First, read this background information from Great Lakes Lessons.

Background
Stratification in lakes prevents surface and bottom waters from mixing (Figure 1). During the summer, bottom water (hypolimnion) is 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. For example:

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 (central Lake Erie, bays) have less total dissolved oxygen in bottom layer.
Very shallow hypolimnion: In very shallow waters (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 – meaning the amount of nutrients available and growth that they can support. Defining trophic (nutrient or growth) status is a means of classifying lakes in terms of their productivity. For example:

Oligotrophic: (ol-i-goh-trof-ik) An oligotrophic lake has low nutrient concentrations and low plant growth.
Eutrophic: (yoo-trof-ik) A eutrophic lake has high nutrients and high plant growth.
Mesotrophic: (meso-trof-ik) Mesotrophic lakes fall somewhere in between eutrophic and oligotrophic lakes.
Eutrophic lakes may have Dead Zones in the summer. Dead Zones are hypoxic or anoxic areas without enough dissolved oxygen to support fish and/or zooplankton. Increased organic matter from both internal inputs (e.g., algae production) and external inputs (e.g., sewage) 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.

Ecological Impact 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 2, Ecological Impact of Dead Zones, above).


Fish can be very sensitive to changes in water temperature or dissolved oxygen concentrations. (Example: Figure 3, Lake Trout, above). 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 do not know exactly how these ‘dead zones’ impact the health of fish populations.
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.

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ASSIGNMENT
Look closely at Figure 2.
This shows a typical water temperature curve present during water stratification.

Go to Great Lakes Coastal Forecasting System, GLCFS to view current lake temperature collected by buoys throughout the Great Lakes

Create a graph that compares the current water temperatures of the Great Lakes.
Hint: Use the buoy readings with the greatest depth of each Great Lake in order to have more data points and changes in temperature.

Remember to choose the most appropriate type of graph for the data and labels (title, legend, axises).