Historical Changes in Sediment and Phosphorus Loading to the Upper Mississippi River; Mass-Balance Reconstructions from the Sediments of Lake Pepin
by Daniel R. Engstrom and James E. Almendinger, St. Croix Watershed Research Station - Science Museum of Minnesota - Marine on St.Croix - Final Research Report April 2000
ABSTRACT Click here for entire research report.
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, a large natural impoundment downstream of the Twin Cities metropolitan area. Cores were dated and stratigraphically correlated using 210Pb, 137Cs, 14C, 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 presettlement 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 presettlement 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 µg l-1 during this period.
Historical Trends Affecting Accumulation
of Sediment and Phosphorus in Lake Pepin
by D.J. Mulla, A.S. Sekely, D. Wheeler, and J.C. Bell
Department of Soil, Water, and Climate University of Minnesota
Final Report April 2000
ABSTRACT Journal Article Available of entire research report.
This study was conducted to collect historical land use information that would help explain the historical patterns in accumulation of sediment and phosphorus in Lake Pepin. Engstrom and Almendinger (2000) found that since 1830, sediment accumulations in
Lake Pepin have increased more than ten fold, while phosphorus inflows to Lake Pepin have increased more than fifteen fold. A wide range of historical factors including cropping systems, phosphorus applications from fertilizer and manure, human and animal
populations, river flows, and phosphorus discharges from wastewater treatment plants were studied using statistical methods. The results showed that sediment losses from the Minnesota River Basin are significantly correlated with historical increases in row-crop production acreage, river flows, and basin population. Total phosphorus accumulations in the sediments of Lake Pepin are significantly correlated with increased phosphorus discharges from the metropolitan-area wastewater treatment plants, increases in row-crop acreage, and increases in river flows. Total phosphorus inflows to Lake Pepin are significantly correlated with increased river flows, increases in row-crop acreage, and increases in phosphorus fertilizer applied to agricultural lands.
At current rates of deposition, Lake Pepin will be filled in with sediment within about 300 years (Engstrom and Almendinger, 2000). Approximately 85% of this sediment arises in the Minnesota River Basin (Nater and Kelley, 1998). Sediment delivery ratios from water erosion of cultivated uplands and stream-bank erosion in the Minnesota, Upper Mississippi, and St. Croix River Basins to Lake Pepin are estimated to be less than 4%.
Historically, from 23-47% of the phosphorus entering Lake Pepin originates in effluent discharged from wastewater treatment plants. The corresponding amounts transported to Lake Pepin from fertilizer, manure, soil erosion, and other diffuse sources ranged from 53-77 %. Although this is the majority of the contribution, it is a small fraction (roughly 2%) of the 100,000 tones of fertilizer and manure phosphorus applied annually to agricultural lands in the Minnesota, Upper Mississippi and St. Croix River Basins. Even though this is a small percentage, it (along with phosphorus from wasteware treatment plant effluent and sediment- bound phosphorus accumulated Lake Pepin is enough to cause eutrophication in Lake Pepin, especially during dry years.
At the peak of phosphorus discharges from wastewater treatment plants in 1974, all wastewater treatment plants upstream of Lake Pepin discharged roughly 1,830 tons/yr of total phosphorus (47% of the phosphorus entering Lake Pepin). After the ban on phosphate detergents, this load decreased to about 1,225 tons/yr total phosphorus in 1992 (23% of the phosphorus entering Lake Pepin). Surprisingly, the large decrease in phosphorus loads from wastewater treatment plants after the ban on phosphate detergents was accompanied by a significant increase in phosphorus inflows into Lake Pepin. It is not clear why total phosphorus inflows to Lake Pepin have increased dramatically since 1974, since neither phosphorus fertilizer or manure applications increased significantly after 1974. It is possible that the decrease in phosphorus discharge from wastewater treatment plants due to the ban on phosphate detergents was offset by increased erosion and river flow from the Minnesota River Basin during the wetter climate of the 1980s and 1990s.
of Sediment and Phosphorus in Lake Pepin
by D.J. Mulla, A.S. Sekely, D. Wheeler, and J.C. Bell
Department of Soil, Water, and Climate University of Minnesota
Final Report April 2000
ABSTRACT Journal Article Available of entire research report.
