Linking species to ecosystems: Effects of spawning salmon on aquatic ecosystem function in Bristol Bay, Alaska

The loss of biodiversity world-wide is occurring at a rate unprecedented in recent Earth history. With the loss of species there is concern that many of the ecosystem goods and services provided to human societies by natural systems will deteriorate. Research into the connections between biodiversity and ecosystem functions is necessary to potentially predict and avoid dramatic degradation of important ecosystem services. In this dissertation, I describe the role of spawning anadromous salmon in controlling ecosystem function with implications for conservation and resource management.;Chapter 2 demonstrates that brown bears feeding on salmon create localized hotspots of biogeochemical activity and that metrics of nitrogen cycling are higher during the salmon run. These effects are limited to small areas where bears actively feed on salmon and persist no more than 11 months. Chapter 3 presents a Bayesian statistical model of aquatic ecosystem metabolism that expands upon traditional open-system measurements using diel oxygen curves. This method allows for the direct estimation of key ecosystem metabolic parameters and incorporates uncertainty of unknown parameters into metabolism results. Chapter 4 uses diel oxygen data and the Bayesian model to consider the relative importance of salmon nutrients versus disturbance in controlling aquatic ecosystem metabolism. My results show that net ecosystem metabolism switched from roughly equivalent gross primary productivity (GPP) and community respiration (CR) to strong net heterotrophy (GPP << CR) in response to bioturbation of benthic habitats by salmon. Chapter 5 asks if the geomorphic complexity of streams regulates the salmon disturbance effect on benthic sediments. Changes in benthic chlorophyll density before versus after salmon in 18 streams that vary in the distribution of rock sizes indicated that salmon decrease chlorophyll on rocks greater than 65 mm in width less than rocks smaller than 65 mm. Because streams in this region are dominated by smaller rocks the overall net effect of salmon is disturbance.;Together the results presented in this dissertation demonstrate how salmon have extremely large impacts on ecosystem functions. The decline in salmon populations throughout much of their range is likely to impact ecosystem function in ways science is only beginning to understand.