SILICA IN THE ST. CROIX AND BEYOND

Laura D. Triplett, University of Minnesota Department of Geology and Geophysics
Daniel R. Engstrom, St. Croix Watershed Research Station
Scott Schellhaass, Metropolitan Council Environmental Services

Silica is a common element in rock and soil minerals, so when those minerals weather (break down) dissolved silica is released into surface or ground water. Silica is also a critical nutrient for diatoms. While dissolved silica is plentiful in rivers and some lakes, it is a limiting nutrient for diatom growth in the oceans as well as for other siliceous marine organisms like radiolarians and sponges. Recent research has shown that the construction of dams on large rivers around the world has had the effect of trapping diatoms in reservoir sediments and preventing them – and their silica – from reaching the ocean (Tréguer et al. 1995; Conley 1997; Humborg et al. 2002; Conley 2003; Humborg et al. 2006). Eutrophication of those reservoirs drives increased diatom growth and thus even higher rates of silica sequestration on the continents. Parts of the Mediterranean Ocean (near the mouth of the Nile River) and the Black Sea (near the mouth of the Danube River) now receive less silica than they did before dams were built, resulting in a shift from diatoms to other types of algae. That change has had a cascade effect on local ecosystems, including deleterious effects on long-standing fisheries.

Sediments from Lake St. Croix and Lake Pepin – two large natural riverine lakes – provide a unique long-term perspective on this global change. Using sediment cores and modern water quality data, we show that as eutrophication progressed in each lake during the twentieth century the lakes’ silica trapping efficiency increased: Lake St. Croix’s by a factor of 5 and Lake Pepin’s by a factor of 10. In addition, a comparison of the modern silica mass balances in the two lakes shows that Lake Pepin is about twice as productive (areally) as Lake St. Croix in terms of diatoms. Furthermore, the rivers themselves are very productive; the St. Croix River supplies as much biogenic silica to Lake St. Croix as is produced in the lake itself and the Mississippi River carries twice as much biogenic silica into Lake Pepin as is produced in that lake.

References

Conley, D. J. 1997. Riverine contribution of biogenic silica to the oceanic silica budget. Limnology and Oceanography 42: 774-7.

Conley, D. J. 2003. Terrestrial ecosystems and the global biogeochemical silica cycle. Global Biogeochemical Cycles 16(4): 61-1 - 68-8.

Humborg, C., S. Blomqvist, E. Avsan, Y. Bergensund and E. Smedberg. 2002. Hydrological alterations with river damming in northern Sweden: Implications for weathering and river biogeochemistry. Global Biogeochemical Cycles 16(3): 12-1 - 12-12.

Humborg, C., M. Pastuszak, J. Aigars, H. Siegmund, C.-M. Morth and V. Ittekkot. 2006. Decreased silica land-sea fluxes through damming in the Baltic Sea catchment - significance of particle trapping and hydrological alterations. Biogeochemistry 77: 265-281.

Tréguer, P., D. M. Nelson, A. J. van Bennekom, D. J. DeMaster, A. Leynaert and B. Queguiner. 1995. The silica balance in the world ocean: A reestimate. Science 268: 375-379.