Phosphorus and nitrogen deposition within a large transboundary watershed: Implications for nutrient stoichiometry and lake vs watershed budgets

Abstract

Atmospheric deposition is an important source of both phosphorus (P) and nitrogen (N) to lakes and their watersheds, but the two nutrients are rarely reported together. For the first time, we measured total P (TP) and N (total, ammonium, NH4-N and nitrate, NO3-N) bulk deposition to the Lake of the Woods (LoW) watershed, a large international lake that experiences frequent, lake-wide cyanobacterial blooms. Phosphorus deposition was highly variable both spatially and seasonally, with on average >75 % of annual deposition occurring in the spring and summer months, associated with local biogenic input. In contrast, winter TP deposition was relatively low and spatially invariant, and not affected by local sources. Importantly, these results suggest that P deposition may be a much larger source of input to the LoW as a result of its tortuous shoreline and abundance of forested islands (>5000), which are concentrated in the northern, Canadian waters and greatly extend the ‘shoreline influence’ within this lake. We suggest that a single annual TP load estimate is not appropriate for the LoW because winter deposition is likely much lower than past estimates whereas spring/summer deposition is substantially higher and could be an important, but previously unrecognized source, of bioavailable P to the nearshore waters. Further research is needed on the spatial and temporal patterns of P vs N deposition to the lake surface and their influence on primary productivity and algal species composition.