Understanding the directions and impacts of major nutrient fluxes that cross ecosystem boundaries is fundamentally important in ecological studies.  Nitrogen (N)-fixing alder can potentially increase the availability of N and phosphorus (P) in adjacent soils, streams, and lakes. Southwestern Alaska is ideal for studying the effects of alder on lakes because alder cover has been a prevalent constituent of terrestrial vegetation over the past 8000 years.  In addition, extensive baseline information on limnology, terrestrial vegetation, and landforms exists for many lakes and their watersheds within the Togiak National Wildlife Refuge (TNWR) in that region. Paleo-evidence revealed that alder invasion 8000 years ago markedly increased lake productivity, as inferred from changes in biogenic silica, C:N and N and C isotopic values in lake-sediment cores. We conducted complementary research to investigate how alder cover (Alnus viridis subspecies sinuata and crispa) affects modern lake chemistry and productivity of seven lakes in the TNWR. The percentage of alder cover within the watershed of each lake was estimated from digital images of vegetation. Water samples from the lakes were analyzed for a suite of nutrient and productivity indicators, including chlorophyll a (Chl a), zooplankton mass, bacterioplankton counts, dissolved N and P, phytoplankton C:N ratios, and d¹⁵N of phytoplankton.

Results/Conclusions

Alder cover is positively related to phytoplankton C:N ratios  (r² = 0.86), Chl a concentrations (r² = 0.74), total dissolved organic N (DON; r² = 0.34), and total dissolved phosphorus (TDP; r² = 0.56). DON and TDP are the most important nutrients driving aquatic productivity, as inferred from their highly significant correlations with Chl a concentrations and bacterioplankton counts. Enhanced nutrient availability occurs at sites where alder cover was above a threshold value of 20%.  This increase exerts strong impacts on the first trophic level (phytoplankton and bacterioplankton) and cascading effects on related components of the aquatic food web. Covariance analysis indicates that these results are consistent with a conceptual model linking nutrient inputs from alder to various dissolved nutrients and components of the aquatic foodweb. At the watershed scale, alder contributes much more N than spawning salmon in the Togiak drainage. Thus alder-derived nutrients are important for lake productivity and nutrient transfers among trophic levels of sub-arctic oligotrophic lakes.