Linkage between the phosphorus and silica cycles in Lake Mendota, Wisconsin

Abstract

The phosphorus and silica cycles in Lake Mendota were studied during 1970–1973, bracketing the diversion of all municipal wastewater inputs to the lake, and again in 1977–1980, following 1 to built;1 2 lake hydraulic residence times. Because of the high concentrations of soluble phosphate and inorganic nitrogen, the vernal diatom bloom is limited by silica. The low nutrient uptake ratio ( mean ΔSi ΔP = 5.5 ± 0.2, by atoms ) reflects nutrient availability and the dominance of low silica diatoms (especially Stephanodiscus species) in this eutrophic calcareous lake. The lower ratio, ΔSi ΔP , in the hypolimnion during summer stratification (mean 2.8 ± 0.1) illustrates that phosphorus recycling proceeds more efficiently than in the case for Si. The vertical fluxes of biologically available P and Si into the epilimnion averaged 9.5 ± 1.0 and 33 ± 6 mg m −2 ( A 0) day −1 during summers 1977–1979. Despite the low Si P flux ratio, soluble silica accumulated each summer in the epilimnion whereas epilimnetic P declined. This difference is probably a consequence of seasonal blue-green algal dominance in the epilimnion. The available Si P in the epilimnion shifts downwards by 3 orders of magnitude during September and October, because of entrainment of metalimnetic and hypolimnetic waters with a lower ratio, and the selective uptake of Si by the fall diatom crop. Because of the seasonal demands by diatoms, coupled with the comparative inefficiency of Si recycling (vs P), Lake Mendota's annual retention coefficient for soluble silica is 0.98 ± 0.01, as compared to 0.40 ± 0.05 for biologically available P. Based on various nutrient and hypolimnetic redox indices, Lake Mendota failed to respond during the eight years following nutrient diversion. Nor has the lake changed significantly in the twentieth century.