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Phytoplankton productivity and growth rate kinetics in the Cedar River lakes

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  • Lakes Findley, Chester Morse and Sammamish, Washington, are characterized by one major outburst of phytoplankton productivity and biomass (mainly diatoms) with usually no or low fall activity. Vernal outbursts were often delayed in the monomictic lakes by inadequate light because of unfavorable climate and/or a lack of thermal stratification. Strong inhibition by light (probably u.v.) was observed in Findley such that average maximum productivity occurred at 10% of surface intensity while maximum was customarily at 60% in the other lakes. Annual productivity was 369C/m2 in Findley, 479C/m2 in Chester Morse and 1989C/m2 in Sammamish. The range in mean chlorophyll a content was 0.8 to 10 ug/,for the same lakes respectively. Although more than three fourths of the productivity in the four lakes was contributed by nanoplankton (5-50u), a tendency for increased contribution from netplankton was observed with increasing trophic state. In vitro experiments during all parts of the growing season show that nitrogen (N) and phosphorus (P) were simultaneously limiting productivity increase in the three lakes. Growth rate kinetics experiments showed increasing half-saturation constants for P (0.17 to 2.8pgP/A) for the natural phytoplankton progressing from oligotrophy to eutrophy. Growth rate models using these parameters were evaluated in Findley Lake subsequent to iceout in 1973. The best agreement was obtained with a model using light (with a function that included inhibition) N and P in contrast to several other combinations of those variables. Light was the most important factor and adaptation problems to low experimental light necessitated increasing the maximum growth rate by a factor of 10 in order to obtain the best agreement with in situ growth rate.
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