1.2.1. Ecological Impacts of Detectable Elements
Trace metals and other elements are natural and present in all ecosystems and water concentrations are influenced by geology, land use, erosion, ecology, and geochemistry. Ecological processes also affect concentrations as aquatic biota ingest and absorb metals. These organisms are affected in both beneficial and adverse ways by the concentrations of these elements. These trace elements typically have no visible indicators in surface waters, although at toxic or deficient levels can cause visual impacts on the ecology [
51]. The toxicity of trace elements is a particular concern for UL as it hosts a large migratory bird population along with the endemic and threatened June Sucker fish species. All metals are toxic at certain thresholds [
52] and impair the survival, reproduction, and behavior of aquatic life.
Some common toxic DE in lakes and reservoirs, including metals, found at concentrations which can cause detrimental impacts include arsenic (As), cadmium (Cd), chromium (Cr), lead (Pb), nickel (Ni), selenium (Se), copper (Cu), and zinc (Zn) [
51]. Even at non-toxic concentrations, As, Cd, and Pb, can affect mobility, feeding, and navigation behaviors of invertebrates [
53,
54,
55]. These effects cascade through aquatic ecosystems [
56,
57]. In larger organisms such as fish, Pb, Cd, Ni, and Cr can impact growth rates, biological processes, and reproductive health [
58,
59]. Impacts of aluminum (Al), Cd, Cu, Zn, Pb, Ni, As, and Se on birds include behavioral impairments and reduced reproductive success [
60,
61,
62].
Some metals are toxic at excess levels and also harmful when deficient. Zn, Cu, and Ni, negatively impact biological activity if they are present in excess levels as well as if they are missing or below toxic deficit concentrations [
41,
42,
43,
56,
57,
63]. Some elements, such as boron (B), iron (Fe), manganese (Mn), and molybdenum (Mo), are beneficial at certain concentrations and act as micronutrients that are essential for aquatic life. Micronutrients have been found to be limiting or colimiting factors for algal growth [
47,
64,
65,
66,
67]. In 2008, Downs
, et al. [
43] conducted a review of micronutrients in 56 freshwater lakes and found that “
the proportion of the lakes analyzed in which micronutrient limitation was found was 76% for molybdenum; 74% for iron; 67% for boron, 67% for cobalt, and 20% for copper.” Similar to the effects of toxicity, nutrient deficiencies for algae and phytoplankton affect the entire biota of lake systems, including invertebrates, migratory birds, fish, and other organisms [
56,
57].
The US Environmental Protection Agency (EPA) states that sources and activities “such as mines, smelters, firing ranges, municipal wastewater treatment outfalls, industrial point sources, urban runoff, landfills, and junkyards are potential sources for heavy metals and other harmful elements that can cause impairment” [
51]. Other sources include combustion of fossil fuels, phosphate fertilizers, metallo-pesticides, and road salts [
52,
68]. All of these sources are currently or have historically been present within the UL watershed. The Geneva Steel plant operated from 1944 to 2002 on the northeastern shore of UL. This plant produced steel using “coal-derived coke,” with effluent drains near the lake that served as a substantial source of metals to UL [
39,
69,
70]. Anthropogenic nutrient sources include seven wastewater treatment plants (WWTP) that discharge into the lake [
71] while the southern end of UL is surrounded by agricultural land, and the western side has active gravel mines. The population in the last 40 years has nearly tripled, increasing from ~220,000 in 1980 to ~640,000 in 2020 [
72].
In addition to the potentially toxic DE, ICP-OES measures several other elements that have important functions or impacts on aquatic ecosystems, including barium (Ba), calcium (Ca), potassium (K), sodium (Na), sulfur (S), B, silicon (Si) and P.
B and S are naturally present in aquatic environments and serve as essential nutrients. However, human activities can lead to elevated concentrations, potentially reaching toxic levels [
73,
74,
75]. Toxic S concentrations are typically rare in surface waters unless there is a direct source of pollution. B is also influenced by human activities, and the range between toxic deficit and excess B concentrations is relatively small compared to other nutrients [
76].
Ba occurs naturally and is also released by industrial processes primarily in the form of airborne particles that can eventually settle in nearby surface waters. UL is particularly susceptible to capturing atmospherically deposited dust due to its location and large surface area [
16,
17,
18,
20,
21]. Ba usually exists in a precipitated form in aqueous systems, with alkaline environments, such as UL, further limiting its solubility [
77].
K and P are essential nutrients for aquatic primary production. Since plants require less K than P, K is not usually a limiting nutrient. Neither element becomes toxic at the concentrations typically found in surface waters. Although high concentrations of P can lead to eutrophication, which negatively impacts water quality. Natural P and K [
78] sources include weathering of geologic formations. In the UL watershed, the Delle Phosphatic and Meade Peak geologic formations contribute large amounts of P to the lake [
1,
19,
79,
80].
Na is a key component of salinity and can severely degrade freshwater systems if present in high concentrations. It enters surface waters through natural weathering, but also from road salts [
81].
The impacts of Se are not well understood in freshwater systems due to the complexity of its geochemical interactions and cycling. However, it is toxic at very low concentrations and can degrade aquatic systems when it enters surface waters through anthropogenic activity [
82]. While Se is necessary for aquatic biota, there is a very narrow concentration window in which Se turns from beneficial to toxic [
83].
These discussions on the origins and impacts of DE on the UL ecosystem are not comprehensive. We included this review to demonstrate the potential impacts and functions of these DE in the UL ecosystem and illustrate the importance of understanding their ambient concentrations and behaviors.