Internal Phosphorus Loading in Lake Pepin, Upper Mississippi River

ABSTRACT

We examined rates of internal phosphorus (P) loading from sediments in Lake Pepin, an impoundment on the upper Mississippi River. Laboratory-determined rates of P release from the sediment, measured using intact sediment core systems, were very high, with mean values ranging from 3.8 to 15 mg m⁻² d⁻¹ under oxic and anoxic conditions, respectively. Independently-determined Fickean diffusional fluxes (mean of 4.9 mg m⁻² d⁻¹), measured in situ under oxic conditions using sediment peepers, were not statistically different from those rates determined in the laboratory under similar temperature, pH, and oxygen conditions. The dominant inorganic P fractions in the sediments were NaOH-extractable P and HCl-extractable P. Under both oxic and anoxic conditions, significant correlations existed between rates of P release and the NH₄Cl- and NaOH-extractable P fractions, suggesting mat loosely-bound and iron- and aluminum-bound sediment P contributed to P release from the sediments in this lake. Correlations between total sediment P and extractable sediment P fractions indicated that loosely-bound and iron- and aluminum-bound P increased, while calcium-bound P decreased, with increasing total P content of the sediment. Our results suggest that internal P loading from die sediments under oxic, as well as anoxic, conditions can play a very important role in the P economy of this system.