Invasive bighead carp (Hypophthalmichthys nobilis) and silver carp (H. molitrix), hereafter, bigheaded carp, pose a major threat to the Great Lakes ecosystem as they advance toward Lake Michigan via the Illinois River. However, a series of navigation dams may deter their upstream movement. Starved Rock Lock and Dam (SRLD) is the most downstream gated dam on the Illinois River, therefore presenting the first navigation challenge for upstream migrating bigheaded carp. Before 2015, five acoustic receivers near SRLD detected successful upstream passage of only two of 900 tagged bigheaded carp. I further investigated the permeability of SRLD to bigheaded carp migration as a function of temperature, gage height, and dam operation. In 2015, I added 12 receivers around SRLD and acoustically tagged an additional 118 bigheaded carp downstream of SRLD. I also investigated the timing of bigheaded carp arriving downstream of SRLD using a generalized linear model. The explanatory variables influencing bigheaded carp arrival were average water temperature, average tailwater elevation, and the change in average water temperature. During 2015-2016, acoustic receivers recorded a total of 11 upstream passage events through SRLD, with nine through the dam gates, one through the lock chamber, and one undetermined. Passage through the dam gates occurred most frequently at high water levels when the dam gates were completely out of the water. The probability of upstream migrating bigheaded carp approaching SRLD was positively correlated with rising temperature and high gage. No upstream migrants approached SRLD between mid-September and late March. Overall, dam gates were more susceptible to upstream passage than the lock chamber, and environmental factors accurately predicted the arrival of bigheaded carp at SRLD. Modifying gate use during times of bigheaded carp arrival will likely prevent upstream passage through SRLD and other lock and dam structures.

Lateral habitats provide a multitude of benefits to riverine fishes, including invasive Silver Carp (Hypophthalmicthys molitrix) and Bighead Carp (H. nobilis), hereafter bigheaded carp. Harvesters have focused removal efforts in lateral habitats (e.g., backwaters and side channels); however, little research has examined the lateral habitat use of bighead carps. The Starved Rock Pool (SRP) is the downstream most pool in the upper Illinois River where contracted commercial fishermen target bigheaded carp to reduce dispersal pressure towards the Laurentian Great Lakes. To examine bigheaded carps’ movement between the main channel and lateral habitats in SRP, fish were implanted with transmitters and tracked using acoustic telemetry. The ranges and detection probabilities of acoustic telemetry receivers have typically been modelled and examined in with linear distance tools. To derive more realistic receiver ranges and detection probabilities, this study used minimum bounding geometry on detected transmissions obtained from boat-mounted transmitters. Receiver detection ranges estimated using minimum bounding geometry were smaller than those estimated using the linear distance method, but estimated detection probabilities within receiver ranges were higher using the minimum bounding method compared to the linear distance method. Detection histories of bigheaded carp implanted with transmitters were examined to assess fish habitat use from June 2016 to April 2018. During 2017, multiple environmental variables (temperature, river discharge, chlorophyll a, dissolved oxygen, total dissolved solids, and turbidity) were measured weekly and zooplankton samples were collected during June and August to assess potential associations between environmental variables and bigheaded carp habitat use that might be useful for informing locations and times for focusing contracted harvest of bigheaded carps. Habitat selection was also examined using a resource selection index (W) and a mark-recapture multistate model in program MARK. Across all seasons, bigheaded carp used lateral habitats more frequently than main channel habitats. Habitat use was strongly influenced by temperature and marginally by main channel discharge. No strong associations between zooplankton and bigheaded carp habitat use were observed during this study; however, there were some differences in zooplankton community structure and abundance among lateral habitats related to rotifers that may have potentially been related to use of specific lateral habitats by bigheaded carp. While having high return percentages to all lateral habitats, bigheaded carp disproportionately selected for a few individual habitats, possibly due to those habitats being locations where tagging was conducted. Harvest efforts in lateral habitats, especially prior to spawning (spring staging), during lower temperature periods (overwintering), and during higher discharges should be most effective because of bigheaded carps’ particularly high use of lateral habitats during these times. Recurring removal efforts in lateral habitats within SRP may facilitate targeting of individuals that frequently return to these lateral habitats and may also reduce bigheaded carp abundance in nearby locations.

