A Study in the Ecosystems of Aquatic Insects

This topic submitted by Timothy Breihan, Andrew Davis, Christopher Haedt, Michael Loeffelman (breihatd@miamioh.edu) at 5:07 pm on 12/10/99. Additions were last made on Wednesday, May 7, 2014. Section: Zinn

Abstract

This lab will seek to quantify and qualify the insect populations in two distinct regions of the Western Pond. By analyzing both the number of insects present as well as the species diversity in two areas of the pond with apparently different ecosystems, we hope to garner conclusive data supporting our hypothesis that an increase of plant matter in a subsurface ecosystems would in fact indicate an increase in both the number and diversity of aquatic populations. Furthermore, our results will aid us in determining if in fact the specific ecology of each respective region of the pond has an impact on the quantity and quality of the insect life present there.


Introduction

In attempting to develop a lab as the capstone of this course, we decided that a study revolving around the Western Pond would be a valuable exercise in research for several reasons. First, such an experiment would test our prowess at data collection and analysis. Secondly, an experiment in aquatic ecosystems, a subject that none of us had extensive previous experience in, would most likely promise to one of great interest and learning. Finally, the amount of data necessary to obtain an accurate result, fit well with the format of the assignment; that is, and experiment in which the class is to participate. While the amount of data needed is certainly of substance, it is by no means beyond feasibility in class of our size.

Given these facts and the timeframe in which we were given to work, we felt that a study in the ecosystems of aquatic insects and the manner in which these ecosystems affect both the quantity as well as the quality of insect populations would be ideal. One of our main motivations in a choosing a subsurface revolves around the changing weather conditions at this time of year, and an aquatic ecosystem is able to exist with relative independence of the radical changes occurring above the surface of the pond.

Much as terrestrial insects are primary consumers of plant matter, so do aquatic insects account for a large amount of the breakdown of plant material in subsurface ecosystems (Resh and Rosenberg 164). This fact, while illustrating the relative importance of insects in an aquatic ecosystem, also provides clues as to where they can be found. The simple fact of the availability of food would tend to suggest that areas of greater vegetable matter accumulation would be more conducive to larger insect populations. Supporting this assertion is the fact that leaf decomposition tends to increase water temperature slightly, resulting in greater algal growth, another source of food (Cummins and Merritt 42).

Objective

Our primary objective in the organization and execution of our experiments consists of a quantitative and qualitative study of the number and diversity of species of aquatic insects in the Western Pond. An artificially created freshwater body, Western Pond provides a nearly ideal platform for extended observation. It is entirely self-contained, minimizing the effects of large chemical, temperature, and pH shifts from external water sources that creeks and stream-fed ponds are sometime subject to. Furthermore, the pond contains two distinct areas, an underdeveloped region, bordered by a dense thicket of deciduous, secondary growth trees, and a developed region, bordered by a cultivated lawn.

Our activities will attempt to determine the specific ecological qualities of each region and the manner in which these unique conditions affect the number and diversity of the aquatic insect populations that inhabit them.
Our observations will seek to ascertain data from two separate aspects of the question at hand:

First, to determine the environmental conditions present in the two aforementioned ecosystems, including sediment level and structure, water temperature, water pH, and dissolved oxygen levels, and

Second, to gather quantitative and qualitative data on the insect populations present in these two aforementioned ecosystems.

Hypothesis Given the environmental variance of the area immediately surrounding the Western Pond, and

Given that one area of the pondís surrounding is more densely populated by secondary-growth tress, and

Given that an increased amount of trees would lead to greater leaf accumulation in the areas of the pond immediately adjacent, and

Given that increased leaf accumulation would result in increased decomposition, and

Given that increased decomposition would result in
1) Greater sediment accumulation, and
2) An increase in water temperature, and
4) A decrease in the dissolved oxygen content of the water;

We propose that insect populations in the area of the pond adjacent to the trees will be positively affected, in both number and diversity, by the aforementioned conditions present in this ecosystem.


Resources
& Materials

Sediment corers
Collection vessels
Strainers
Microscopes
Aquatic insect keys
Thermometers
Hach kit test for dissolved oxygen
Digital pH meter (pHTestr 1)


Procedure & Methods

The execution of this study will take place in two distinct phases, with a third phase of data analysis. The first of these will involve only our groups members and will take place on a weekly basis from the date of proposal acceptance to roughly one week prior to the due date of the final report. The second will take place on the scheduled class day, and will involve all students in Section D of the Natural Systems 121/123 class.


