PHOSPHORUS SOURCES AND LAKE PEPIN WATER QUALITY.  

William F. James 1, John W. Barko 1, 2, Harry L. Eakin 1, and D. Kent
Johnson 3.   1 USAE Waterways Experiment Station, Eau Galle Aquatic
Ecology Laboratory, Spring Valley, WI 54767; 2 USAE Waterways
Experiment Station, Environmental Laboratory, Vicksburg, MS
39180;  3 Metropolitan Council Environmental Services, Mears Park
Centre, 230 East Fifth Street, St. Paul, MN 55101.  

As one component of the 1994-1998 environmental studies of
phosphorus sponsored by the Metropolitan Council Environmental
Services (MCES) of St. Paul, MN, and with partial funding from
MCES, the U.S. Army Corps of Engineers (USAE) examined suspended
seston and phosphorus dynamics in the Upper Mississippi River
(UMR) and naturally-impounded Lake Pepin during a three-year
period (1994-96).  The study focused on the external and internal
phosphorus loading processes which may be contributing to
impaired water quality conditions in Lake Pepin during low-flow
periods.  The Minnesota River, located ÷60 miles upstream of Lake
Pepin, accounted for most of the annual and summer suspended
seston and total phosphorus loads.  The Metropolitan Wastewater
Treatment Plant (Metro Plant), located ÷50 miles upstream of Lake
Pepin, accounted for much of the annual soluble reactive
phosphorus (SRP) loading to the UMR.  Lake Pepin retained a
substantial portion of the summer suspended seston load, and was
also a sink for total phosphorus.  While total phosphorus
concentrations generally decreased in Lake Pepin from the
headwaters to the outflow, SRP exhibited a trend of increasing
concentration from headwaters to outflow, with net SRP export
during the summer.  This spatial SRP trend suggests the
occurrence of internal phosphorus loading from lake sediments,
and/or phosphorus transformations in the water column from
particulate to soluble phases.  Internal diffusive phosphorus
flux from profundal sediments, estimated in laboratory incubation
systems under different temperature and redox conditions,
averaged ÷7.5 mg m-2 d-1 in the summer, with predominately oxic
conditions during all three years.  Internal sediment phosphorus
flux accounted for 30 to 56% of the net SRP export from  the lake
during the summer.  Recently-deposited sediments in Lake Pepin
also had a relatively high equilibrium phosphorus concentration
(EPC) and native adsorbed phosphorus pool at equilibrium.  During
the summer, SRP concentrations in flows entering Lake Pepin were
lower than the EPC due to dilution effects by the St. Croix
River, located 10 miles upstream of the lake.  This indicates a
strong potintial for phosphorus desorption from suspended seston
entering the lake.  When converted to a rate of internal
phosphorus loading, estimated phosphorus desorption from
suspended seston ranged from 2.3 to 3.7 mg m-2 d-1.  When
incorporated into the overall phosphorus budget for Lake Pepin,
phosphorus desorption from suspended seston potentially accounted
for 25% to nearly 40% of the measured internal phosphorus load
(i.e., diffusive plus kinetic) to the lake.  As such, this may be
an important internal flux that is not commonly included in the
phosphorus budgets of lakes.  
Keywords:  Upper Mississippi River, Lake Pepin, external
phosphorus loading, internal phosphorus loading