This study was conducted to collect historical land use information that would help explain the historical patterns in accumulation of sediment and phosphorus in Lake Pepin. Engstrom and Almendinger (2000) found that since 1830, sediment accumulations in
Lake Pepin have increased more than ten fold, while phosphorus inflows to Lake Pepin have increased more than fifteen fold. A wide range of historical factors including cropping systems, phosphorus applications from fertilizer and manure, human and animal
populations, river flows, and phosphorus discharges from wastewater treatment plants were studied using statistical methods. The results showed that sediment losses from the Minnesota River Basin are significantly correlated with historical increases in row-crop production acreage, river flows, and basin population. Total phosphorus accumulations in the sediments of Lake Pepin are significantly correlated with increased phosphorus discharges from the metropolitan-area wastewater treatment plants, increases in row-crop acreage, and increases in river flows. Total phosphorus inflows to Lake Pepin are significantly correlated with increased river flows, increases in row-crop acreage, and increases in phosphorus fertilizer applied to agricultural lands.
At current rates of deposition, Lake Pepin will be filled in with sediment within about 300 years (Engstrom and Almendinger, 2000). Approximately 85% of this sediment arises in the Minnesota River Basin (Nater and Kelley, 1998). Sediment delivery ratios from water erosion of cultivated uplands and stream-bank erosion in the Minnesota, Upper Mississippi, and St. Croix River Basins to Lake Pepin are estimated to be less than 4%.
Historically, from 23-47% of the phosphorus entering Lake Pepin originates in effluent discharged from wastewater treatment plants. The corresponding amounts transported to Lake Pepin from fertilizer, manure, soil erosion, and other diffuse sources ranged from 53-77 %. Although this is the majority of the contribution, it is a small fraction (roughly 2%) of the 100,000 tones of fertilizer and manure phosphorus applied annually to agricultural lands in the Minnesota, Upper Mississippi and St. Croix River Basins. Even though this is a small percentage, it (along with phosphorus from wasteware treatment plant effluent and sediment- bound phosphorus accumulated Lake Pepin is enough to cause eutrophication in Lake Pepin, especially during dry years.
At the peak of phosphorus discharges from wastewater treatment plants in 1974, all wastewater treatment plants upstream of Lake Pepin discharged roughly 1,830 tons/yr of total phosphorus (47% of the phosphorus entering Lake Pepin). After the ban on phosphate detergents, this load decreased to about 1,225 tons/yr total phosphorus in 1992 (23% of the phosphorus entering Lake Pepin). Surprisingly, the large decrease in phosphorus loads from wastewater treatment plants after the ban on phosphate detergents was accompanied by a significant increase in phosphorus inflows into Lake Pepin. It is not clear why total phosphorus inflows to Lake Pepin have increased dramatically since 1974, since neither phosphorus fertilizer or manure applications increased significantly after 1974. It is possible that the decrease in phosphorus discharge from wastewater treatment plants due to the ban on phosphate detergents was offset by increased erosion and river flow from the Minnesota River Basin during the wetter climate of the 1980s and 1990s.
Science Abstracts
Synthesis of Phosphorus/Seston Fluxes and Phytoplankton Dynamics in Lake Pepin (Upper Mississippi River)
1994-1996 by William F. James, John W. Barko and Harry L. Eakin
U. S. Army Engineer Waterways
Experiment Station, Eau Galle Aquatic Ecology Laboratory
Final Report February 2000
ABSTRACT: We examined overall water quality conditions, nutrient/seston fluxes, and phytoplankton dynamics in Lake Pepin, located on the Upper Mississippi River (UMR), during the years 1994- 96. Our most prominent objective was to determine the relative importance of various external and internal sources of phosphorus (P) to the P economy and phytoplankton community of Lake Pepin.
The Minnesota River accounted for most of the annual suspended seston (~81%), total phosphorus (~44%), and viable chlorophyll a (~46%) loads externally to the UMR. The Metropolitan Wastewater Treatment Plant (Metro Plant) dominated loading of soluble reactive phosphorus, accounting for ~45% of the annual load.
