Internal phosphorus loading contributions from deposited and resuspended sediment to the Lake of the Woods

ABSTRACT

James WF. 2017. Internal phosphorus loading contributions from deposited and resuspended sediment to the Lake of the Woods. Lake Reserv Manage. 33:347–359.

The Lake of the Woods exhibits cyanobacterial blooms despite substantial declines in watershed phosphorus (P) loading since the 1970s, suggesting that internal P loads are contributing to the P budget. A better understanding of internal P loading contributions and dynamics is needed to refine management strategies and water quality goals. Sediment cores were collected from several stations within the US portion to examine potential internal P contributions from deposited and resuspended sediment. Although laboratory-derived diffusive P fluxes were highest under anaerobic conditions (8−12 mg/m² d), aerobic conditions at the sediment–water interface may regulate in situ diffusive P fluxes due to frequent polymixis. Although much lower under aerobic conditions, modest fluxes of 0.2−0.6 mg/m² d could play an important role in the P budget. Simulated resuspension indicated that the critical shear stress of deposited sediment was relatively low at 1.3−2.3 dynes/cm². The wind-exposed long fetches and shallow morphometry of regions of Lake of the Woods could result in frequent resuspension of P-enriched clays and silts that contribute inorganic P to the water column. Resuspended sediments exhibited a low equilibrium P concentration (near zero) and high linear adsorption coefficient (700−3200 L/kg), suggesting they may act more as a sink rather than a source for soluble P. Resuspension could compete with cyanobacterial uptake by sequestering soluble reactive P. Resuspended inorganic P could also constitute a significant portion of the water column total P composition in addition to organic P, incorporated as cyanobacterial biomass, and needs to be considered in the P budget and chlorophyll prediction.

KEYWORDS:

• Sediment P release
• phosphorus equilibrium concentration
• Lake of the Woods
• internal P loading

Acknowledgments

I gratefully acknowledge N. Baratono and C. Hernandez of the Minnesota Pollution Control Agency for providing field support for sediment core collection on very windy days in September. I also thank A. Carlson (Engineer Research and Development Center - Eau Galle Aquatic Ecology Laboratory) for analytical support and G. Nürnberg and other anonymous reviewers for comments that greatly improved this manuscript.

Funding

Funding was provided by the Minnesota Pollution Control Agency.