Tempe Town Lake Water Quality
Constructed in 1997, the Tempe Town Lake is a small, man-made reservoir that transforms a section of the typically dry Salt River bed into a 224-acre lake in the heart of Tempe, Arizona. To accommodate the river when it flows, the lake features hydraulically operated steel gates that allow water to pass through the system unimpeded. The lake has been a remarkable success as a community amenity and as a driver of economic growth in the area around the lake. The lake provides an ideal model system for the many artificial lakes constructed in arid-land cities owing to management decisions, such as draining, that affect their operation and ecology. At the same time, dramatic shifts in hydrology and chemistry when the lake is transformed to a flowing river and back into a lake during and after floods, provide opportunities to study the system’s dynamic evolution to new limnological steady states.
The CAP LTER began measuring water quality, including temperature, pH, conductivity, dissolved oxygen, dissolved organic carbon (DOC), and total dissolved nitrogen (TDN) in the lake starting 2005. Initially, these data were collected through a mix of hand-held meters and analyses of water samples collected during regular visits to the lake (e.g., see knb-lter-cap.630). In summer 2018, the CAP LTER installed a in situ datasonde to measure water quality, including optical DOC characteristics, at high temporal resolution (~ 30 minutes). After a brief period during which data were collected both with the datasonde, and by hand-held meters and analyses of water samples, the project transitioned to collecting data exclusively with the datasonde.
Analyses suggest variable impacts of extreme events, climate variability, and management decisions. Complex seasonal and inter-annual patterns in DOC quantity and quality suggest that carbon cycling in the lake responds both to meteorological/climatological events and to anthropogenic activity. Time-series analysis is revealing that water flow into the lake and rainfall have a positive effect on DOC concentration and composition. Further, seasonal patterns in dissolved oxygen concentration from 2005–2015 revealed supersaturation more than 70% of the time, suggesting that the lake is autotrophic and, thus, is a sink for atmospheric carbon dioxide.