My research is a part of an experimental ecosystem manipulation of Crystal Lake, Vilas County, WI. Crystal Lake is an oligotrophic (nutrient poor), dimictic (fully mixes in spring and autumn), seepage lake (ground water inputs), with no surface water inlets or outlets. It is a small lake with a surface area of 37 ha and maximum depth of 20 m (66 ft). During summer months thermal stratification partitions the lake into a shallow warm epilimnion, and a deep cold hypolimnion. In the summers of 2012 and 2013 we warmed the hypolimnion of Crystal Lake to test the thermal limits of a coldwater invasive fish Rainbow Smelt (Osmerus mordax; Gaeta et al. 2012, Lawson et al. 2015). We warmed the hypolimnion by mixing heat down from the epilimnion with the novel GELI lake mixing technology (Gradual Entrainment Lake Inverter; Read et al. 2011). We hypothesized the smelt would be extirpated from Crystal Lake if we could raise hypolimnetic water temperatures above the fishes critical thermal limits, while the warm water tolerant native species (Yellow Perch; Perca flavescens) would be unaffected. As Crystal Lake's hypolimnetic temperatures warmed, drastic changes in the smelt's behavior were observed and elevated rates of mortality were measured. We estimate smelt mortality increased 40% above natural rates (90% total) during each year of the thermal manipulation. It’s unclear how a portion of the population was able to survive the thermal stress but a study is currently investigating this.
The thermal manipulation of Crystal Lake provided opportunity to investigate other research questions, two of which have become the focus of my research: (1) How does reduced water column stability and hypolimnetic warming affect lake ecosystem metabolism? (2) How does the efficiency and operational costs of the GELI compare to other lake mixing technologies?
Metabolism at the lake ecosystem scale is a measure of the total production and consumption of biologically available energy within an lakes boundaries. The metabolic balance of a lake provides fundamental insights into the energetics and biogeochemical processes of the lake. It is well known that the metabolic balance of a lake is dependent on multiple variables that are regulated by thermal stratification. It is however less well known how the metabolic balance of a lake is affected by modified stratification. By manipulating the timing, duration, and strength of thermal stratification our research team created conditions expected to alter the metabolic balance of Crystal Lake. I explore these potential shifts and contrast them to a nearby reference lake.
The natural process of thermal stratification can sometimes lead to problems in water quality. Artificial mixing to reduce stratification is a tool commonly used by lake and reservoir managers to improve water quality. Beyond water quality issues, artificial mixing has been demonstrated to be useful in the restoration of mine pit lakes, decreasing bioaccumulation of mercury, and increasing aquaculture yields. One problem with artificial mixing is the high operational costs that become substantial in long-term projects. To assess the efficiency and cost savings of the GELI lake mixing technology, I use hydrodynamic models to simulate the unmanipulated thermal regime of Crystal Lake as well as simulate the mixing of Crystal Lake with other technologies for comparison.