Hypoxia, or low oxygen, is commonly defined as dissolved oxygen levels (DO) at or below the 2-4 mg/L range. It occurs in the bottom layer (hypolimnion) of some highly productive regions of the Great Lakes, including the central basins of Lake Erie and Green Bay in Lake Michigan. Hypoxia typically occurs during the late summer. As organic matter such as algae decomposes, bacteria consume oxygen in the water column, leading to oxygen depletion. Human-induced nutrient enrichment, particularly from elevated phosphorus inputs, has increased the frequency and areal extent of hypoxia.
Impacts of hypoxia on aquatic systems
- Fish kills are the most apparent and extreme impact of hypoxia, but these events are infrequent and tend to be limited in scale.
- Shifts in species distribution in reposnse to hypoxic conditions are more common, along with changes to the benthic community structure. Bottom-dwelling organisms that are sessile (e.g., mussels) will die if oxygen levels are too low, while mobile organisms will shift their habitat. Shifts in community structure and loss of habitat can have long-term, bottom-up effects on food web structure.
- Exposure to hypoxic conditions can limit fish growth, survival, and reproductive capacity.
Mapping hypoxia as a Great Lakes stressor
DO levels > 4 mg/L are unlikely to be stressful to the majority of species.1 We estimated relative stress due to hypoxia using the continuous range of DO values and scale from 0 (no stress, > 4mg/L) to 1 (maximum stress, 0 mg/L). We mapped DO data for Lake Erie from September 20052 and Green Bay for September 2010.3 We also examined data for Saginaw Bay but found it did not meet our criteria for hypoxia.
Point data were kriged to develop a continuous surface using ArcGIS algorithms with a neighborhood of 5 data points.
Spatial distribution of hypoxia as a stressor in the Laurentian Great Lakes (Inset: Green Bay, Lake Michigan).