Trophic interactions within aquatic ecosystems are complex, with many different pathways facilitating transfer of energy and nutrients among trophic levels and many different mechanisms that influence energy and nutrient transfer. This is illustrated in the “top down” and “bottom up” regulatory effects on aquatic food webs, through which primary producer biomass and, therefore, herbivore and carnivore densities, are influenced by both nutrient availability (bottom up) and densities of consumers at higher trophic levels (top down). In an aquatic food web, planktivore presence can directly alter zooplankton density via consumption, while indirectly shaping phytoplankton biomass via reduced herbivore abundance and the release of nutrients due to excretion, egestion, and decomposition. Novel species introduced into an established food web may have important consequences. An invasive species may impact an invaded food web through competition, predation, alteration of nutrient cycling, or, potentially, through facilitation of native species or other invasives. For example, an invasive planktivore may shift zooplankton density or community composition, thereby facilitating phytoplankton blooms. Such a planktivore may also compete with and, potentially, replace native species. Moreover, an invasive species that reaches high densities within its invaded range may serve as an important nutrient sink as it consumes a high biomass of native species or a nutrient source via excretion or decomposition. Two such invasive species with the capacity to dramatically alter native food web dynamics are bighead (Hypophthalmichthys nobilis) and silver carp (H. molitrix; collectively, bigheaded carp). Bigheaded carp are large-bodied, planktivorous fishes that were introduced into the United States in the 1970s and have since spread throughout much of the Mississippi River and its tributaries. These species currently threaten the Great Lakes, where they may constitute a threat to native planktivores such as gizzard shad (Dorosoma cepedianum) and commercially important species such as walleye (Sander vitreus), although there remains a great deal of uncertainty surrounding their potential ecosystem impacts. Consumption of both zooplankton and phytoplankton has been observed in bigheaded carp, although their impact on primary producer biomass is not well understood. Although field observations suggest that condition and abundance of native planktivores, including gizzard shad and bigmouth buffalo (Ictiobus cyprinellus), as well as zooplankton density, have declined following the bigheaded carp invasion, there is little direct, experimental evidence of bigheaded carp food web impacts. Therefore, I sought to examine the effects of bigheaded carp on native ecosystems through a series of mesocosm experiments at the Southern Illinois University pond facility. My primary objectives were to 1) observe potential competition between bigheaded carp and the native gizzard shad, 2) evaluate effects of bigheaded carp predation on zooplankton and phytoplankton communities, 3) assess impacts of bigheaded carp decomposition on nitrogen and phosphorus availability, and 4) measure the rate at which bigheaded carp excrete nitrogen and phosphorus. In order to elucidate the impacts of bigheaded carp on gizzard shad growth and survival, zooplankton and phytoplankton densities, and nitrogen and phosphorus availability in the pelagic and benthic pools and to determine whether gizzard shad experience a diet shift in response to bigheaded carp presence, I performed two mesocosm experiments with three treatments: gizzard shad only, gizzard shad, bigheaded carp, and fishless control (Chapter 1). I predicted that bigheaded carp would reduce zooplankton densities but that gizzard shad, which are both detritivorous and planktivorous, would be unaffected due to their ability to use detritus as an alternative food source. Additionally, both predator release via zooplankton consumption and increased nutrient availability from bigheaded carp excretion would stimulate phytoplankton. I found that gizzard shad survival was reduced by bigheaded carp presence but that surviving gizzard shad did not experience a decline in growth in the bigheaded carp plus gizzard shad treatments. This may have been due to the ability of gizzard shad to consume detritus, as foreguts of sampled gizzard shad in Experiment 2 contained mostly detritus. Moreover, phytoplankton density declined in the presence of silver carp in Experiment 2, suggesting silver carp herbivory. In addition, nitrogen and phosphorus availability in either the pelagic or benthic pools did not appear to be impacted by bigheaded