Physics
Scientific paper
Feb 1986
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1986gecoa..50..215s&link_type=abstract
Geochimica et Cosmochimica Acta, vol. 50, Issue 2, pp.215-229
Physics
Scientific paper
Geochemical studies of Fe, Mn, Si, P, Ca, and K were made in two basins of Shagawa Lake, Minnesota during summers 1977-1979. Within the metalimnion, upward and laterally diffusing Fe(II) oxidizes and co-precipitates a large fraction of soluble P. Nevertheless, the soluble Fe / P ratio decreases while progressing upward through the oxidation zone. The mean paniculate Fe / Si ratio is 3.0 ± 0.5 in the thermocline, but because of the stoichiometric excess of dissolved Si over Fe, and preferential associations between Fe and P, the oxidation of Fe has a negligible effect on the dissolved SiO 2 profiles. The lake sediments are net sources of Ca, K, and P during summer stratification. These elements are transported upward through the thermocline and exported from the lake. Despite the very active sediment release of dissolved SiO 2 , and upward transport through the thermocline, the lake acts as a net sink for biogenic opal in summer. The prominent soluble Mn maxima in the thermocline of the east basin result from antecedent sediment release (Mn 2+ ) and vertical transport (primarily in the west basin), followed by dispersion and oxidation in the epilimnion, and finally downward settling and dissolution within the east basin thermocline. Rhodochrosite solubility does not influence Mn 2+ profile development. Very large percentage increases in epilimnetic Fe, Mn, Si, and P occur simultaneously during episodes of wind-activated entrainment of metalimnion water. These increases are in stoichiometric agreement with the entrained water mass, provided account is also taken of the preformed slurries overlying the metalimnetic shelf sediments in mid-summer. The bathylimnetic soluble Fe / P ratio remains low since the cultural inputs of P were terminated in early 1973. This geochemical stability is explained by the large reserves of potentially mobile P in the upper post-cultural sediments. Phosphorus release from these sediments and transport phenomena account for the persistent eutrophic condition of the lake during mid-late summer, despite the 80% reduction in external P loading.
Armstrong David E.
Stauffer Robert E.
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