ANALYSIS OF NUTRIENT/SESTON FLUXES AND PHYTOPLANKTON
DYNAMICS IN LAKE PEPIN

John W. Barko, William F. James and Harry L. Eakin.  
USAE Waterways Experiment Station, 3909 Halls Ferry Rd.,Vicksburg, MS
39180-6199.

The objective of the investigation, reported here based on studies conducted in
1994, was to examine nutrient/seston fluxes in relation to phytoplankton
dynamics in Lake Pepin, a eutrophic natural impoundment located on the
Upper Mississippi River. The Metropolitan Wastewater Treatment Plant in St.
Paul, MN, and the Minnesota River co-dominated loadings of soluble reactive
phosphorus (SRP). The Minnesota River accounted for most of the annual
suspended seston, particulate organic matter (POM), and total phosphorus
(TP) loadings to the lake.  Internal loading of SRP via release from bottom
sediments averaged 4 mg m-2 d-1 during the summer, which accounted for only
3% and 6% of TP and SRP loadings, respectively, from the watershed. During
periods of elevated flow, longitudinal and vertical variations in turbidity,
suspended seston, POM, and TP indicated the presence of density currents in
the lake. The lake provided a net sink for seston and TP, and a net source for
SRP. SRP appeared to derive from suspended seston, involving a complicated
series of sorption/desorption processes.  Mean concentrations of chlorophyll a
in Lake Pepin often exhibited maxima shortly after peaks in inflow.
Production of the phytoplankton community (principally diatoms and blue-green algae) was not limited by phosphorus, based on measured alkaline
phosphatase activity. Increases in chlorophyll a in the water column occurred
during temporary periods of thermal stratification, suggesting the importance
of physical factors in regulating phytoplankton dynamics in the lake. Based on
results of this first year of a 3-yr effort, it appears that management of
phytoplankton in Lake Pepin will require attention to physical conditions,
including availability of underwater light and water column stability.

Keywords: Phytoplankton, Phosphorus, Nutrient loadings, Sediment, Lake
Pepin

1. John W. Barko, USACE Waterways Experiment Station, 3909 Halls Ferry
Road, Vicksburg, MS  39180-6199.  Phone (601) 634-3654; fax (601) 634-3528; e-mail barkoj@ex1.wes.army.mil

2. Platform presentation.

3. Non-student paper.