LIFE IN A WINTER POND OR LAKE

PURPOSE: To describe the organisms and the physical conditions in a lake or pond in winter and determine if winterkill conditions exist.

MATERIALS: Field -- Ice auger, ice spud, ice sieve, meter stick, carpenter's rule or wire coat hanger, thermometer, Hach (dissolved oxygen) kit, plankton net, wash bottle, collecting jar, bottom sampler, large plastic bag, white enamel pan. Lab -- 500 um sieve, petri dishes, stereoscope, eyedropper. Optional: Compound microscope, slides.

INTRODUCTION: A pond or lake is a dynamic ecosystem with complex interactions between living things and the environment. Microscopic, free-floating, photosynthetic organisms known as phytoplankton are producers in the aquatic food web. Zooplankton are the free-floating, animal-like consumers in the food web. They are often important food items for fish. Benthic or bottom-dwelling organisms range in size from tiny protozoans to comparatively large aquatic insects. They are consumers or decomposers in the aquatic food web.

The kinds and numbers of organisms in a pond or lake depend on environmental conditions such as temperature, depth of light penetration, and dissolved oxygen content. It's no surprise that winter conditions under water are very different from summer:

(1) Thick ice and snow cover reduce light penetration, which decreases the amount of oxygen in the water produced by photosynthesis.

(2) The type of ice cover influences light penetration, photosynthesis, and oxygen content. Black ice is transparent, allowing good light transmission. It appears black to the viewer because of the darkness below. Opaque white ice permits little light transmission because of trapped air bubbles and snow frozen into the ice. A white ice layer usually forms on top of the black ice layer.

(3) Oxygen depletion may occur in winter because the oxygen that is consumed by respiration and decomposition is not replaced by photosynthesis.

Low oxygen levels in winter ponds and lakes are a key factor in what fisheries biologists call winterkill conditions that lead to fish death. Shallow lakes with high productivity and decomposition rates are most likely to experience winterkill. In this investigation, you will make a hole in the ice covering a pond or lake. You will sample the organisms and measure the physical conditions in this ecosystem to determine if winterkill conditions exist.

PREDICTION: Use what you know about the size and depth of the pond or lake and ice fishing in the local area to make a prediction.

1. Do you think that many plankton and benthic organisms will be found in this lake or pond? Do you think you will find evidence of winterkill?

FIELD PROCEDURE: Be sure to dress properly for the winter weather. Solid, unfractured ice with a thickness of at least 8 cm (3 in) is considered safe to walk on.

A. Select a central location over deep water in a pond or lake. Use the ice auger to cut a hole in the ice. Use the ice spud to enlarge the hole to fit the sampling gear. Remove the ice chips from the hole with the ice sieve.

B. Measure and record the thickness of the snow cover, the black ice, and the white ice.

C. Record the temperature of the air and surface water.

D. Fill a dissolved oxygen bottle with water. Be careful to avoid air bubbles. Fix the sample immediately using the following method:

(1) Add the contents of one pillow each of Reagent Powder 1 and 2. Stopper the bottle and shake vigorously to mix. Allow the flocculent to settle.

(2) Shake the bottle again. Let the contents settle a second time.

(3) Remove the stopper and add the contents of one pillow of Reagent Powder 3 and shake to mix. A yellow-brown color will indicate that oxygen was present, while a white color denotes oxygen depletion.

E. Collect a plankton sample as follows:

(1) Lower the plankton net to the bottom. Then raise it with a slow but steady hand-over-hand motion (0.5 m/sec).

(2) Transfer the plankton sample to your collecting jar. Use the wash bottle to assure a complete transfer of organisms by washing down the net.

F. Obtain a benthic sample using the bottom sampler. Empty the sediment and water into a large plastic bag. Seal the opening and put the bag into a white enamel pan for transport.

G. If you are unable to examine your plankton and benthic samples immediately, store them in the refrigerator or add alcohol preservative (1 part 70% isopropyl alcohol to 6 parts water) for later examination.

LAB PROCEDURE:

H. Determine the dissolved oxygen content of the fixed water sample using the following titration method:

(1) Add the fixed water sample up to the fill line of the measuring tube and then pour the measured amount into the mixing bottle.

(2) Add the PAO titrant counting one drop at a time, while swirling the bottle to mix, until the solution changes from yellow to colorless. The number of drops added is equal to the number of parts per million (ppm) of dissolved oxygen.

(3) Use the Dissolved Oxygen Nomogram to determine the percent saturation of dissolved oxygen in the water.

I. Transfer your plankton sample to a petri dish and examine using a stereoscope. Under such low magnification most phytoplankton are too small to distinguish clearly. However, you should be able to identify the larger, actively mobile zooplankton. The most common freshwater zooplankton are rotifers, cladocerans, and copepods, such as those shown in the diagrams. Identify, sketch, and give the relative abundance (few, many) of any zooplankton you find.

J. Describe the benthic organisms you collected as follows:

(1) Note the color, texture and odor of the sediment from the benthic sample. Record your observations on the data sheet.

(2) Separate out the larger benthic organisms by passing the sample through a 500 um sieve. Use water and gentle agitation to speed up the process.

(3) Transfer the larger benthic organisms to a petri dish and examine using a stereoscope. The organisms you find will be strongly influenced by the type of sediments you sampled -- some organisms cling to rocks and gravel, while others burrow in sand and silt. Identify and sketch any organisms you find, and give their relative abundance (few, many).

CONCLUSIONS:

2. How did the temperatures of the air and the surface water compare?

3. Which zooplankton organism had the highest relative abundance? Which benthic organism had the highest relative abundance?

DISCUSSION:

4. Which of the physical factors that you examined and recorded is the best indicator of potential winterkill? According to your data, is winterkill a possibility in this pond or lake?

5. Does your biological data indicate that winterkill is occurring? Use your data to explain your answer.

6. What effect do phytoplankton have on the dissolved oxygen content of the water? What effect do zooplankton have on the oxygen content?

7. Some benthic organisms are red because their bodies contain hemoglobin. How do you think hemoglobin helps these organisms survive low oxygen levels in the sediment?

8. Why do ice fisherman usually avoid lakes that even occasionally experience winterkill?

9. Why does water in a pond or lake freeze from the top down and not from the bottom up?

DATA SHEET

POND/LAKE STATION	Date	Snow Depth (cm)
Cover Thickness	Black Ice (cm)	White Ice (cm)
Temperature	Air (C)	Water (C)
Dissolved Oxygen	D.O. (ppm)	D.O. (%)
Sediment (Color, Texture, Odor)
BENTHIC ORGANISMS Worms Midges Others	Sketch and Relative Abundance (Few, Many)
ZOOPLANKTON Rotifers Cladocerans Copepods Others	Sketch and Relative Abundance (Few, Many)