Oxygen in Water
Oxygen is the key to life — most organisms cannot survive without it, even those under water. Seasonal weather patterns and the physical properties of water can affect temperature and dissolved oxygen levels throughout the water column. Why is this important? Because the seasonal weather patterns and cycles are directly related to how much life an aquatic environment can support.
For example, during the summer, bottom water (hypolimnion) can be cut off from new supplies of dissolved oxygen from the air until fall. Therefore, the size of the hypolimnion affects the ecology of a lake. By examining and graphing water temperatures and the amount of dissolved oxygen in a water column, students will be able to make a connection between the life a lake can support to the amount of oxygen found in stratified layers of water.
- National Science Education Standards, 5-8 grade
- Michigan Grade Level Content Expectations, 5-7 grade
- MS-ESS3-4 Earth and Human Activity: Construct an argument supported by evidence for how increases in human population and per-capita consumption of natural resources impact Earth’s systems.
For alignment, see: NGSS Summary
- Describe how properties of water are related to productivity in a lake.
- Describe how dissolved oxygen and temperature levels can influence populations of organisms.
- Graph dissolved oxygen levels and graph water temperatures.
- Analyze water temperature versus depth graphs to answer questions.
- Analyze dissolved oxygen versus depth graphs to answer questions.
Stratification in lakes prevents surface and bottom waters from mixing. (See: Layer upon Layer Lesson for more background on stratification.) During the summer, Great Lakes’ bottom water (hypolimnion) is typically blocked from new supplies of dissolved oxygen from the air until fall. Adequate concentrations of dissolved oxygen are necessary for the life of fish and other aquatic organisms. The size of the hypolimnion affects the ecology of a lake.
- Big Hypolimnion: In the Great Lakes, areas with a big (deep) hypolimnion (e.g., eastern Lake Erie, Lake Michigan) will have plenty of dissolved oxygen to last the summer.
- Shallow Hypolimnion: Areas with a shallow hypolimnion (e.g., central Lake Erie, bays) have less total dissolved oxygen in the bottom layer.
- Very Shallow Hypolimnion: In very shallow waters (e.g., western Lake Erie and most nearshore waters), the whole water column warms. As a result, a hypolimnion may not form at all and bottom waters remain well-oxygenated.
Figure 1. Water Temperature and Lake Stratification
Lakes have different levels of productivity. Productivity refers to the amount of nutrients available in a lake and the growth that they support. Defining trophic (nutrient or growth) status is a means of classifying lakes in terms of their productivity.
- Oligotrophic (olig-oh-trof-ik) – An oligotrophic lake has low nutrient concentrations and low plant growth (e.g., Lake Superior). It is usually considered to have low productivity.
- Eutrophic (yoo-trof-ik) – A eutrophic lake has high nutrients and high plant growth. (e.g., Lake Erie). It is considered to have high productivity.
- Mesotrophic (meso-trof-ik) – Mesotrophic lakes fall somewhere in between eutrophic and oligotrophic lakes. It is considered to have average productivity.
Too Much of a Good Thing
Really productive, eutrophic lakes may develop areas that are called dead zones in the summer. Dead zones do not have enough dissolved oxygen to support fish and zooplankton. Dead zones are also called hypoxic or anoxic areas. Organic matter like excess algae from either internal inputs (they come from the aquatic environment itself) or external inputs (stemming from an external force like human-related runoff) contributes to dead zones. The increase can accelerate the depletion of dissolved oxygen in the hypolimnion in the summer. Organisms living and breathing in the hypolimnion and the decomposition of algae and other organisms can also speed up the loss of oxygen in the hypolimnion. Thus, the area is referred to as a dead zone because it no longer can support life.
Ecological Impacts of Dead Zones
Figure 2. Two-story fishery in stratified lakes
Fish: Lakes that become stratified in the summer may have a two-story fishery: warm- and cool-water fish living in the epilimnion and cool/cold-water fish in the cold, oxygen-rich hypolimnion.
Figure 3. Lake Trout
Fish can be very sensitive to changes in water temperature or dissolved oxygen concentrations. For example, yellow perch cannot tolerate low dissolved oxygen levels and need dissolved oxygen concentrations of at least 2.0-3.0 mg/l. When oxygen concentrations get too low in the hypolimnion, fish may move vertically or horizontally out of the hypoxic area. Fisheries scientists know that dead zones impact the overall health of fish populations, however, it is not fully understood in what ways.
Zooplankton: Zooplankton tolerate hypoxic conditions better than fish. Zooplankton can live in waters with dissolved oxygen concentrations as low as 0.1-0.2 mg/l, but not much less.
- Graphing Temperatures
Summary: Graph Lake Erie water temperatures from the surface to the bottom of the lake.
Time: One 50-minute class period
Dead Zones – Lesson 3 Activity A: Standards and Assessment
- Air Supply: Graphing Dissolved Oxygen
Summary: Graph dissolved oxygen from the surface to the bottom of Lake Erie.
Time: Two 50-minute class periods
Dead Zones – Lesson 3 Activity B: Standards and Assessment
- Lake Erie Dead Zone – EPA
- Facebook article: Dead Zone Growth is an Ecological Time Bomb, researcher Donald Scavia
- Nitrogen and Lake Erie Dead Zone – Ohio Sea Grant
- USGS Identifies Top Gulf Dead Zone Pollution Sources
- Lake Erie Bathymetry (source: NOAA)
Louisiana Marine Education Resources – Gateways to Aquatic Science. On Again, Off Again – The Dead Zone. Louisiana Sea Grant. Louisiana State University, Baton Rouge, LA 70803. Authors: Lindstedt, D. Website, accessed December 1, 2009.
Water on the Web – Monitoring Minnesota Lakes on the Internet and Training Water Science Technicians for the Future – A National Online Curriculum using Advanced Technologies and Real-time Data. University of Minnesota-Duluth, Duluth, MN 55812. Authors: Munson, BH, Axler, R, Hagley C, Host G, Merrick G, Richards C. Website, accessed December 1, 2009.
Great Lakes Coastal Forecasting System. NOAA-Great Lakes Environmental Research Laboratory (GLERL) Ann Arbor, MI 48108. Authors: Schwab, DJ, Beletsky, D, Bedford, KW, Lang, GA.
Great Lakes Water Data Sets for Teachers. Eastern Michigan University, Ypsilanti, MI 48197. Project supported by the Office of Education and Outreach at NOAA’s Great Lakes Environmental Research Laboratory, Ann Arbor, 48108. Authors: Rutherford, S, Coffman, M, Marshall, A, Sturtevant, R, Klang, G, Schwab, D, LaPorte, E