Lake Pepin retained, presumably as sediment, a substantial portion of the suspended seston load entering the UMR. It
was also a sink for total P; however, net retention of P was much less both annual and seasonally (13 and 9%,
respectively), than retention of suspended seston. A large portion of soluble P, which accounts for most of the total P mass, was flushed from the lake rather than retained as sediment or biomass. While total P concentrations generally declined in Lake Pepin from headwaters to outflow, soluble reactive P (SRP) exhibited a trend of increasing concentration from headwaters to outflow with net SRP export during the summer. These contrasting patterns in total and SRP suggested the occurrence of internal P loading and/or transformations in the water column of P from particulate to soluble phases.
Internal loading of phosphorus from profundal sediments, estimated from laboratory incubation systems under different temperature and redox conditions, averaged ~ 7.5 mg m-2 d-1 during the summer under predominantly oxic conditions over all years. Although this oxic rate is high relative to other eutrophic lakes, it accounted for <15% of both the external total P and SRP loads to Lake Pepin, and accounted for only a portion of the net SRP export from the lake during the summer.
Overall, the phytoplankton community did not appear to be limited in its growth by phosphorus, based on measurements of alkaline phosphatase activity. Alkaline phosphatase activity was extremely low, coincident with high concentrations of soluble reactive phosphorus > 50 mg/L in the water column throughout the summer. Increases in viable chlorophyll a in the
water column could be partially explained by the occurrence of temporary stratification, higher residence time, increased stability, and the storage of heat in the water column, suggesting that physical/hydrological factors were, in part, regulating phytoplankton biomass in Lake Pepin.
1994-1996 by William F. James, John W. Barko and Harry L. Eakin
U. S. Army Engineer Waterways
Experiment Station, Eau Galle Aquatic Ecology Laboratory
Final Report February 2000
ABSTRACT: We examined overall water quality conditions, nutrient/seston fluxes, and phytoplankton dynamics in Lake Pepin, located on the Upper Mississippi River (UMR), during the years 1994- 96. Our most prominent objective was to determine the relative importance of various external and internal sources of phosphorus (P) to the P economy and phytoplankton community of Lake Pepin.
The Minnesota River accounted for most of the annual suspended seston (~81%), total phosphorus (~44%), and viable chlorophyll a (~46%) loads externally to the UMR. The Metropolitan Wastewater Treatment Plant (Metro Plant) dominated loading of soluble reactive phosphorus, accounting for ~45% of the annual load.
Lake Pepin retained, presumably as sediment, a substantial portion of the suspended seston load entering the UMR. It
was also a sink for total P; however, net retention of P was much less both annual and seasonally (13 and 9%,
respectively), than retention of suspended seston. A large portion of soluble P, which accounts for most of the total P mass, was flushed from the lake rather than retained as sediment or biomass. While total P concentrations generally declined in Lake Pepin from headwaters to outflow, soluble reactive P (SRP) exhibited a trend of increasing concentration from headwaters to outflow with net SRP export during the summer. These contrasting patterns in total and SRP suggested the occurrence of internal P loading and/or transformations in the water column of P from particulate to soluble phases.
Internal loading of phosphorus from profundal sediments, estimated from laboratory incubation systems under different temperature and redox conditions, averaged ~ 7.5 mg m-2 d-1 during the summer under predominantly oxic conditions over all years. Although this oxic rate is high relative to other eutrophic lakes, it accounted for <15% of both the external total P and SRP loads to Lake Pepin, and accounted for only a portion of the net SRP export from the lake during the summer.
Overall, the phytoplankton community did not appear to be limited in its growth by phosphorus, based on measurements of alkaline phosphatase activity. Alkaline phosphatase activity was extremely low, coincident with high concentrations of soluble reactive phosphorus > 50 mg/L in the water column throughout the summer. Increases in viable chlorophyll a in the
water column could be partially explained by the occurrence of temporary stratification, higher residence time, increased stability, and the storage of heat in the water column, suggesting that physical/hydrological factors were, in part, regulating phytoplankton biomass in Lake Pepin.