1. Introduction
Calcium (Ca) is an essential element in ecosystems at many levels of the food chain to maintain ecosystem health and sustain biodiversity. In general, calcium is abundant in rivers, lakes, and coastal waters. Calcium is a key structural component in building the inner and outer skeleton of invertebrates and vertebrate animals [
1,
2,
3,
4,
5,
6,
7]. Freshwater and marine mollusks depend on dissolved calcium for reproduction and growth [
8,
9,
10,
11]. Mollusk shells are predominantly composed of calcium carbonate. Many species of bivalve mollusks survive, reproduce, and maintain normal populations in rivers, lakes, and oceans only where calcium concentrations are sufficient. For many bivalve species, dissolved calcium concentrations in water can be a limiting factor for survival, growth, and reproduction. Although calcium concentration data are available for major rivers throughout the world [
12,
13,
14], they are still comparatively rare for numerous lakes and reservoirs [
13], The present study examined spatial and temporal changes in dissolved calcium that may have led to the establishment of a reproducing zebra mussel population in Kentucky Lake, the terminal and largest reservoir in the Tennessee River system.
The zebra mussel (
Dreissena polymorpha (Pallas); Dreissenidae, Bivalvia) is an exotic invasive aquatic mollusk that came to the United States from the Caspian Sea via ballast waters discharged into the Laurentian Great Lake System in 1985-1986 [
15,
16]. The mussel quickly colonized the Great Lakes, moving down the Mississippi River and up the Ohio River in the early 1990s, eventually arriving in the Tennessee River and several other freshwater ecosystems in the southeastern and mid-western United States [
16,
17,
18,
19]. By 1994, the following states had reported zebra mussels within their borders or in water bodies adjacent to their borders: Alabama, Arkansas, Illinois, Indiana, Iowa, Kentucky, Louisiana, Michigan, Minnesota, Mississippi, Missouri, New York, Ohio, Oklahoma, Pennsylvania, Tennessee, Utah, Vermont, West Virginia, and Wisconsin [
19].
Numerous studies have documented that zebra mussels can displace native aquatic species, disrupt energy flow through ecosystems, and negatively affect people’s quality of life. Zebra mussels hinder boating, swimming, fishing, and other recreational activities leading to ecological and economic impacts [
16,
20,
21]. Zebra mussels are notorious for their “biofouling” capabilities costing billions of dollars in their removal from industrial and power plant systems and public water supply lines [
16,
20,
21]. The adverse effects of dreissenid mussels on freshwater systems have led to their ranking as one of the world’s most invasive aquatic species [
22,
23]. Zebra mussels are now found abundantly in the lower Ohio River, but only sporadically in the Tennessee River. The Cumberland, Green, and Tennessee rivers (including Kentucky Lake) have the highest diversity of unionid mussel species in the world [
24,
25]; thus, zebra mussels have the potential for disrupting their populations [
26,
27].
Kentucky Lake (
Figure 1) was created by the impoundment of the Tennessee River by the Tennessee River Authority (TVA) in 1944. Kentucky Lake is mesotrophic to oligotrophic (based on N, P, and chlorophyll concentrations). It is a moderately turbid reservoir that drains north into the Ohio River. Kentucky Lake covers an area of about 65,000 ha. It is the last and the largest of a series of nine mainstem reservoirs on the Tennessee River and is the largest reservoir in the southeastern United States [
28,
29]. Kentucky Lake was created specifically for flood control, power generation, and commercial navigation. Surrounding communities also depend on Kentucky Lake for recreational activities, regional development and growth, and domestic and industrial water supplies [
30]. Tourism and sport fisheries are important contributors to the economy. The Tennessee River basin encompasses portions of seven southeastern states with a land use comprised primarily of row crop agriculture and smaller portions of second-growth forests. Population density is 19 people km
-2 concentrated primarily in Knoxville and Chattanooga, both of which are several hundred river km upriver from Kentucky Lake [
29,
30]. The Tennessee River empties into the Ohio River at Paducah, Kentucky, approximately 25 km downstream from the Kentucky Lake dam, thus the portion of Kentucky Lake actually in Kentucky is relatively small (
Figure 1).
