Past and present sockeye salmon productivity in the Kenai River Watershed, Alaska

Sockeye salmon are one of Alaska's most valuable natural resources, and are supported by a multitude of different habitats. Understanding changes in the relative productivity of different habitat types can inform our understanding of how habitat diversity contributes to ecosystem stability, which allows managers to anticipate future changes in abundance. The Kenai River's sockeye runs are supported by production from both glacial and clear-water nursery lakes, providing sharply contrasting rearing habitats in which productivity may respond differently despite shared climatic conditions. For example, warm summer air temperatures should positively influence zooplankton productivity in clear-water lakes, but negatively impact productivity in glacial lakes due to increased turbidity. Based on hypothesized differential responses to summer air temperatures, productivity regimes in glacial and clear-water nursery lakes would be asynchronous. However, decadal-scale reconstructions of historic productivity showed substantial synchrony between glacial and clear-water runs, presumably due to any differential effect of freshwater rearing conditions was later overridden by common conditions at sea. Over millennial time scales, delta15N in sediment cores from the Kenai River's primary clear-water nursery lake indicated that salmon populations have been relatively stable over the past ~3700 years, except for a 600-year period (50 BCE-550 CE) when salmon appear to have largely been absent. Sedimentary delta15N results were inconclusive in the glacial system, precluding comparison with clear-water productivity regimes over millennial time scales. The decline in salmon abundance observed in the clear-water lake provides additional evidence for a widespread and sustained low-productivity climatic regime in the North Pacific, as evidenced by concurrent declines in salmon populations >400 km away identified in previous paleolimnology work. Results from this thesis demonstrate that ocean conditions have the potential to synchronize the productivity of salmon populations across a range of spatial and temporal scales.