EDLUND, MARK B.1*, LAURA TRIPLETT1,2, and DANIEL R. ENGSTROM1. 1St. Croix Watershed Research Station, Science Museum of Minnesota, 16910 152nd St. N., Marine on St. Croix, Minnesota, 55047, USA; 2Limnological Research Center, Department of Geology, University of Minnesota, Minneapolis, Minnesota, 55455. - Post-European sedimentation and nutrient loading in Lake St. Croix, a natural impoundment on the St. Croix River, USA.
The 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 twenty to fifty-fold since settlement
with a shift from benthic–dominated 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- to 3-fold since presettlement times. Presettlement
values of c. 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 (c. 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.
Key words: diatoms, ecology, land use, large rivers, nutrient loading, paleolimnology