carp presence. After demonstrating experimentally the overall negative impact of bigheaded planktivory on native food webs, I focused my remaining two chapters on the effects of silver carp on nutrient availability. In Chapter 2, I outline a decomposition experiment testing for potential changes in pelagic and benthic nitrogen and phosphorus availability and, in turn, phytoplankton, zooplankton, and macroinvertebrate densities in response to silver carp decomposition. Although silver carp die offs have been reported throughout the Midwest, little is known about the magnitude of those die offs and their consequences for the ecosystem. In this study, silver carp decomposition did not appear to alter nutrient availability or densities of phytoplankton or invertebrates. However, in comparison to northern streams in which salmon spawning and decomposition provide an important nutrient subsidy, the mesocosms used in this study have relatively higher background nutrient concentrations. Thus, silver carp decomposition, at least at the densities studied, may have little importance to in-stream nutrient availability. Lastly, because I am interested in how bigheaded carp, particularly silver carp, alter nutrient dynamics in invaded food webs, it is necessary to calculate silver carp nitrogen and phosphorus excretion rates, as well as body nitrogen and phosphorus content (Chapter 3). Nutrient stoichiometry theory predicts a balance between the relative consumption of nutrients by an organism and the extent to which the organism retains nutrients in its tissues or excretes them. Thus, it is a useful tool in determining how an invasive species may alter nutrient availability via consumption and excretion. In Chapter 3, I describe the body and excretion N:P ratios for silver carp, which exhibit a lower body N:P ratio than excretion N:P, suggesting that these organisms may serve as a sink for phosphorus. Moreover, silver carp body excretion N:P ratios were higher than those reported for gizzard shad, suggesting that, in regions where silver carp may replace gizzard shad or lower gizzard shad population density via competition (Chapter 1), silver carp may alter nutrient cycling processes in aquatic ecosystems by shifting the overall available N:P ratio. Bigheaded carp may pose a significant threat to invaded ecosystems through their potential to compete with native species, reduce plankton densities, and alter nutrient availability. However, although bigheaded carp are expanding in range and approaching the Great Lakes, the full extent of their ecosystem impacts remain uncertain. Through my work on bigheaded carp food web impacts, particularly the influence of silver carp on native species and nutrient cycling processes, I have found that bigheaded carp have the capacity to negatively impact invaded ecosystems overall by reducing zooplankton, phytoplankton, and forage fish densities. Moreover, as bigheaded carp in particular continue to reach high densities as they expand in range, their capacity to alter relative nitrogen and phosphorus availabilities must be monitored to understand the extent of their influence. Due to their ability to disrupt top down and bottom up processes in freshwater ecosystems, bigheaded carp constitute a critical environmental issue in the Great Lakes area and throughout the Midwest and, thus, it is imperative to continue to experimentally assess how bigheaded carp interact with native species to the detriment or benefit of U.S. freshwater communities.

This research investigated hydrologic connectivity, the intricate network of water pathways linking waterbodies, and its implications for biodiversity exchange in floodplains. Chapter 1 provides an exhaustive literature review encompassing factors influencing hydrologic connectivity, assessment approaches, scales, challenges, and management tools. Existing research often focuses on single scales and short-term periods, revealing a need for comprehensive multi-scale and extended temporal analyses. The absence of standardized definitions and methodologies in this field is also considered. Chapter 2 presents an innovative approach quantifying eight key connectivity metrics using remote sensing and GIS within the Lower Mississippi Alluvial Valley (LMAV). This adaptable method assesses connectivity between oxbow lakes and varying stream sizes, revealing spatial variability within the LMAV and enhancing scientific understanding of connectivity dynamics while ensuring portability. This research is crucial for effective ecosystem management and targeted conservation efforts, particularly regarding invasive species like the bigheaded carps (Hypophthalmichthys spp.).