achieving conditions similar to those experienced in 2012, in nine out of 10 years.

Why 2012?

In 2012, the amount of cyanobacteria present in the Western Basin was considered to be a mild bloom; no significant impacts were noted, with the exception of some bloom conditions in inner Maumee Bay. Therefore, a reasonable threshold to limit the cyanobacteria metrics would be at a level below those seen in 2012.

Why Nine Years Out of 10?

In a year with extremely wet conditions in the spring (i.e. heavy rainfall) there will likely be substantial algal blooms despite phosphorus reduction efforts. In consideration of this, the GLWQA Nutrients Annex Subcommittee proposed meeting the 2012 thresholds for algal blooms, on average, nine years out of 10.

In the Central Basin, where hypoxia is the problem, the GLWQA Nutrients Annex Subcommittee determined that the focus should be on Lake Ecosystem Objective #1 (“Minimize the extent of hypoxic zones associated with excessive phosphorus”). The GLWQA Nutrients Annex Subcommittee proposed that in order to reasonably achieve this Objective in the Central Basin, efforts should be directed toward achieving an average late summer (i.e. August to September) hypolimnetic (cold bottom layer) dissolved oxygen concentration in the Central Basin of Lake Erie of 2.0 mg/L or higher.

In the Eastern Basin, where Cladophora is the problem, the GLWQA Nutrients Annex Subcommittee determined that the focus should be on Lake Ecosystem Objective #2 (“Maintain the levels of algae below the level constituting a nuisance condition”). Having reviewed the available science and modeling, the GLWQA Nutrients Annex Subcommittee proposed that in order to reasonably achieve this Objective in the Eastern Basin, efforts should be directed toward achieving Cladophora biomass of less than 50 g/m2.

In certain nearshore areas, where localized cyanobacteria blooms are the problem, the GLWQA Nutrients Annex Subcommittee focused on Lake Ecosystem Objective #3 (“Maintain algal species consistent with healthy aquatic ecosystems in the nearshore waters of the Great Lakes”). Having reviewed satellite imagery and other available data, the GLWQA Nutrients Annex Subcommittee proposed that in order to reasonably achieve this Objective in certain nearshore areas, efforts should be directed toward reducing nearshore cyanobacteria blooms.

With the relevant Lake Ecosystem Objectives identified, and quantified, the GLWQA Nutrients Annex Subcommittee next identified the level of phosphorus reduction required to achieve the desired outcomes.

Modeling experts from the United States and Canada used nine different computer simulation models to correlate changes in phosphorus levels with levels of algal growth. By comparing and contrasting the results of these models, the GLWQA Nutrients Annex Subcommittee was able to arrive at phosphorus load reduction targets, calculated using 2008 data. 2008 was selected due to the quality of the data available for that year and because conditions in Lake Erie in 2008 are considered representative of an “average” year.