TOPICS:  Invertebrates, ecology

LEVEL:  High school and advanced placement

TIME:  One class period for field work (plus travel time), one class period for lab work.

ADVANCED PREPARATION:  Check the weather forecasts to select sampling day; avoid extreme cold and high wind chill.  Check with local officials for advice on ice safety, using at least 8 cm of solid, unfractured ice as a guideline.  Contact local fisherman or the regional fishery biologist for assistance in site selection and logistics.  A lake used by ice fisherman should provide good biological samples whereas a winterkill lake will exhibit more dramatic physicochemical characteristics .

MATERIALS: Conical plankton nets and Ekman bottom samplers are readily available from various biological supply companies. Many homemade designs are available in educational and environmental journals and newsletters. Straighten the wire coat hanger and then bend a few inches at the end at a 90^o angle (see step B note below) .

PREDICTIONS: In class discussion, encourage creative, logical, and critical thinking. We recommend that you introduce the following ideas if they are not suggested by students: Shallow lakes with high summer weed and algae populations are susceptible to winterkill. The presence of ice fisherman probably indicates well-oxygenated water containing plankton and benthic organisms that support the fish populations.  Absence of ice fisherman does not confirm that winterkill has occurred; other factors such as poor water quality, small fish size, or the presence of undesired fish species may be involved .

SAFETY NOTE: Select a day with little or no wind because of the danger of wind chill on an unsheltered frozen pond or lake .

STEP A. A central location in the deepest portion of the pond or lake is considered most representative.  Having the hole drilled prior to the field trip saves class time .

STEP B. A bent wire coat hanger or carpenter's rule hooked on to the bottom edge of the ice facilitates measurement of the ice layers .

STEP E. Dip sampling gear in the hole to thaw or use a wash bottle as necessary .

STEP H. The solubility of oxygen in winter at low temperatures is high; determination of % saturation is the best indicator of how much oxygen is actually in the water .

STEP I. Phytoplankton are small, green, and relatively immobile.  Zooplankton are larger, translucent, and faster moving. You may wish to have students examine phytoplankton and/or zooplankton under the low power of a compound microscope .

STEP J1. A rotten egg odor in the sediment is an indication of potential winterkill conditions. However, oxygen depletion typically occurs in bottom sediments even in lakes that do not exhibit winterkill. 

QUESTIONS 4-9 require higher order thinking skills.  We recommend that you discuss these questions in class before having students write out their answers .

2. Answers will vary.  Typically, water temperature remains quite constant near 0^oC during the period of winter ice cover.  Air temperature fluctuates widely and may be considerably higher or lower than the water temperature .

3. Answers will vary.  Copepods are found all year in lakes and ponds.  Some rotifers and cladocerans may be found in winter, but they are far more abundant in summer. Oligochete worms and red-colored midge larvae (bloodworms) can tolerate the low oxygen conditions typical of sediments in winter .

4. The best indicator of winterkill is extremely low (near 0) dissolved oxygen.  Thick ice and snow cover, and a rotten egg odor in the sediment also indicate potential winterkill conditions .

5. Low or absent plankton and benthos indicate winterkill .

6. Phytoplankton add oxygen by photosynthesis; zooplankton remove oxygen through respiration.  Both contribute to oxygen depletion as a result of bacterial decomposition of dead organic matter .

7. Hemoglobin enables benthic organisms to retain oxygen in their bodies so that they can withstand long periods of low oxygen concentration .

8. Lakes that occasionally experience winterkill often have stunted fish populations because of the disruption in their food supply.  Undesirable fish (bullheads and mud minnows) are better able to survive such conditions than game fish (bass, pike) or pan fish (perch, sunfish) .

9. Water becomes more dense as it cools due to close packing of molecules, causing it to sink.  At 4 ^oC, water reaches its maximum density.  As it cools further toward 0 oC, its molecular structure expands, becoming less dense as ice crystals begin to form.  Ice is less dense than water so it floats.

GOING FURTHER:

1. Sample another lake or pond that exhibits contrasting physical and biological conditions.  Compare the two sets of winter data.

2. Return to the sampling area in summer and repeat the same procedures.  Compare winter and summer data.