LAKE PEPIN SEDIMENT HISTORY.  

Daniel R. Engstrom and James E. Almendinger.   St. Croix
Watershed Research Station, Science Museum of Minnesota, Marine
on St. Croix, MN 55047.  

Long-term changes in sediment and phosphorus loading to the Upper
Mississippi River were quantified from an array of 25 sediment
cores from Lake Pepin (Pool 4), a large natural impoundment
downstream of the Minneapolis/St. Paul metropolitan area.  Cores
were dated and stratigraphically correlated using 210 Pb, 137 Cs, 14 C,
magnetic susceptibility, pollen analysis, and loss-on-ignition. 
All cores show a dramatic increase in sediment accumulation
beginning with European settlement in 1830.  Accumulation rates
are highest and show the greatest post-settlement increases in
the upper end of the lake.  Present-day sediment-phosphorus
concentrations are roughly twice those of pre-settlement times,
and the Fe/Al-bound fraction makes up a greater portion of the
total.  Diatom assemblages record a marked increase in nutrient
availability over the last 200 years, changing from clear-water
benthic forms and mesotrophic planktonic taxa in pre-settlement
times to exclusively planktonic assemblages characteristic of
highly eutrophic conditions today.  Lake-water total-phosphorus
concentrations (total-P), estimated by weighted averaging
regression and calibration, increased from 50 to 200 ęg/l during
this period.  Sediment loading to Lake Pepin from the Upper
Mississippi River has increased by an order of magnitude since
1830.  Modern fluxes are about 900,000 metric tons annually, and
are more than 80% detrital mineral matter.  About 17% of the
lake's volume in 1830 has been replaced by sediment, and at
current accumulation rates the remainder will be filled in
another 340 years.  Phosphorus accumulation in Lake Pepin
sediments has increased 15-fold since 1830, rising from 60 to 900
metric tons annually.  This rise represents a seven-fold increase
in phosphorus loading from the Mississippi River coupled with
more efficient retention of phosphorus inflows by bottom
sediments.  More efficient trapping of phosphorus in Lake Pepin
over the last century resulted from higher rates of sediment
burial.  The most dramatic changes in nutrient and sediment
inputs to Lake Pepin have occurred since 1940, although gradual
increases began shortly following European settlement.  Sediment
and phosphorus accumulation rates rose sharply between 1940 and
1970 and then leveled off, while lake-water total;-P
concentrations and inflows increased continuously up to the
present.  

Keywords:  Upper Mississippi River, Lake Pepin, paleolimnology,
phosphorus, sediment cores