Phase One: A Study of the Ecosystems of Western Pond

A pair of test sites shall first be selected, one adjacent to the wooded area along the eastern edge of the pond, and one adjacent to the cultivated lawn on the western edge of the pond. The tests in this phase will be performed on at least two occasion at different times of day, as to best minimize the effects of direct sunlight or excessive shade on the conditions of the water. These tests will be performed by the group only.

Test One:
Temperature

Using a thermometer, the water temperature of each test site shall be measured and recorded.

Test Two:
Dissolved
Oxygen

Note: This procedure is reproduced directly from the Hach manual.

Collect a water sample in a clean 300-mL BOD bottle.
Note: Allow the water to overflow the bottle for 2-3 minutes to ensure air bubbles are not trapped.

Note: If sample cannot be titrated immediately, perform steps 1-5 and store for up to 8 hours in the dark before titration.

Add the contents of one Manganous Sulfate Powder Pillow and one Alkaline Iodide-Azide Reagent Powder Pillow.

Immediately insert the stopper so that air is not trapped inside the bottle. Invert several times to mix.
Note: A flocculent precipitate will form. It will be orange-brown if oxygen is present.

Wait until the floc in the solution has settled. Again invert the bottle several times and wait until the floc has settled.
Note: Waiting until the floc has settled twice ensures complete reaction of the sample and reagents.

Remove the stopper and add the contents of the Sulfamic Acid Powder Pillow. Replace the stopper without trapping air in the bottle and invert several times to mix.
Note: The floc will dissolve and leave a yellow color if present.

Select a sample volume and Sodium Thiosulfate Titration Cartridge corresponding to the expected dissolved oxygen concentration from Table 1.
Note: See Hach manual.

Insert a clean delivery tube into the titration cartridge. Attach the cartridge to the titrator body.

Turn the delivery knob to eject a few drops of titrant. Reset the knob to zero and wipe tip.

Use the graduated cylinder to measure the sample volume taken from Table 1. Transfer the sample into a 250-mL Erlenmeyer flask.

Place the delivery tube into the solution and swirl the flask titrating with sodium thiosulfate to a pale yellow color.

Add two dropperfuls of Starch Indicator Solution and swirl to mix.
Note: A dark blue color will develop.

Continue the titration to a colorless end point. Record the number of digits required.


Calculate:
Digits Required x Digit Multiplier = mg/L Dissolved Oxygen

Test Three:
Water pH

Note: Procedure reproduced directly from the pHTestr 1 manual.

After obtaining a water sample from the Western Pond, dip the electrode of the pHTestr 1 1/2" to 1" into the test solution.

Stir once and let the display stabilize. Note the pH.

Test Four:
Sediment
Structure

Using a sediment corer, a sample of the sediment at each test site shall be taken and photographed for record.


Phase Two: A Quantitative and Qualitative Study of the Aquatic Insects of the Test Sites

Prior to the date of the class experiment, the procedure below will be performed by the members of our group alone, in an attempt to minimize the margin of error intrinsically present when a large group of assistants is utilized.
On the assigned day of the class experiment, the class will be broken down into a number of small groups, each containing three or four students. Each group will be given a collection vessel, strainer, and identification key of aquatic, freshwater insects.

At the pond, each group will be assigned to one of the two test sites. At their respective sites, the groups will take samples of the sediment in their collection vessels. Each group, as to provide sufficient test results for statistical analysis must collect at least six separate samples.

After samples are collected, the groups will begin to strain the sediment. Water and silt will be discarded, while larger solid matter will be kept for observation.

Groups will then examine the solid matter, looking for aquatic insects that may be inhabiting it. Aquatic insects shall be collected.

Utilizing identification keys, students will attempt to identify the genus and species of insect found. This data, along with the number of insect found, shall be recorded for analysis.

The remaining solid matter will then examined under a microscope for further insect life. Quantitative and qualitative exercises described above will be repeated.


Phase Three: Data Analysis

At the conclusion of data collection, the results of all respective tests will be compared.

In regard to the temperature, dissolved oxygen level, and pH of the water
at the respective test sites, this data will be compared to the hypothetical propositions in order to determine if our reasoning pertaining to the environmental conditions of the respective test sites was accurate.

In regard to the qualitative and quantitative data on the population of aquatic insects at the respective test sites, this data will be examined to determine species richness, evenness and diversity. The resulting information will tell us whether or not the environmental differences in the two subsurface ecosystems do in fact conclusively affect the quantity and quality of insect populations found.

