The difference in pH at nighttime and daytime of the Milwaukee River

Kao Lee Vue, Fatma Al-Sammarraie, and Ta Kyla Johnson


River water has relatively lower pH at nighttime because photosynthetic organisms produce carbon dioxide through dark reaction.On the other hand, pHis higher in the daytime because photosynthetic organisms take in carbon dioxide and produce Oxygen via photosynthesis.We hypothesized that river water is more acidic at nighttime compared to the daytime. Our results were not significant (P=2.94).

Key Words: Cellular respiration, photosynthesis, ATP, Water, River, pH


Water occupies the biggest portion of earth covering over 70%, which is 1.4billion km3 (Chetelat & Frabces, 2001). Around 96.5% of the water on earth is ocean and sea, salty water, while 2.5% is fresh water (Klasseen et al., 1998). Water is very important for all living cells including plants, animal, bacteria, and algae. It is one of the most important ingredients for the production of ATP in photosynthetic cells (Forti, 2008), and it constitutes three quarters of the human body. A single water molecule consists of one oxygen atom and two hydrogen atoms covalently bond together.

River water contains many types of organism, most importantly plants, which are considered a necessary source of nutrients for other water organisms like fish. Plants have to convert sun light into energy, ATP, in order to survive, and they do this in two different processes: photosynthesis and cellular respiration. In the presence of sun light plants take in carbon dioxide and water and produce sugar and oxygen in a process called photosynthesis. This happens in chloroplast. On the other hand, plants preform cellular respiration in the absence of light in the stoma by taking in oxygen to break the sugar and release carbon dioxide , water, and energy(Forti, 2008).

During cellular respiration, plants produce carbon dioxide that is converted to a weak acid when itís mixed with water: carbonic acid. Therefore, riverís water pH would decrease (Richter et al., 2003).pH is a way to measured how acidic or basic the water is , and it is ranked on a scale from 0-14, where 14 is the most basic and 0 is the most acidic. Water pH is very important for living cells because enzymes have optimum pH. Optimum pH is the perfect pH where plants can preforms best therefore; changing the acidity of water can be lethal to organisms. Burning fossil fuel releases carbon dioxide and forms a weak acid when dissolved in water. Manufactures and chemical deposits can also cause change in pH.

The pH of a river can also changes slightly at night when cellular respiration in plants takes place and where glucose molecules are broken down and oxygen is taken in to produce ATP and carbon dioxide. Dissolved carbon dioxide in water forms weak carbonic acid, which increases acidity and lowers pH.


There were two sites that we collected data samples from; the Milwaukee River: Urban Ecology Center and Lincoln Park. The Milwaukee River: the Urban Ecology Center is located on 1500 East Park Place, Milwaukee WI, 53211. Lincoln Park is located at North Milwaukee River Parkway (by picnic area # 8) Milwaukee Wisconsin. The materials that were used in this investigation were flag markers, an Xplorer GLX PasPort Pasco PS-2002, measuring tape. On September 29, 2012, at 1300 hours a total of eight samples were gathered at this site. The starting location was picked, which could be observed on table 1. When the starting point was chosen, a flag marker was placed and the probe of the Xplorer GLX PasPort Pasco PS-2002 was placed into the river to obtain the pH. We continued this until we gathered the remaining seven samples at this location. After promptly gathering the Lincoln Park samples, we went to the second site. There the same procedure was carried out. A staring point was identified and then a total of seven data samples were gathered, resulting in a total of fifteen samples from both sites.On October 2, 2012 at 1850, the sites were revisited starting at the Urban Ecology Center. The pH was gathered at the same spot the initial daytime pH was gathered. Then after completely gathering the samples at this location, we headed to the Lincoln location to gather the remaining eight samples. The data was analyzed using Excel© from Windows 2007©, a type 1, 1-tailed t-Test was utilized.


Figure 1:Mean (+/- S.D.) of pH in Milwaukee River at daytime and nighttime. (P-value: 0.0001)

The data thatís reflected in the graph showed that the daytime pH recorded was greater or more basic than of the pH observed during the evening. Through statistically testing such as error bars and a t-test with 1 type and 1 tail, we identified that the decrease of pH from the morning to the night was significantly different (P=0.0001). The mean of the daytime pH was 9.38 with a standard deviation of+/- 0.34. The mean of the nighttime pH was 8.96 with a standard deviation of+/- 0.16.


The study was performed to determine and analyze whether the river is more acidic at the nighttime compared to the daytime.Our results showed that at night the river pH was lower (P=0.00002). Therefore, our hypothesis, which stated that river water pH would decrease during the nighttime compare to the daytime, was supported. When plants are not able to use the sunlight light as a source, plants would result in using HCO3- (Moss, 2010). Depending on the equilibriaof the many forms of carbon present in the river such as molecular CO2, bicarbonic acid, bicarbonate, and carbonate will determine the pH (Moss, 2010) Our study, which correlates to information provided by Moss, provided insight that photosynthesisoccurs more abundantly at higher a pH (2010 p.163).

The areas that could be improved to better observe the changes in pH from morning to night would be to modify the collection time. Since the data was collected around 1950 in the evening, the sun had just set. There have been findings that found a strong correlation between pH and the moss and lichen distribution present in the river (Aho & Weaver, 2006).If we had collected the data at a later time, then we would have witness the resulting pH after a night full of cellular respiration, which means more oxygen consumption and more carbon dioxide production. We could have also investigated multiple rivers and compare their pH differences from morning to night as well as relate those findings the abundance or rocks present. Carbonate rocks, along with the CO2 in the atmosphere control the pH (Horne, 1978).

Some possible errors would include that there were times it was hard to reach the exact location of the marked spots from the morning due to the decreased/ lower water at night.The resulting factor forced us to take samples from a different spot than the same spot as the daytime. Also there was information provided that the flow of the water could influence pH readings. There is also evidence that the proximity at which the probe was placed in reference to the leaf surface of the algae/plants, pH could be elevated by photosynthesis (Moss, 2010). Therefore, Considering there were areas where the depth of the water was less than in other areas, the probe may have been closer to the leafs surface.



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