PERIODIC ANTHROPOGENIC BIOTURBATION OF A SMALL URBAN POND

We characterized the impact of episodic anthropogenic bioturbation on the chemistry of Mirror Lake, located on The Ohio State University campus. In the 19th century, Mirror Lake had been a spring-fed natural water body, but is now an engineered structure with limestone brick sides. Water level, ~1 meter deep, is maintained by inflow from a fountain on the east side and outflow from a small stream on the west side. Total volume of the lake is ~2600 m³. In November 2008, several thousand Ohio State undergraduates rallied to support the university football team in their upcoming game against the University of Michigan by jumping into Mirror Lake. This event occurs annually, two nights before the big game, and lasts for several hours.

Water samples were collected at two locations before, during, and after the event by the research group. The pH, temperature, dissolved oxygen and conductivity were measured at the time of sampling, and we also noted the abundance and activity of local flora and fauna and the extent of the disturbance during sampling. Water samples were filtered with a 0.45 µm filter and then analyzed for major ions using a Dionex ion chromatography and nutrients (TN, NH₄⁺, NO₂^-, NO₃^-, TP and PO₄³⁺) using a Skalar nutrient analyzer. In excess of 2000 students were observed to have jumped into the lake over a two hour period (from approximately 11 pm to 1 am) resulting in a ~ 1°C increase in temperature, from 3 to 4°C. Results of the chemical analyses showed that the major element concentrations in water were similar at the north and south end of the lake and did not vary significantly over the period of disturbance. The trace nutrients did vary slightly as a result of the event. There was a large spike in both NH₄⁺ and TN concentrations during the disturbance on one side of the pond, with little change in nitrogen species concentrations on the other. This spike was associated with the peak number of intoxicated students in the water. After the event, oxidized nitrogen species (NO₂^- + NO₃^-) increased slightly over the next few days, presumably due to the oxidation of the reduced nitrogen species released to the water during the disturbance. Phosphate concentrations did not vary significantly over time nor between the two sampling sites and appeared to be limited by apatite solubility.