Two rivers, two lakes, two legacies: Anthropogenic alterations to silica cycling and heavy metal loading in Lake St. Croix and Lake Pepin, USA.

摘要

Rivers are the vital connection between land and water, between continents and oceans. Water that falls on land infiltrates the ground or flows across the surface, touching everything on land and picking up dissolved minerals, organic matter, sediment particles and many other substances. River water chemistry reflects the watershed, so if rivers could speak, they could tell us a long history of the earth's surface, of environmental change, and of the rise of human civilizations and how they have impacted water quality. But rivers cannot speak and, unfortunately, rigorous water quality monitoring only began late in the twentieth century. In the first of these studies, 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. Historical rates of silica sequestration in each lake were determined using ASi burial in multiple sediment cores and modeled estimates of historical silica fluxes. The silica trapping efficiency of each lake was found to have increased exponentially with cultural eutrophication (2-5x in Lake St. Croix and 9-16x in Lake Pepin). Also, concentrations of Hg, Pb, Ag, Cd, Cr and Zn were measured in Lake St. Croix sediment cores and converted to whole-lake accumulation rates at 10- or 20-year intervals. Metal fluxes to that lake increased from 3x (Cr) to 18x (Pb) between Euro-American settlement and 2000. Point source discharges from the lakeside city of Stillwater were largely responsible for fluxes of Ag, Cd, Cr and Zn to the lake, as demonstrated by the enrichment of those metals in sediment cores downstream of the city. In contrast, regional atmospheric emissions were the dominant sources of Hg and Pb to the lake. The metal fluxes in Lake St. Croix are 10-50x lower than those in Lake Pepin. Together, Lake St. Croix and Lake Pepin represent a unique historical record of contrasting levels of human impacts on large river systems.