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Nutrient budgets in the lakes of the Cedar River Watershed

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  • Aquatic mineral measurements and sediment characterization of the Cedar River basin lakes and streams reveals two largely distinct physical and chemical environments. Near the headwaters of the Cedar River Watershed the composition of lakes and streams reflect adjacent rocks but the effects of forest and agricultural practices and of processes occurring downstream very quickly alter the composition to a type largely responsive to the urbanization in the lower reaches of the watershed. The relative ionic composition in the watershed is: Ca > Mg ≥ Na > K and HCO₃ > Cl ≥ S0₄. A 2- to 10-fold increase in concentration is measured with most chemical parameters when comparing upper watershed to lower watershed. The lowest increase is observed with total phosphorus reflecting perhaps identical phosphorus solubility control mechanisms. Annual nutrient budgets for Lake Sammamish suggest a reduction in sedimented phosphorus since diversion, but little change in the quantity of phosphorus released from anaerobic sediment. Although the sediment phosphorus supply during the summer and fall months is nearly three times higher than the external phosphorus supply its relative contribution to the trophogenic layer phosphorus is a function of lake mixing. An iron precipitation mechanism appears to control the mean winter phosphorus contents of Lake Sammamish. The prevailing anoxic conditions in the lake during late summer and fall bring about large increases in hypolimnetic iron concentrations which upon destratification precipitate phosphorus as an iron (111) hydroxy complex. Lake sediment characterization indicate differences between lakes with respect to carbon/nitrogen, nitrogen/phosphorus and particle size distribution reflecting drastic variations in the depositional environments and trophic status of the upper and lower watersheds. Higher carbon/nitrogen and nitrogen/phosphorus ratios and coarser sediments are measured in oligotrophic Findley Lake. With the exception of Findley Lake, all sediments appear to have the same composition with respect to Fe, Mn, Ca, Mg, Na, and K contents. Simulation of sediment phosphorus release at the sediment-water interface In enclosed in situ Lake Sammamish sediment-water columns appears to parallel the phosphorus release measured in the lake hypolimnion at the peak of the lake stagnation period. Preliminary studies indicate that it will be possible to relate the nutrient release results from the in situ columns to those of laboratory studies, arriving thus at a faster more economically obtained elucidation of the nutrient exchange mechanisms and potentials at lake sediment-water interface.
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