Linking watershed alder coverage to lake nutrient availability: Biogeochemical records from southwestern Alaska

Background/Question/Methods

Understanding the directions and impacts of nutrient fluxes across ecosystem boundaries is a central goal of ecosystem studies.  Nitrogen (N)-fixing alder can 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 lake nutrients and productivity because of the prevalence of alder thickets on the landscape.  To assess the effects of Holocene alder expansion, we analyzed lake sediments for pollen, biogenic silica (BSi), organic C and N content, d¹⁵N and d¹³C .  Furthermore, we conducted complementary research to investigate how variation in the coverage of alder (Alnus viridis subspecies sinuata and crispa) affects the chemistry and productivity of modern lakes. Alder coverage within the watershed of each lake was estimated from digital images of vegetation. Lake-water samples 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

Pollen records provide evidence for the rapid and widespread range expansion of Alnus around 8000 years before present.  Coincident with this vegetational change, the N content and d¹⁵N of sediments increase substantially, suggesting enhanced N availability and accelerated N cycling.  BSi also increases from ~5 to >100 mg/g with the arrival of Alnus, reflecting increased diatom productivity as a result of Alnus N fixation in the watershed soils and the associated increase in N flux to lakes. The ultimate cause of these ecosystem changes was probably a regional increase in effective moisture.

On the modern landscape, 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 is above a threshold value of 20%.  Alder exerts strong impacts on the first trophic level and has cascading effects on related components of the aquatic food web. Covariance analysis indicates that our results agree with a conceptual model linking nutrient inputs from alder to various nutrients and components of the aquatic foodweb. Thus alder-derived nutrients are important for lake productivity and nutrient transfers among trophic levels of sub-arctic oligotrophic lakes.