Laura Triplett

| Geology

Biogeochemical cycling of Si in large polluted rivers

Human activities are likely altering how much silica is transported down rivers and delivered to oceans.  Following are the abstracts of two articles collaborators and I published about our work on silica cycling in the St. Croix and Mississippi Rivers.  Please contact me to get the full articles.

Triplett, L.D., Engstrom, D.R., and Conley, D.J. 2011.  Changes in amorphous silica sequestration with eutrophication of riverine impoundments.  Biogeochemistry, DOI 10.1007/s10533-011-9608-9

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 one 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 2- to 5-fold 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.

Triplett, L.D., Engstrom, D.R., Conley, D.J., Schellhaass, S.M.  2008.  Silica fluxes and trapping in two contrasting natural impoundments of the upper Mississippi River.  Biogeochemistry 87, p.217-230

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.