pH of Water Samples Obtained from the Milwaukee River Demonstrate Weak Inverse Correlation with Distance Upstream from Lake Michigan

Tina Jeselun and Henan Zaal

Abstract

In this experiment, we hypothesized that water samples obtained from the river at greater distances upstream from the lake would have an increase in pH.Our hypothesis was based on our understanding of how photosynthesis affects pH.We believed that an increase in plant growth would result in a higher reading of pH.There was not a correlation between pH of the river and distance upstream from the lake (R2 = 0.1628).

Key words: Milwaukee River, Lake Michigan, pH, photosynthesis

Introduction

From previous observations and understanding of lake and river ecosystemís we believed that river water samples obtained from further upstream from the lake would be exposed to more plant life and this would result in a higher pH.We predicted that an increase in plant life, including algae and phytoplankton, would cause an increase in pH.Upstream, the water is shallower and more optimal for algae and plant growth.Photosynthesis increases pH (Carru et al., 1999).This is because plant life takes up nutrients from the water including carbon and releases oxygen into the water.Carbon dioxide content lowers pH and the resulting increase in oxygen in a ratio with carbon dioxide increases pH.†† The effects of the growth on the bottom of the river become more concentrated as the body of water becomes increasingly shallow.Upstream from the lake we expected the resulting shallow rivers would produce an ecosystem with a higher pH.When the water is shallower the growth on the bottom of the river obtains more sunlight allowing for the increase in photosynthesis (Anten and Hirose, 2001).We also expect that the shallow waters at the edge of the river from which we obtained our samples would be less turbulent, allowing for more plant life to thrive.


 

Methods

On October 8, 2011 from 1100 to 2000 and on October 9, 2011 from 1200 to 1500 we collected 25 different samples of river water from the Milwaukee.To decide where each of the samples would be taken a map that showed 25 kilometers of the river and its surrounding land was produced.First the photo of the land along with street names was obtained from Map Quest, an online mapping site.The scale provided by Map Quest allowed for each kilometer to be marked with the use of Photoshop.

In Photoshop, the starting point was chosen, because it was where Lake Michigan becomes the Milwaukee River.On the enlarged photo of this map represented in figure 1, we marked the starting point with a large red dot.The green letter A represents the first spot that we would be driving to in order to reach our first sampling area.††† As seen on our map, this is located near the Summerfest festival grounds in the city of Milwaukee.At this point, there was a large drop off from the land to the river and to obtain water sample, one of us had to climb down a ladder and reach the measurement dropper into the water to obtain 2 mL of water.We did not test the pH immediately, because of the difficulty of dipping the pH paper into areas of waters such as this and reading it correctly in a consistent amount of time.We believed that collecting all the samples and then reading them in one place where variables such as light did not affect our judgment of the color that would determine the reading of pH.We did not realize that obtaining the water samples and then testing them later would produce more variables that we did not consider.This is considered in the discussion of this paper.

Using Photoshop, the rest of the river was indicated with a white line and at 1 kilometer intervals each sample area was marked by a smaller red mark.Using the streets provided on the map, we were able to find each sample site.Due to the river sometimes being restricted by personal properties and fences, we were not always able to reach the exact spots indicated by the map.Also, some points marked for obtaining readings on the map were not very close to roads.They required that we park and hike down to the river, possible skewing our understanding of where we were located exactly on our map.

We continued this process for 25 kilometers, obtaining a two mL sample every kilometer.Our final sample was obtained near River Road in the city of Mequon and is indicated as a second large red dot.An enlarged photo of this area is provided in figure 2.

After collecting all of the samples the pH of each was measured with the use of pH paper.We graphed the data in a scatter plot with the use of excel.As a statistical test, we plotted a linear trend line and obtained an R-squared value.


 

Fig. 1 (above) the starting point at which the first water sample was obtained indicated by the large red dot.

Fig. 2 (above) the point of the river at which the last sample was obtained indicated by a large red dot

Text Box: Fig. 3 (Left) full map of river area measured

Results

Fig. 1 pH of water samples shows very weak inverse correlation with distance obtained upstream from Lake

The data obtained, as shown in figure 3, shows a very weak inverse correlation between water sample pH and the distance at which the sample was obtained upstream from Lake Michigan.The pH of the river, according to our readings, ranged from 5 to 7 over the 24 kilometer region.

Discussion

The results do not support our hypothesis.We believe that this was due to mistakes made in our methods and multiple outside factors that could have affected the results.First, the pH of each water sample obtained should have been read immediately.pH can change over time once removed from its source.One of the reasons for this is that unseen organisms in the water can continue to grow, expelling oxygen or die and dissolve into the water, increasing carbon dioxide content.Because some of the samples actually showed a lower pH, it is possible that dead organisms in the water altered our measurements.

A point that was not considered when producing our hypothesis was that seasonality greatly effects plant growth in water systems.During algae blooms pH rises, because the algae is removing carbon dioxide from the water, but during seasons of plant death the dead algae returns to the water along with its nutrients increase the riverís carbon content and therefore lowering the pH (Carru et al., 1999).Beier et al. (2006) demonstrated in one ecosystem that there was a change of 22 to 33 percent in ecosystem respiration.In 0 degree Celsius water, photosynthesis produced around 5 percent of the respiration while in 20 degree Celsius up to 35 percent of respiration.They concluded that season was important to the carbon cycle of an ecosystem.We would expect larger algae blooms in the spring and more algae death in the fall when the samples were taken.

Also, we observed that during the time that these samples were obtained, many of the surrounding trees were losing their leaves.Often leaves fell directly into the river to die, likely resulting in their decomposition and an increase in the waterís pH (Abelho et al., 2005).

While lake ecosystems are generally conducted by biotic factors, rivers are exposed to many more outside factors, such as land runoff, because there is much less water per land area (Carpenter et al., 2002).Toxic run off from the land surrounding the rivers can greatly affect the river ecosystem and cause a change in pH (Nekola et al., 1999).As shown in figure 5 some of the river that we measured went through the city, exposing it to factors not considered.

Text Box: Fig. 5 the Milwaukee River running through downtown Milwaukee

Because of the additional information that we have learned related to season and cellular respiration in ecosystems, we would be interested in performing the same test again, but at different times of the year.It would also be important that we recorded the temperature of the water.We would expect to then be able to see the predicted correlation between distance upstream from the lake and pH as a result of increased photosynthesis during the spring and summer months and much less of a correlation during other parts of the year.We would test the pH of each sample at the site that it was obtained, to avoid the possible changes in pH over time once the sample is removed.When repeating the test we would also record other variables in the surrounding environment that could affect our results.

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