Evaluating alder and salmon as drivers of nutrient availability and lake productivity in southwestern Alaska

Background/Question/Methods

Nitrogen (N)-fixing alder (Alnus spp.) and Pacific salmon (Oncorhynchus spp.) provide key nutrient subsidies to ecosystems in southwestern (SW) Alaska. This region supports the greatest salmon runs in the world. Alder is a prevalent constituent of the regional vegetation, and may contribute dissolved nutrients to adjacent aquatic systems. This importance is expected to increase as alder cover expands under climate warming, and salmon harvesting reduces marine-derived nutrient (MDN) concentrations in salmon-spawning habitats. We investigate the relative roles of salmon and alder in controlling nutrient dynamics and lake productivity in this region at 13 lakes and 26 streams in the Togiak National Wildlife Refuge (TNWR) in SW Alaska. Measurements of alder cover and watershed features were extracted from satellite images of the TNWR in ArcGIS. Resin-based lysimeters were installed in soils under alder-dominated and non-alder communities to assess nitrate (NO_x) and dissolved reactive phosphorus (DRP) leaching from alder. Water samples from lakes and streams were collected in the summer and fall in 2010 and 2011 for the analysis of a suite of nutrients, including dissolved N and P and chlorophyll a (Chl a). Lake sediment samples were analyzed for biogenic silica (BSi). Salmon-spawner density in each lake was estimated using nitrogen isotopes (d¹⁵N) in lake surface sediments and via aerial salmon counts.

Results/Conclusions

Soil-lysimeter samples reveal that alder patches leached approximately 25 kg NO_x  ha^-1 yr^-1 and 0.5 kg DRP ha^-1 yr^-1 more than non-alder patches (F=29.2, p=0.001 and F=4.70, p=0.06 respectively). Stream and lake N is positively correlated with alder cover across both seasons (NOx; r²=0.36-0.64, TDN; r²=0.25-0.36, p<0.01, n=56 and 26). Watershed elevation and alder cover explain 48% of the variation in stream N concentrations in the fall of 2011. In lakes, NH₄⁺ is significantly correlated with Chla concentrations in the fall (r²=0.76, p<0.05, n=26), suggesting NH₄⁺ as a key driver of lake productivity. BSi fluxes showed no relationship with alder in both years. Sediment d¹⁵N was positively related to TDP (r²=0.48, p=0.02, n=11), and aerial salmon counts were positively related to lake zooplankton biomass values in 2011 (r²=0.36, p<0.05, n=26), reflecting the impact of MDN on lake productivity. The lack of strong relationships of alder cover with lake productivity may reflect more complex nutrient cycling occurring in lakes compared to streams. Measures of the physical characteristics of each watershed need to be further integrated into the data analyses to assess the factors controlling variation in aquatic nutrients and productivity.