Post‐European Sedimentation and Nutrient Loading in Lake St. Croix: A Natural Impoundment on the St. Croix River, USA

Abstract

AbstractThe final 37 km of the St. Croix River were naturally dammed by the Mississippi River to form Lake St. Croix approximately 9500 years ago. The St. Croix River is currently regarded as having ‘exceptional resource value’, as one of the least impacted large Midwest river systems. Twenty‐four 2‐m piston cores were recovered in 1999–2001 from Lake St. Croix sub‐basins to identify post‐European settlement signals of land use, trophic change, and sedimentation using a whole‐basin approach to reconstruct loading history of nutrients, sediments, heavy metals, and organics. Dating chronologies based on 210Pb inventories indicated both cores recovered a sediment sequence dating from pre‐ and post‐European settlement (c. 1850) in the St. Croix River basin. Select cores were subjected to magnetic susceptibility, loss‐on‐ignition, and diatom microfossil analysis. Sedimentary increases in magnetic susceptibility were indicative of increased erosion and transport of ferromagnetic mineral grains due to initiation of settlement, logging, and agricultural activities in the basin. A three‐fold increase in sediment accumulation began in the mid‐1800s in the northern basin and by 1900 in the southern basin. Diatom accumulation increased 20–50‐fold since settlement with a shift from benthic‐ to planktonic‐dominated assemblages. Simultaneous with the assemblage shift were the introduction and establishment of many planktonic diatoms considered ubiquitous indicators of eutrophy. The fossil diatom assemblages were further analyzed using weighted‐averaging calibration and reconstruction of historical water column total phosphorus (TP). Reconstructed TP values showed that water column nutrient values have increased 2.5–3‐fold since presettlement times. Presettlement values of about 0.02 mg/L TP were found in both cores with TP increases beginning c. 1910 and especially dramatic increases after World War II. Modern reconstructed TP values (about 0.055 mg/L) were similar to TP concentrations reported from monitoring during the last few decades; however, the river was clearly impacted well before monitoring efforts were begun.