Zebra mussels are commonly found in the lower Ohio River, with well-established colonies. While small numbers of primarily adult zebra mussels have been present in the Tennessee River and its impoundments since 1992, only one short-lived reproducing colony had been recorded in Kentucky Lake [
31]. The recent infestation in Kentucky Lake most likely began at least as early as 2016 based on mussel size distributions. Unfortunately, mussels were not observed until late 2017, Calcium levels in Kentucky Lake had not been routinely measured before 2018; therefore, we were limited to analyses during the 2018-2022 period when mussels were present and after the infestation subsided. Zebra mussels largely disappeared by the end of 2018, and by 2022, only occasional adults were found on monitoring equipment.
Understanding calcium patterns and potential causes may help predict the occurrence of this notorious biofouling exotic mollusk in the future. Previous studies had reported that mussels can survive in waters with calcium concentrations as low as 12 mg/L but that reproducing populations usually occur at 25-28 mg/L [
32], we were motivated to investigate calcium dynamics in Kentucky Lake waters focusing on spatial and temporal changes during and after zebra mussels were present.
We took advantage of an already existing Kentucky Lake Monitoring Program (KLMP) [
33] and added dissolved calcium sampling at several of the regular monitoring sites. Sites were monitored monthly from early 2018 through 2022 (
Figure 1C). Sites included the Kentucky Lake main channel (CH, CW, CB,), several tributary embayments (WL, 1N, UA), and two tributary streams (5L, PS, 2N). Two sites on the Ohio River (
Figure 1D) were added, one at the confluence of the Tennessee and the Ohio Rivers at Paducah, Kentucky (OH-P) and one in the Ohio River at Brookport, Illinois (OH-B) that represents primarily Ohio River water and known for zebra mussel infestation. The objectives of our study were to (i) determine spatial and temporal variations in dissolved calcium levels in Kentucky Lake and Ohio River waters during and after zebra mussel colonization (ii) evaluate whether calcium concentrations reached a threshold level that would allow for zebra mussels' growth and reproduction in the Kentucky Lake, and (iii) determine if we could identify potential sources of increased dissolved calcium concentrations.
4. Conclusions
A recent global assessment report by the Intergovernmental Platform on Biodiversity and Ecosystem Services (IPBES) has documented that the number of alien species is growing across the globe, leading to dramatic changes to biodiversity and ecosystems and costing the global economy US
$423 billion annually [
57]; and furthermore, number of established alien species projected to increase by 36% in the next three decades [58, 59]. In order to reduce economic and ecological impacts from aquatic alien species, studies are warranted across major taxonomic groups, geographic regions habitat types. Our study addressed the dissolved calcium profile in a man-made freshwater lake in the southeastern United States and its implications for one of the top-ranked invasive species, the zebra mussel, and its colonization. The results revealed several important findings with respect to spatial and temporal variations in calcium levels in streams, Kentucky Lake, and the Ohio River. Possible sources of this element to these water resources, as well as its relevance for the prevalence of zebra mussels in Kentucky Lake, were explored. At this time, we can only speculate on possible sources. The findings are as follows: (i) detectable concentrations of dissolved calcium were found in all selected sampling sites in tributary streams, Kentucky Lake, and Ohio River; (ii) results provided evidence that dissolved calcium levels varied with location and year(s) of sampling; (iii) tributary streams from rural (Ledbetter) sites had relatively lower calcium levels and did not serve as sources of calcium to the Kentucky Lake; (iv) the forested site (Panther Spring) consistently had the highest dissolved calcium levels, however, was not a significant source of calcium to Kentucky Lake because of low discharge rates. Future studies may be focused on the Panther Spring chemistry and geological makeup of this region as well as loading estimates of calcium into the Kentucky Lake, and (v) dissolved calcium in Kentucky Lake water exceeded the threshold level for establishing zebra mussel colonies of > 28 ppm in 2018. Calcium monitoring did not begin until 2018 when zebra mussel populations were already present in Kentucky Lake. It appears that zebra mussel reproduction could have begun as early as 2016 and that populations had gone undetected. Therefore, we can assume that calcium probably had exceeded the threshold at least by 2016.
The results of this study emphasize the value and need for future monitoring of Kentucky Lake and its watershed to detect any abnormal spikes in calcium as well as monitoring for zebra mussel populations. Kentucky Lake is an integral part of the landscape and economy of Western Kentucky and Tennessee. Understanding the calcium levels and their influence on zebra mussel’s biological cycles could provide useful information about their relationship within a man-made freshwater reservoir ecosystem such as Kentucky Lake and predict zebra mussel outbreaks.