Graduate Thesis Or Dissertation


Carp, Climate, and Water Quality: Applying Systems Modeling to Ecosystem Recovery in Malheur Lake Public Deposited

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  • Shallow lakes exist in either a clear or turbid state, with the clear state characterized by an abundance of aquatic macrophytes, diverse aquatic biota, low water column nutrients and phytoplankton biomass, whereas the turbid state is characterized by the opposite. These two distinct states are maintained by reinforcing (positive) feedback loops that pull the system towards one of the two states. I used a mechanistic modeling approach to better understand the drivers of the currently turbid state of Malheur Lake, a large shallow lake-wetland ecosystem in southeast Oregon. Currently this system exists in a turbid state, and there has been great interest in restoring the system to a clear state by influencing two agents suspected in causing turbidity: benthic foraging by non-native carp (Cyprinus carpio) and wind resuspension of lakebed sediments. I began by simulating the non-native carp population in Malheur Lake and focused on controlling carp via removal efforts aimed at suppressing carp biomass below the desired 50 kg/ha threshold. These simulations indicated that individual carp removal actions would likely fail due to compensatory density dependent responses (recruitment, mortality, growth) within the carp population. Simulations further demonstrated that combinations of two or all three active removal methods could reduce the biomass below the desired threshold, however the carp reduction rate would have to be maintained ≈40% at each life-stage, in perpetuity. Furthermore, adding environmental fluctuations into the carp population model ultimately demonstrated that the carp population in Malheur Lake is more affected by the interactions within the population brought on by environmental fluctuations than the human capacity to impose mortality rates via removal efforts. Ultimately these results demonstrated that focusing management actions solely on the reduction of carp would likely be ineffective, and thus investigations of other mechanisms helping to maintain the turbid state was necessary. Therefore, I shifted the modeling to investigate the deleterious effects of the wind and wave energy, with simulations ultimately demonstrating that the wind-wave energy is a major driver of the turbid state, and that restoration efforts in the form of wave reduction barriers may be used to decrease the suspended sediment concentrations and increase the water clarity. Collectively, simulation results reinforce the notion that future restoration actions in Malheur Lake must be more broadly focused (i.e. systems perspective) and guided by the principles of the alternative stable state theory.
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