Human activities have altered riverine silica cycling and diminished the supply of silica to the oceans, but few rivers have been intensively monitored to evaluate the magnitude of these changes. In this study we measured dissolved silica (DSi) and amorphous silica (ASi) fluxes into and out of two large, culturally-impacted natural impoundments of the upper Mississippi River, Lakes St. Croix and Pepin, USA. ASi sedimentation rates and sediment-water fluxes of DSi were calculated for each lake, and a mass-balance approach was used to determine in-lake ASi production. ASi from terrestrial phytoliths in the lake sediments was determined to be only partially available to biotic recycling, and in-lake ASi dissolution was small relative to the total silica budgets. The river reaches upstream of the two lakes were found to have abundant DSi, and riverine diatom production was found to contribute significant amounts of ASi to each lake. The average total phosphorus concentration in Lake Pepin is four times that in Lake St. Croix but ASi production in Lake Pepin is only 2.3 times higher than in Lake St. Croix, indicating that diatom growth in Pepin is limited by factors such as turbidity. Lake St. Croix currently traps about 10% of the inflowing total bioavailable silica (TSib = DSi + ASi) while Lake Pepin traps closer to 20% of its inflowing TSib, clearly demonstrating the importance of silica retention in lakes and reservoirs along the land-ocean continuum.
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Acknowledgements We thank staff of the Limnological Research Center (LRC) at the University of Minnesota (U of MN), Jill Coleman Wasik at the St. Croix Watershed Research Station (SCWRS) for analytical assistance, Mark Edlund (SCWRS) for helpful insights, Rob Lusteck (U of MN) for initial phytolith identification, Robert Burdis (Minnesota Department of Natural Resources) for sampling assistance, and Amy Myrbo (LRC), Tony Case and two anonymous reviewers whose comments greatly improved this manuscript. DJC was supported by the European Union, Marie Curie program. Major support was provided by the Canon National Parks Science Scholars program.
Copyright 2008 Elsevier B.V., All rights reserved.
- Biogenic silica
- Dissolved silicate
- Mississippi River
- Nutrient cycling
- Riverine lake