A Comparative analysis of Nitrogen Content in Sediments of Lakes and Wetlands

Sarah Grochowski & Melissa Peart

Department of Biology, Alverno College, Milwaukee WI

Abstract

In this experiment we compared the levels of nitrogen in wetland and lake sediments. Samples were collected from 8 different lakes and wetland sites, and then tested for levels of nitrogen. Our results indicated that lakes and wetlands have similar trace levels of nitrogen. A p-value of 0.3 was obtained that signifies that the data obtained is not statistically significant. The results of this experiment do not support the hypothesis of lakes having lower levels of nutrients than wetlands.

Introduction

The purpose of this experiment is to compare nutrient content found in the sediment of shallow lakes and wetlands. Plant growth in lakes and wetlands depends upon the nutrients available in the top most layers of soil. In this experiment we will measure nitrogen levels in the sediment. Nitrogen is a key nutrient that is responsible for the growth of plants. An over abundance of nutrients will lead to excessive plant growth, and scarcity of nutrients will lead to plant loss (Scholes et.al, 2007). Wetlands have the ability to retain heavy metals, sediments, salts, nutrients (Zedler & Kercher, 2004). This may be why wetlands seem to have more vegetation than lakes. According to Drenner, et al. (1997) while some nutrients such as nitrogen from feces or run-off sink into the lake sediment some is taken in by nitrogen-fixing bacteria. Lakes may have less vegetation on the bottom due to microorganism activities, but this could also be due to the shortage of light caused by deeper waters. This experiment will help speculate whether lakes have less bottom vegetation due to the shortage of light, nutrients, or both.

Our hypothesis is that sediment in lakes will retain more nutrients than sediment in wetlands due to wetlands going through dry and wet periods. The saturation and drying cycles of wetlands would increase the amount of leaching from the top layer of sediment.

Methods

On October 19, 2008 starting at approximately 10:00am sediment samples from 8 different lakes and wetlands were collected. Samples were obtained by taking sediments from the edge of the wetland or lake. A dredge was used to get the samples from the bottom to make sure collections methods were the same. The lakes that the sediment was collected from were Veterans Lake, Lake Michigan, Jackson Park Lake, Scout Lake, Mallard Lake, Wind Lake, Greenfield Park Lake, and Muskego Lake. The wetland sediments were taken from various wetland regions in northwestern Waukesha County.

The soil samples were then allowed to dry over a one week period. Those soil samples that were not dry after one week were placed under a heat lamp for approximately one day. Once dry, the sediment samples were tested for amounts of nitrogen using a LaMotte combination soil testing kit. The nitrogen was extracted using extracting solutions and then nitrogen indicators were added. The colors of the soil solutions were then compared to color charts to determine the amount of nitrogen.

Results

The results for the nitrogen test showed no difference between the amounts in lakes and wetlands. Both had trace amounts of nitrogen for nearly all of the sediment samples collected. The data for this analysis is shown in Tables 1 & 2.

Table 1: Lake sediment data Table 2: Wetland sediment data

Lake	Nitrogen	Kg/ha
Scout	trace	11.25
Michigan	low	22.5
Jackson	trace	11.25
Veterans	trace	11.25
Greenfield	trace	11.25
Mallard	trace	11.25
Wind	trace	11.25
Muskego	trace	11.25

Wetland	Nitrogen	Kg/ha
Highway P1	Trace	11.25
Highway P2	Trace	11.25
Pewaukee Park	Trace	11.25
Washington Ave	Trace	11.25
Oconomowoc 1	Trace	11.25
Oconomowoc 2	Trace	11.25
Nashatoh Park	Trace	11.25
Backyard	Trace	11.25

Figure 2: Wetland – Nitrogen Test Results

Figure 1: Lake – Nitrogen Test Results

The photographs of both sets of test results show very little variance, but when the averages of both sites nitrogen content are graphed there appears to be a difference. However the low variance is shown in the standard deviation values in Table 3 for the wetland. The lake had a higher standard deviation of 3.9, but variability is due to one sample being different from the rest. A t-test was performed to see if there was a statistical difference in the results, and a high p-value of 0.35 was obtained.

Table 3: Data Analysis Figure 3

	Lake	Wetland
	Kg/ha	Kg/ha
	11.25	11.25
	22.5	11.25
	11.25	11.25
	11.25	11.25
	11.25	11.25
	11.25	11.25
	11.25	11.25
	11.25	11.25
Average	12.65625	11.25
Standard Deviation	3.977476	0
T-test	0.350617

Discussion

In this experiment we found that the levels of nitrogen were similar in both lakes and wetland sites. Because plants rely on the abundance of nutrients, water, and light, our initial question of what the limiting agent in lake growth is can be answered. There is an abundance of water in both areas and the p-value of 0.35 concludes that the data is not statistically significant. This experiment was not able to prove that the nutrient content in lakes and wetlands was different.

There was little variation in the data, this could have been caused by taking samples from areas within close proximity or the low variation could be caused by the lakes being man-made. It was found out after sampling at Muskego Lake that it had recently been drained for a period of time and then refilled. This would have impacted our data, making that lake have less nutrient content due to the disturbance.

For future study in this, we propose performing other tests for the nutrient content in the sediment such as a phosphorus test. We also propose looking at the nutrient content in the waters of both wetlands and lakes because this would also impact the amount of life in the body of water. Another study could involve comparisons between man-made lakes and natural lakes in their nutrient content since there was no consideration on that in this experiment.

Literature Cited

Drenner, R. Day, D. Basham, S. Smith, J. & Jensen, S. (1997). Ecological water treatment system for removal of phosphorus and nitrogen from polluted water. Ecological Applications, 7:2, pp381-390

Scholes, M., de Villiers, S., Scholes, R., & Feig, G. (2007, July). Integrated approach to nutrient cycling monitoring. South African Journal of Science, 103(7/8), 323-328. Retrieved September 24, 2008, from Academic Search Elite database.

Zedler, J., & Kercher, S. (2004, September). Causes and Consequences of Invasive Plants in Wetlands: Opportunities, Opportunists, and Outcomes. Critical Reviews in Plant Sciences, 23(5), 431-452. Retrieved September 24, 2008 using EBSCOhost, the Academic Search Elite Database.