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Chemical Analysis of North Pond


Rebecca Hager and Andrew Vande Griend | December 11, 1998 | Biology 345

Introduction | Methods | Results | Discussion | Figures | Tables

INTRODUCTION

By studying the chemical composition of a pond, one can better understand the ecological processes that take place in the pond. The purpose of our project was to study the chemical composition of North Pond in the Ecosystem Preserve of Calvin College over the course of the fall semester, 1998.

Most ponds have extremely low concentrations of phosphates and nitrates because they are limiting nutrients for plants. Ammonia concentrations are also very low. They are normally less than 1mg/L (Reid 1961). In small shallow ponds, the pH can vary from 4.0 - 9.0. These fluctuations are quite common (Welch 1952). We assume that North Pond is a typical pond; therefore, we expect similar results. In addition, we expect dissolved oxygen levels to be very high initially because the pond is very eutrophic. We also expect dissolved oxygen levels to decline as the season progresses due to the die off of phytoplankton.

METHODS

Our study was conducted on North Pond of the Calvin College Ecosystem Preserve, Grand Rapids, MI. Samples were collected and analyzed on four dates in the fall of 1998. These dates were September 24, September 29, October 28, and November 17. Samples were collected in three places and averaged together to get a mean for the whole pond. Samples were also collected at two different depths to get a vertical profile of the pond. Samples were collected with a VanDoorne sampler.

Turbidity was measured at each site using a Secchi disk. Dissolved oxygen and temperature were measured at depths of surface, 0.5 meters, 1 meter, and bottom using a dissolved oxygen meter. pH was measured in the field using color changing indicator strips.

Each sample was analyzed in the lab for different chemicals using Hach kits. The Hach kits used were HAC-DT for hardness, NI-11 for nitrate, AL-DT for alkalinity, CD-DT for chloride, PO-19 for phosphate, and NI-8 for ammonia. Ammonia, phosphate, and nitrate measurements were taken twice and the results were reported as means.

RESULTS

Turbidity remained nearly the same early in the fall. Once the weather turned cold, however, turbidity decreased (Fig.1). Dissolved oxygen levels decreased with increasing depth. As the fall progressed the difference between the surface level and the bottom level decreased. By the middle of November the difference was less than 4 mg/L (Fig. 2). Throughout the entire season the dissolved oxygen levels were very high. The water at the surface was supersaturated with oxygen.

pH fluctuated from 5.0 to 8.5 (Table 1). The pH 15 cm above the bottom was always the same as or lower than the pH 30 cm below the surface. Greater variation existed between the surface and bottom levels after it had rained.

Hardness levels had little variation (Fig. 3). They averaged around 20 mg/L of CaCO3. Alkalinity levels started higher 15 cm from the bottom than 30 cm from the surface. As the season progressed these levels switched and by mid-November the alkalinity levels were higher at the surface than at the bottom (Fig. 4).

Chloride levels started high and by October they had decreased by about half (Fig. 5). No nitrate or phosphate was detected in any of the samples. Ammonia levels dropped to 2/3 of the original levels by late October. They then increased again by mid-November to higher levels than were originally found (Fig. 6).

DISCUSSION

The decrease in turbidity was probably due to plankton dying. Levels of phytoplankton decreased between September and November, making the water clearer (De Young and Dyk, unpublished data). This was also seen in the color change of the pond. In September the pond was dark green and by November the pond was much clearer, although still green. Ammonia decreased when the phytoplankton died. It rose again when decomposition took over because decomposers release ammonia from the plankton.

Dissolved oxygen should have also been affected by phytoplankton. We expected to see a decrease in the levels of dissolved oxygen when the plankton died because they produce oxygen as a by-product of photosynthesis. Also, decomposers use oxygen in respiration, which should also have decreased the dissolved oxygen levels. However, the overall dissolved oxygen levels of the pond did not decrease. The water at the upper levels was supersaturated with oxygen. This means that the levels of oxygen were much higher than normal.

The variations in pH were within the normal range for a small, shallow pond. These variations were expected. They were probably so drastic because of the low alkalinity values. Alkalinity is a measure of the buffering capacity of the water to resist changes in pH. Since North pond has a low buffering capacity the pH will vary from day to day.

Alkalinity values were possibly low because there was no runoff from any road into North pond. Road salts can raise alkalinity. Chloride levels were also low because of this. Chloride levels probably dropped because of dilution. There was a lot of rainfall in October. This caused the level of the pond to rise since there is no major outlet. Hardness levels were also very low, which means that North Pond is a soft water pond.

No nitrate or phosphate was found, which was expected. Nitrate and phosphate are readily taken up by plants. Phosphate was found in the sediments (Post, Postema, and Wetzel, unpublished data) but no dissolved phosphate was found in the water because of it's low solubility in water.

Literature Cited

Ried, George K. 1961. Ecology of Inland Waters and Estuaries. Reinhold Publishing Co., New York, USA.

Welch, Paul S. 1952. Limnology. McGraw-Hill Book Co., New York, USA.


FIGURES

Figure 1 Figure 2
Figure 3 Figure 4
Figure 5 Figure 6

TABLE 1

Depth 9/24/98 9/29/98 10/28/98 11/17/98
30 cm from surface 6.5-7.0 8.5 6.0-7.0 5.5
15 cm from bottom 6.0-6.5 5.5-7.0 5.5-7.0 5.5