Tempe Town Lake is an artificial lake located near the urban core of Tempe, AZ, derived from the damming of the Salt River. It was constructed in 1999 primarily for the use of recreation such as fishing and boating. This lake is Arizona’s second most visited tourist attraction that generated a cumulative economic impact of over $1.5 billion dollars since its creation, so it is critical to monitor and maintain its viability.
Since 2005, it has been subject to long-term water quality monitoring. Dissolved oxygen levels are generally above 5 mg/L, suggesting the lake is not eutrophic. The lake also trends alkaline, with an average pH around 8, likely due to its high productivity and carbonate-poor basement rock.
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Arizona State University
For the past thirteen years, the pH, dissolved oxygen content, temperature, conductivity, dissolved organic carbon (DOC), and nitrogen content of the lake has been measured on a weekly basis non-contiguously. These measurements required water to be taken from the lake and brought back to the lab for analysis, which limited the temporal resolution of the dataset.
Funding was obtained to equip Tempe Town Lake with a data sonde, capable of detecting dissolved organic matter via fluorescence, that would allow in situ measurements (measurements generated at the origin) at a much higher frequency than was previously possible. A major goal of this project was to determine if the parameters that can be measured by the sonde - in particular, colored dissolved organic matter (CDOM) - can be used as a proxy for DOC, which is not practical to measure in situ.
A customized Eureka Manta 35+ water probe was procured to investigate the dynamics of Tempe Town Lake at a higher temporal resolution. The Manta+ sonde was equipped with sensors for temperature, pH, conductivity, dissolved oxygen, turbidity, CDOM/FDOM, and Chlorophyll a.
After calibration, the Manta 35+ water probe was lowered into Tempe Town Lake on June 25, 2018 off a floating pier on the northeast side of the lake. Six months of data was collected, in two three-month increments. Unsurprisingly, lake temperature showed a strong diurnal cycle, as did oxygen levels; oxygen lagged temperature, likely due to the time required for oxygen levels to reach saturation. More surprising was that the levels of CDOM appeared to cycle as well, especially during the summer (Fig 3). This diurnal cycle weakened during the winter but was still detectable in Fourier analysis.
Preliminary results from the Manta demonstrated that there are patterns in the behavior of Tempe Town Lake’s biogeochemistry that require high-resolution monitoring.
Additionally, there appears to be a correlation between temperature and the level of CDOM present in the lake, which may be driving the diurnal cycling. Understanding this relationship remains an important future goal for the project.
Looking ahead, having a robust time series for Tempe Town Lake may make it possible to conduct more deeper analysis for critical slowing down and other transitions in the ecology of the lake, to aid efforts in keeping it a healthy and valuable resource.