Historical phosphorus dynamics in Lake of the Woods (USA–Canada) — does legacy phosphorus still affect the southern basin?

ABSTRACT

Edlund MB, Schottler SP, Reavie ED, Engstrom DR, Baratono NG, Leavitt PR, Heathcote AJ, Wilson B, Paterson AM. 2017. Historical phosphorus dynamics in Lake of the Woods (USA–Canada) — does legacy phosphorus still affect the southern basin? Lake Reserv Manage. 33:386–402.

A historical phosphorus (P) budget was constructed for southern Lake of the Woods. Sediment cores from 7 bays were radioisotopically dated and analyzed for loss-on-ignition, P, Si, diatoms, and pigments. Geochemical records for cores were combined using focusing factors for whole-basin estimates of sediment, total P, and P fraction accumulation. Although historical monitoring shows that external P loads decreased since the 1950s, sediment P has continued to increase since the mid-20^th century. Much sediment P is labile and may be mobile within the sediments and/or available for internal loading and resuspension. Two mass-balance models were used to explore historical P loading scenarios and in-lake dynamics, a static one-box model and a dynamic multi-box model. The one-box model predicts presettlement external loads were slightly less than modern loads. The dynamic model shows that water-column P was higher in the 1950s–1970s than today, that the lake is sensitive to external loads because P losses from burial and outflow are high, and that the lake is moving to a new steady state with respect to water-column P and size of the active sediment P pool. The active sediment pool built up in the mid-20^th century has been depleted through outflow and burial, such that its legacy effects are now minimal. Comparison of historical nutrient dynamics and sediment records of algal production showed a counterintuitive increase in production after external P loads decreased, suggesting other drivers may now regulate modern limnoecology, including seasonality of P loading, shifting nutrient limitation, and climate warming.

KEYWORDS:

• Cyanobacteria
• internal phosphorus loading
• large lakes
• paleolimnology
• shallow lakes

Acknowledgments

We thank the Minnesota Pollution Control Agency (MPCA; Contract 41642) and the Lake of the Woods Sustainability Foundation for providing funding for this project. Fieldwork was supported by the MPCA, the Sportsman's Lodge, and Captain Randy Beebe from WolfsHead Research Logistics aboard the R/V Arctic Fox. Devin Hougardy and Aaron DeRusha provided field assistance. Kathryn Hargan (PEARL, Queens University), Geoff Kramer (RESPEC), and Jesse Anderson (MPCA) provided historical loading data, Brittany Store (MPCA) provided GIS data, Matt DeWolfe (lwcb.ca) provided outflow data for the Winnipeg River. Norman Andresen (Andresen LLC, Ypsilanti, MI) analyzed diatoms in cores. Erin Mittag, Michele Natarajan, Erin Mortenson, and Alaina Fedie of the St. Croix Watershed Research Station coordinated lab analyses. Reviewers provided valuable comments that significantly improved the project.