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SFS Annual Meeting

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DO INTRODUCED FISH AND NUTRIENTS AFFECT THE PREVALENCE OF CYANOBACTERIA BLOOMS IN MOUNTAIN LAKES?

Mountain lakes are important freshwater resources due in part to their perceived pristine state, yet their isolation does not preclude them from anthropogenic disturbances such as fish stocking. Fish introductions can increase algal biomass and the frequency of algal blooms through changes in: 1) bottom-up forcing as fish increase rates of phosphorous recycling; and 2) top-down forcing as fish reduce the abundance of large-bodied grazers. The resilience of mountain lakes, like those in the Cascade Range, to fish stocking or other perturbations remains uncertain, especially in the context of varying landscape and climate. Variation in nutrients across this region may be a significant factor in phytoplankton growth. We analyzed existing monitoring data on 30 historically fishless lakes in the Oregon Cascades across a gradient of ultra-oligotrophic to eutrophic systems, either with or without stocked trout. Preliminary analysis suggests cladoceran abundances increase with decreasing fish stocking densities. Cladoceran grazers reduce the abundance of diatoms and in a potential competitive release, cyanobacteria levels were positively correlated with cladoceran abundance, possibly due to altered ratios of N:P availability via recycling by zooplankton. These results will guide upcoming field surveys and mesocosm experiments.

Lara Jansen (Primary Presenter/Author), Portland State University, ljansen@pdx.edu;


Angela Strecker (Co-Presenter/Co-Author), Portland State University, strecker@pdx.edu;