The 2012 Great Lakes Water Quality Agreement states that “the Waters of the Great Lakes should be free from other substances, materials or conditions that may negatively impact the chemical, physical or biological integrity of the Waters of the Great Lakes.”
Assessment highlights
Land-based stressors which can affect water quality are assessed as Fair with an Unchanging trend. Status is not currently assessed for the Climate Trend sub-indicators; however long-term trends are presented.
Watershed Impacts
The northern portion of the Great Lakes basin remains largely undeveloped and dominated by natural cover. The southern portion is more populated with less natural cover. Development, agriculture and road density are stressors on the Great Lakes ecosystem, especially in areas with larger populations. Urban and agricultural lands are important to the Great Lakes region because they help support people and the economy, however, the water quality in these areas is at higher risk of impairment.
Most of the lake basins experienced an overall increase in population over the current 10-year period that census data are available (2010-2020 for U.S. and 2011-2021 for Canada), with the greatest population growth occurring in the Lake Ontario basin, especially in Canada. In fact, it is expected that the Lake Ontario basin’s population will continue to increase rapidly, particularly in the Greater Toronto Area, further impacting the basin.
Based on 2015 data, overall Great Lakes basin land cover is classified as approximately 66% natural land cover. There is a high degree of variability in the percentage of land cover type between the lake basins. For example, Lake Superior has a high amount of natural land cover (97%), compared to the Lake Erie basin (21%). From 2000 to 2015, there was an estimated net increase in developed land of 2,893 km2. Various other land cover types changed in overall extent over this period, including an estimated net decrease in forest land of 2,900 km2 and in wetlands of 583 km2.
Research has shown that Great Lakes water quality benefits from forest cover within a riparian zone (i.e., land along a lake, river or stream). Forested riparian zones provide essential ecosystem services such as decreasing runoff and erosion and regulating water temperatures.
The amount of Great Lakes shoreline that has been hardened (any placement of material used to armor the shoreline to offer protection from waves and water level changes) has increased. Shoreline hardening can cause changes in habitats and sediment transport. Currently almost one quarter of the assessed Great Lakes shoreline is either moderately or highly hardened.
Tributaries play an important role in transporting surface water, however, watershed land use and stressors directly impact water quality in receiving tributaries. Water quality data from 72 Canadian Great Lakes tributaries were evaluated using an index based on ammonia, chloride, copper, iron, nitrate, nitrite, phosphorus and zinc. The results of the index scores confirm that overall tributary water quality is influenced by land use with poorer scores typically being seen in more urbanized or agricultural watersheds.
Historical Climate Trends
Long-term climate data generally show basin-wide increases in precipitation, increases in summer surface water temperatures and a reduction in Great Lakes ice cover. Studies have shown that Lake Superior is one of the fastest warming large lakes in the world. Lake Superior also has the greatest long-term decline in ice cover in the Great Lakes, experiencing a 35% decrease in maximum ice cover from 1973 to 2020.
In general, water levels in lakes Superior, Michigan, Huron and Ontario show no significant overall average change over the last 100 years, while Lake Erie has seen increasing water levels. However, short-term trends are quite variable. For example, water levels in all the Great Lakes have increased over the past 10 years and Lake Ontario experienced its highest monthly mean levels in over 100 years during June of 2019. Due to the many hydrological influences on lake levels, it is difficult to determine with certainty if these water level trends are within natural climatic variability or are longer-term trends that will continue in the future. However, the recent increasing water levels align with the very high precipitation amounts experienced over the most recent ten-year period. In fact, the total precipitation amounts measured from 2011 to 2020 for the Great Lakes basin were higher than any other ten-year period since 1950.
Shifts in climate can affect Great Lakes habitats, including impacting spawning areas and other habitats for fish species, the extent and quality of coastal wetlands and forest composition. Shifts in climate can also alter biological communities, such as contributing to the northward migration of native and invasive species and creating conditions that favor some invaders over native species. Great Lakes water quality can also be impacted by increases in runoff, changes to contaminant and nutrient cycling and increases in algal blooms.
Sub-indicators supporting the Watershed Impacts assessment
Sub-Indicator | Lake Superior | Lake Michigan | Lake Huron | Lake Erie | Lake Ontario |
Good and Improving | Fair and Unchanging | Fair and Unchanging | Poor and Unchanging | Fair and Unchanging | |
Good and Unchanging | Fair and Unchanging | Fair and Unchanging | Poor and Deteriorating | Fair and Deteriorating | |
Good and Undetermined | Good and Deteriorating | Good and Deteriorating | Poor and Deteriorating | Poor and Deteriorating | |
Undetermined | Not Assessed | Fair and Unchanging | Poor and Unchanging | Fair and Unchanging | |
Unchanging | Increasing | Increasing | Increasing | Increasing |
Sub-indicators supporting the Climate Trends assessment
Sub-Indicator | Lake Superior | Lake Michigan | Lake Huron | Lake Erie | Lake Ontario |
Unchanging | Increasing | Increasing | Increasing | ||
Unchanging | Unchanging | Increasing | Unchanging | ||
Increasing | Increasing | Increasing | Increasing | Increasing | |
Decreasing | Decreasing | Decreasing | Decreasing | Decreasing | |
Climate information is not assessed in the same manner as other indicators in this report, as thresholds for Good, Fair or Poor have not been established at this time. Therefore, climate trends are simply assessed as Increasing, Unchanging or Decreasing. Note that the datasets used to calculate these trends span different time periods and are therefore not directly comparable.
Status
