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Limnology Program
Introduction
This summary will present an overview of results for the annual limnology program for the Great Lakes which began in 1983. The limnology program provides information on key environmental factors that influence the food chain and fish of the Great Lakes. The annual monitoring of the Great Lakes began in 1983 for Lakes Michigan, Huron, and Erie; in 1986 in Lake Ontario; and in 1992 for Lake Superior. The sampling strategy is to collect water and biota samples at specific water depths from a limited number of locations in each lake twice every year.
Objectives of the annual program are:
- Assess the state of water quality in the open lake basins (water greater than 30 meters in depth, or greater than 3 miles from shore.)
- Provide data to detect and evaluate trends and annual changes in chloride, nitrate nitrogen, silica, phytoplankton, total phosphorus, chlorophyll a, and secchi disc depth.
- Provide data sufficient to verify or modify water quality models.
- Provide data to calculate the Trophic Index of each lake.
Overview of Results
Chloride
Anthropogenic (human generated) input of chloride compounds (brines, road salt) has
resulted in increased chloride ion concentrations in the Great Lakes. In Lake Michigan,
the observed chloride ion concentration continues to increase at a slow rate of about 0.1
mg/l/year. Models predict increasing chloride ion concentrations in Lake Ontario, Erie,
Michigan, Huron and Superior over the next 500 years. Chloride inputs to Lake Huron and
Lake Erie have apparently decreased over the last twenty years resulting in lower chloride
levels in Lakes Erie and Ontario.
Table1. Chloride changes from Richardson and Rockwell,
Chloride pollution of the Great Lakes,
(in preparation, data from GLNPO's annual spring program.)
Table 2. Chloride trends in the Great Lakes from
1983 to 2006
(data from GLNPO's annual spring program.)
Nitrate and Nitrite
Nitrate concentrations in the waters of the Great Lakes continue to increase in most
basins.
Table 3. Nitrate and Nitrite trends in the Great
Lakes from 1983 to 2006
(data from GLNPO's annual spring program.)
Silica
Dissolved reactive silica (building blocks for diatom shells) has increased significantly in Lake Michigan, and in the Eastern Basin of Lake Erie while remaining stable in the other Great Lakes. Below is a graph of lake-average silica trends. For more detail, see the maps of station-average SiO2 for the Great Lakes.
Table 4. Silica trends in the Great
Lakes from 1983 to 2006
(data from GLNPO's annual spring program.)
Phytoplankton
Phytoplankton species in Lake Erie show transitions from species associated with eutrophic conditions (heavy nutrient enrichment) to species associated with mesotrophic conditions (moderate nutrient enrichment).
Phosphorus
Phosphorus concentrations have stabilized in all of the Great Lakes except for Lake
Ontario where total phosphorus concentrations are slowly declining at a rate of 0.3 ug/l
/year. Total phosphorus concentrations are below the objectives set by the United States
and Canada in all of the Great Lakes except in Lake Erie. Lake Erie's Western Basin has
exceeded the target concentration of 15 ug/l by about 60% while both the Central and
Eastern Basins have exceeded their target concentration of 10 ug/l by about 10 to 20%.
Table 5. Total Phosphorus trends in the Great
Lakes from 1983 to 2006
(data from GLNPO's annual spring program.)
Table 6. Total Dissolved Phosphorus trends in the Great
Lakes from 1983 to 2006
(data from GLNPO's annual spring program.)
Chlorophyll A
Phytoplankton biomass can be indirectly estimated through the measurement of the concentrations of Chlorophyll a in the water. Chlorophyll a concentrations in all the lakes are stable with Lake Superior having the lowest levels and Lake Erie the highest levels. Central and Western basins of Lake Erie have constantly exhibited elevated/enriched chlorophyll a concentrations.
Table 7. Chlorophyll-a trends in the Great
Lakes from 1983 to 2006
(data from GLNPO's annual spring program.)
Dissolved Oxygen
Oxygen depletion in Lake Erie's bottom waters is a persistent problem. Anoxic conditions occur when dissolved oxygen falls below 0.5 mg/l. Bottom dwelling fish would suffocate at these dissolved oxygen levels. Dissolved oxygen concentrations fall below 0.5 mg/l by late August in areas of the central basin and remain at or below this level until autumn mixing occurs in September. Oxygen depletion rates decreased in the late 1980s. Higher oxygen concentrations were also observed. Together these suggest a moderation in the oxygen depletion of the bottom waters.
More on Lake Erie Dissolved Oxygen
Limnology Questions and Answers
1. How are the Great Lakes doing?
Conditions in the Great Lakes are improving. State and federal agents have instituted controls to limit nutrient transfer into the basin. Excessive nutrients were fertilizing the water column, stimulating phytoplankton growth, and ultimately stripping the water column of necessary oxygen levels. Today, nutrient enrichment is under control in all the lakes except Lake erie. Visit the trophic index page for more detailed information on the status of the lakes. See the Trophic Index page for detailed information on the status of the lakes.
2. Is water quality better or worse in particular lakes?
The upper Great Lakes are generally less impacted by human inputs and subsequently, have cleaner water. Although each lake has its trouble spots, Lake Superior, Lake Michigan, and Lake Huron face fewer chemical stressor issues than Lake Erie and Lake Ontario. Lake Erie, the shallowest of all five lakes, is surrounded by urban centers and agriculture. It continues to be a receptacle for great quantities of phosphorus. Lake Ontario battles high chloride concentrations as a result of its location downstream from the other four lakes.
3. Can we identify what sources are polluting our lakes?
Generally, yes. GLNPO scientists use mass balance models to track pollution inputs and movement through the lakes. The concept of mass balance is based on the premise that the amount of a pollutant entering a system should equal the amount of that pollutant leaving, trapped in, or chemically changed in the system. These models enable GLNPO scientists to estimate the relative magnitude of particular pollution sources.
4. What has our research taught us?
"Man-made substances can be controlled once they are identified, but pollution effects on the ecosystem can never completely be eliminated"
----Dr. Glenn Warren, Aquatic Biologist
"Chemical and biological disruptions can occur quite quickly in the Great Lakes despite their large size."
---David Rockwell, Environmental Scientist
"Certain pollutants, such as chloride, are not affected by metabolic or other degradation processes. Therefore, the best way to curtail their lake-wide impact is through limiting the source inputs."
---Dr. Paul Bertram, Environmental Scientist
5. What is the biggest Great Lakes problem today?
Chemical contaminants still threaten the system, which is exemplified by the plethora of fish consumption advisories in effect. In recent years, environmental scientists have been very concerned with the growing number of new species found in the Great Lakes, transported from other regions of the world. The Sea Lamprey, zebra mussels and others are upsetting the very delicate Great Lakes ecological balance. There may be some difficult battles with these exotic species yet to come.
6. Can the R/V Lake Guardian withstand rough weather?
The R/V Lake Guardian is extremely stable in all sea conditions. It has sampled the Great Lakes in all seasons, under a multitude of weather conditions. Sampling operations are halted and the ship will seek shelter in a nearby port when wave heights exceed six feet high.