Nitrogen (N)-fixation by alder (Alnus spp.) and Pacific salmon (Oncorhynchus spp.) provide key nutrient subsidies to freshwater systems. In southwestern (SW) Alaska, alder is a prevalent constituent of the regional vegetation. This region also supports the greatest salmon densities in the world. The relative importance of alder-derived nutrients in the region is expected to increase because shrub cover is expanding in response to climate warming. Also, salmon declines from commercial harvest and habitat degradation are reducing marine-derived nutrients (MDN) in salmon-spawning habitats. We analyzed the impacts of gradients in alder cover and salmon abundance on lake chemistry, bacterial community structure, and microbial metabolism from 13 lakes in the Togiak National Wildlife Refuge, SW Alaska, from 2012-2013. We also conducted bioassay experiments to determine microbial nutrient limitation and factors modulating microbial response to nutrient inputs.
Results/Conclusions
Seasonal shifts in bacterial community composition (F = 7.47, P < 0.01) were related to changes in lake N and phosphorus (P) concentrations (r2 = 0.19 and 0.16, both P < 0.05), and putrescine degradation (r2 = 0.13, P = 0.06), suggesting the influx and microbial utilization of MDN. Temporal and spatial patterns in salmon abundance were also associated with increased microbial metabolism. In contrast, alder cover was not related to microbial metabolic activity, likely because alder-derived N provided less resource diversity than MDN. In response to nutrient additions, greater substrate utilization occurred among microbial communities from lakes with elevated Chl a concentrations (β estimates for +N, +P and +NP treatments = 0.78, 0.92, 0.81 respectively, all P < 0.07) and large relative catchment areas (β estimates for +N, +P and +NP treatments = 0.57, 0.54, 0.53 respectively, all P < 0.05) in the spring. These findings demonstrate that ongoing shifts in the quantities of alder- and salmon-derived nutrients are likely to affect aquatic microbial community structure and metabolic activity, but that ambient watershed and lake features will potentially mediate these responses to nutrient loading.