EFFECTS OF PHOSPHORUS LOADINGS ON THE WATER QUALITY  OF  THE  UPPER
MISSISSIPPI RIVER, LOCK AND DAM NO. 1 THROUGH LAKE PEPIN

Catherine E. Larson, D. Kent Johnson, Rececca J. Flood, Michael L. Meyer, Terrie J. 
O'Dean, and Scott M. Schellhaass.  
Metropolitan Council Environmental Services, Environmental Planning and Evaluation 
Department, 230 East Fifth Street, St. Paul, MN 55101-1633.

During the 1988 drought, low river flows and high nutrient levels led to excessive 
algal blooms in Lake Pepin (Pool 4), accompanied by surface scums, bad odors, low 
oxygen levels, and localized fish kills.  The Metropolitan Wastewater Treatment Plant 
(Metro Plant), owned and operated by the Metropolitan Council Environmental 
Services (MCES) is the largest point source of phosphorus upstream of Lake Pepin.  In 
1990-92 and 1994-1998, MCES and its partners conducted studies to determine the 
effect of phosphorus (P) loadings from the Metro Plant and other sources on the 
water quality of the Mississippi River, specifically algal blooms in Lake Pepin and 
Spring Lake (Pool 2).

The 1994-98 study had six major components.  The Science Museum of Minnesota 
examined sediment cores from Lake Pepin to estimate historical changes in sediment 
and P loadings and diatom communities.  The University of Minnesota studied 
historical trends in agricultural practices and wastewater discharges to relate human 
activities to loading changes.  MCES assessed a 21-year water-quality database to 
determine sources and patterns of sediment, P, and chlorophyll loadings.  The U.S. 
Army Engineers conducted limnological studies of Lake Pepin to analyze nutrient and 
seston fluxes.  The MN-WI Boundary Area Commission lead a volunteer monitoring 
program to evaluate water quality from the users' perspective.  HydroQual, Inc. 
developed an advanced model to project future water-quality conditions under various 
P management strategies.

Over the past 200 years, algal communities in Lake Pepin have changed from clear-
water benthic and mesotrophic planktonic taxa to mostly planktonic assemblages 
characteristic of highly eutrophic conditions.  The major factors are likely increased P 
concentrations and decreased light.  P loadings to the lake have increased more than 
five-fold to over 4000 mt/yr.  Wastewater discharges and fertilizer applications are 
the likely causes.  P concentrations in Lake Pepin have increased approximately four-
fold, from 50 to 200 ęg/L.  Sediment loadings have increased ten-fold for 850,000 
mt/yr, most likely due to increases in row-crop acreage.

Currently, nutrients are abundant in the Mississippi River, Lock & Dam 1 through Lake 
Pepin, and rarely decline to concentrations low enough to limit algal growth.  When 
physical and hydrological conditions are favorable, severe nuisance algal blooms occur 
especially in lower Pool 2 and Lake Pepin.  During periods of low river flows, point 
sources contribute the majority of P loads upstream of Lake Pepin (e.g., 89% in 
1988).  However, at high flows, nonpoint sources dominate P loads (e.g., 75% in 
1993).  At average flows, P loads are roughly split between point and nonpoint 
sources.  The Metro Plant contributed 20% of the TP and over 40% of the SRP 
during the past two decades.  Nonpoint-source loads were highest in the Minnesota 
River.  Only a small fraction of P is retained in Pools 2-4; most is flushed through and 
transported downstream.  During 1985-96, the overall P retention rate in Lake Pepin 
was 10%.  In all 12 years, Lake Pepin was a net sink of particulate P and a net 
source of SRP.  Internal SRP loading represented 10% of the total SRP loading to 
Lake Pepin during this 12-year period.  The fraction climbed to a third in low flow 
years and two-thirds in low flow summers.














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In a future low flow summer, water-quality conditions in Lake Pepin would improve 
somewhat under the P reduction strategies tested.  P concentrations would decrease 
dramatically but would remain high enough to support excessive algal growth.  Even 
with P removal to 1.0 mg/L at point sources and moderate controls at nonpoint 
sources, algal levels in Lake Pepin would remain excessive (i.e., viable chlorophyll a> 
30 ęg/L) over half of the summer.  The main benefit would be to reduce peak algal 
levels during low flow periods.  Biological P removal will be fully implemented at the 
Metro Plant by the end of 2003.  However, long-term improvements in water quality 
will only be achieved through basin-wide reductions in P loadings from point and 
nonpoint sources.

Keywords:  Mississippi River, Lake Pepin, phosphorus, algae, water quality
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