Physical impacts of wind and boat traffic on Clear Lake, Iowa, USA

Clear Lake is a shallow (Z_mean=2.9 m), eutrophic (TP_mean=188 μgL⁻¹) lake that is intensively used forrecreation. After a century of intense agriculture in the watershed, the bottom is covered with nutrient-rich organic sediments. We monitored wind, boat traffic and turbidity and found that resuspension of this sediment by wind-induced waves and recreational boat traffic contributes to daily, often substantial, nutrient fluxes. Intensive monitoring over a wind-event showed that total phosphorus concentrations can increase by 100% over a diel period and ammonia concentrations increase to levels near to those toxic to fish at the peak of winds. GIS of the digitally analyzed lake basin coupled with physical models show that when wind speeds exceed 10 ms⁻¹ (22 mph), >46% of the lake's benthic surface area may become mobile. Wind speeds >20 ms⁻¹ (44 mph) can mobilize >98% of the lake bottom sediment surface area. The correlation between boat traffic and sediment resuspension was weak (r² = 0.23) in near-shore monitoring due to confounding by wind and other factors but heavy boat traffic appears to exacerbate wind-induced resuspension and may slow the resettlement of resuspended sedimente. Boat traffic correlated with up to 50% increases in turbidity. Resuspension of sediments by boats is likely to occur across 56% of the lake area. Benthic sediment resuspension may contribute to the suppression of fish and macrophyte communities, domination of the phytoplankton community by Cyanobacteria, suspension of toxic ammonia, and increased restoration time-scales.