Data

The data represented in the table below is the result of our Phase One tests;

Data Table 1: Water Conditions of the Western Pond
Afternoon Night
Area 1 Area 2 Area 1 Area 2
Temperature (ºC) 12 ºC 13ºC 2ºC 5ºC
pH 6.9 6.9 6.9 6.9
Dissolved Oxygen
(mg/L) 4.2 mg/L 6.0 mg/L 4.0 mg/L 6.0 mg/L


The data represented in the table below is a result of our Phase Two tests;

Data Table Two: Quality and Quantity of Insects Inhabiting the Western Pond
Area 1 Area 2
Coiled Snail 0 1
Crawfish 0 1
Freshwater Clams 3 0
Larva 1 0
Spiral Snail 2 7
Threadworm 1 12
Tubifex 0 3


Data Analysis

To gauge the accuracy of our environmental predictions for each of the subsurface ecosystems studies, we first compared the data recorded in Table 1 to the expected conditions stated in the hypothesis. While tested pH of the water in both sites was not consistent with the parameters outlined in the hypothesis, the tested water temperature and dissolved oxygen content of the respective test sites was congruent with our hypothesized conditions. Therefore, our expectations of the environmental changes that occur in an aquatic ecosystem when large amounts of plant-matter decomposition occur were correct.

In order to gauge the differences in the quantity and quality of the insect populations present in the two tested ecosystems, we first arranged our data in the form of a bar graph for visual analysis;



While the data collected would tend to suggest that certain species, particularly spiral snails, threadworms and tubifex are more abundant in regions of increased plant-matter decomposition, there is no conclusive evidence that either the quantity or diversity of populations is greater in Area 2 than in Area 1.

The data was then forwarded to our instructor, Lisa Zinn, and analyzed to find indexes for richness, evenness and diversity, as well as the standard Simpsonís diversity index. This data is as follows;


Indexes for Richness, Evenness and Diversity
Area 1 Area 2
S 7 24
E 4.921 5.765
H 1.277 1.231
D 0.69 0.65

S=Richness; E=Evenness; H=Diversity; D=Simpsonís Diversity Index

While the first figure would indicate that species richness is far greater in Area 2 than in Area 1, all other figures are inconclusive in determining a difference in species quantity and diversity between the two test sites.


Conclusion

While the results of our Phase One testsóthose gauging the environmental conditions of the respective subsurface ecosystemsówere successful in that our findings supported the conditions initially proposed in our hypothesis, the results of our Phase Two tests were unsuccessful in conclusively establishing a link between the amount of plant matter present in an aquatic ecosystem and the quantity and quality of the insects found there.

There are several aspects of human error that may account for this lack of success. Most importantly, we believe, are the methods we employed to collect sediment for analysis. As we were limited by the length of our collection devices, we were mainly restricted to those sediment samples that were found within an armís reach of the bank. As these areas are the shallowest areas of the pond, they are also the most susceptible to temperature changes. If we were able to have collected sediment from the deeper and therefor more climactically stable regions of the pond, we may have had more success in establishing a conclusive base of data.

There is, of course, the issue of the pond itself. At the time of sediment analysis, our instructor, Lisa Zinn, commented that there was an anomalous scarcity of insects present in the samples collected. Upon further reflection, we believe that there are two primary causes of this anomaly. Not only is this body of water artificially aerated in an attempt to minimize algal blooms, it is located within close proximity to an often-traveled roadway. We therefore suspect that the pond may have higher than average levels of hydrocarbon pollution, as the exhaust and petroleum residue present on the roadbed undoubtedly washes into the pond in the rain. Therefore, the insect populations of the pond may be affected adversely by the aforementioned conditions.

In hindsight, an interesting experiment would have been made out of comparing the results of the Western Pond experiment with a similar experiment conducted in a more secluded and less manipulated pond. Such an experiment may well address some of the new questions that have arisen as a result of our experiment.



Bibliography

Beroza, Morton: Chemicals Controlling Insect Behavior. New York:
Academic Press, 1970.

Cummins, Kenneth W. & Merritt, Richard W.: An Introduction to the
Aquatic Insects of North America. Dubuque, Iowa: Kendall/Hunt Publishing Co, 1996.

Lanham, Url: The Insects. New York: Columbia University Press, 1964.

Lehmkuhl, Dennis M.: How to Know the Aquatic Insects. Dubuque,
Iowa: Wm. C. Brown and Co. Publishers, 1979.

Pimentel, David: Insects, Science, and Society. New York: Academic
Press, Inc, 1975.

Resh, Vincent H. & Rosenberg, David: The Ecology of Aquatic Insects.
New York: Praeger Scientific, 1984.

Samways, Michael J.: Insect Conservation Biology. London, Chapman &
Hall, 1994.

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