The effect of eutrophication on particulate amorphous silica (ASi) sequestration was isolated and quantified in Lake St. Croix and Lake Pepin, two natural, human-impacted impoundments of the upper Mississippi River. In contrast to impoundments behind engineered dams, where silica (Si) fluxes may be changed by various aspects of dam construction, these two riverine lakes have long (9,000+ years) sedimentary sequences that record the entire span of cultural eutrophication and the resulting silica sequestration. The concentrations of dissolved silicate (DSi) and ASi in the lake inflows were measured for 1 year to obtain the total flux of bioavailable silica (TSi b = DSi + ASi) to each impoundment. Historical rates of Si sequestration in each lake were determined using ASi burial in multiple sediment cores and modeled estimates of historical TSi b fluxes. The Si trapping efficiency of each lake was found to have increased exponentially with cultural eutrophication (estimated two- to fivefold increase in Lake St. Croix and 9- to 16-fold increase in Lake Pepin over the last 100 years), indicating the degree to which eutrophication of impoundments can reduce silica export to downstream coastal and marine ecosystems. Because these two lakes presently exhibit different degrees of eutrophication, together they depict a relationship between phosphorus concentration and Si trapping efficiency that may be applied to other impoundments, including human-made reservoirs.
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Acknowledgments We thank Scott Schellhaass of the Metropolitan Council Environmental Services (St. Paul, MN) for his assistance collecting samples. We thank the staff of the Limnological Research Center (LRC) at the University of Minnesota and Jill Coleman Wasik at the St. Croix Watershed Research Station for analytical assistance, and Amy Myrbo (LRC) whose comments improved this manuscript. We thank two anonymous reviewers for their thoughtful comments. LDT was supported by the Canon National Parks Science Scholars program. DJC was supported by the European Union, Marie Curie program and the Swedish Research Council (VR).
- Biogenic silica
- Mississippi River
- Riverine lake
- Trapping efficiency