Assessment of anthropogenic impacts on the trophic dynamics of Babine Lake: Implications for the production of sockeye salmon

Abstract

Babine Lake, British Columbia, is Canada’s largest sockeye salmon (Oncorhynchus nerka) nursery lake, sustaining numerous ecosystem services (e.g., fisheries, recreation, cultural and spiritual benefits). The lake and its watershed have experienced significant anthropogenic and natural disturbances since the early 1900’s, including extensive logging, pine beetle infestations, mining, fisheries enhancements, and climate change. To help understand the cumulative impacts of local and regional factors on the trophic ecology of this large freshwater ecosystem, we used the paleolimnological approach to reconstruct changes over the past ∼ 200 years of primary production (subfossil pigments, diatom microfossils), secondary production (cladoceran zooplankton microfossils) and geochemistry (carbon and nitrogen isotopes, elemental analyses, % organic matter). Between ca. 1950 and the 2000’s changes in the sedimentary proxies are consistent with an enhanced influx of nutrients and dissolved organic carbon likely due to the cumulative landscape disturbances and an increase in precipitation. These changes include shifts in diatom composition ca. 1950 to higher abundances and taxa indicative of decreased light penetration and nutrient enrichment. Increases in δ¹⁵N (∼ +1‰) and cladoceran concentrations, concurrent with increasing sockeye salmon escapement post ca. 1975, is indicative of a higher pelagic fish production that occurs concurrently with both fisheries enhancements and sustained higher precipitation. The largest and most abrupt change in algal pigments occurred post ca. 2000, characterized by increases in motile and buoyant algae (i.e., chrysophytes, cryptophytes, colonial cyanobacteria), a time of increasing regional air temperatures, reduced spring and autumn winds, and resultant changes in patterns of lake mixing.