Sedimentary legacy of sockeye salmon (Oncorhynchus nerka) and climate change in an ultra-oligotrophic, glacially-turbid British Columbia nursery lake

Anadromous Pacific salmon (Oncorhynchus spp.) populations are strongly regulated by climatic regimes and human activities across numerous spatial and temporal scales. The carcasses of adults returning to spawn provide important marine derived nutrients (MDN) to freshwater and terrestrial ecosystems through multiple trophic pathways. Sockeye salmon (O. nerka) rear extensively in lakes and recent studies of sockeye nursery lake sediments in Alaska have used indicators of spawner density (ä15N) and algal production (fossil pigments and diatoms) to show that lake trophic status is often regulated by climate and harvest via MDN from adult spawners. However, the strength of these controls and the utility of the paleolimnological techniques for measuring them are not well understood for sockeye nursery lakes in coastal British Columbia (BC). We examined relationships between climate, harvest, sockeye population dynamics, and lake trophic status from 1958 to 2005 using ä15N, ä13C, C:N, and fossil carotenoids in a radioisotope-dated sediment core from Kitlope Lake, BC. This ultra-oligotrophic, glacially-turbid nursery lake has a large (~872 km^2) pristine catchment and historically high, but currently depressed, sockeye returns. Climate and fisheries data supplemented with local and traditional knowledge (LTEK) indicated that sockeye escapements were regulated by both harvest and climate over the period of record. The sedimentary record of Kitlope Lake indicated extremely high sedimentation rates, significant inputs of terrestrial organic matter and periphytic diatoms from the main tributary, and the lowest ä15N yet measured in a sockeye nursery lake. Nevertheless, sedimentary ä15N, C/N, and fossil pigments were coherent with order-of-magnitude changes in sockeye populations prior to the mid-1970s, after which time escapements fell below management targets while air temperature remained significantly correlated with sedimentary proxies of lake trophic status. Proxy-inferred algal production fluctuated but generally increased throughout the sediment core, likely due to an increased growing season caused by a warming climate, especially in the past decade. Despite potentially elevated productivity due to climate warming, Kitlope Lake remains nutrient limited with a depressed sockeye salmon population. A substantial increase in annual sockeye escapement is required to alleviate nutrient limitation in this system and ensure that the abundant rearing habitat is fully utilized