A trophic-dynamics approach was used to examine the relationships of phytoplankton production, other autochthonous primary producers, and allochthonous organic matter to the organic carbon supply necessary to support the fish community production in 17 southeastern and midwestern U.S. reservoirs. Fish community carbon requirements were estimated using average efficiencies of 10 and 15% per trophic transfer to bracket the range of efficiencies that probably occur in reservoir food webs. At 10% efficiency, most of the reservoir food webs considered appeared to be supplemented significantly by allochthonous organic matter. At 15% efficiency, phytoplankton production accounted for the organic carbon required for fish production in 10 of the 17 reservoirs. In this case, supplementary organic carbon sources were invoked to meet fish production requirements in the remaining seven reservoirs. Our results suggest that, depending on the actual food web efficiencies, autochthonous primary production can often account for the organic carbon supply required for reservoir fish production and that the importance of allochthonous organic matter to reservoir biological productivity may have been previously overemphasized. Although both the morphoedaphic index (MEI) and phytoplankton productivity were strongly correlated with reservoir fish production, a simple relationship does not appear to exist between the MEI and phytoplankton productivity, probably because of the complex aggregate nature of the MEI descriptor.
DEB75-19777 to the University of Wisconsin. tus 1977). Since there is a pH effect on the proportion of DIC fractions (CO,, IICOn-, and COn2-) at equilibrium, the differential capability to use HC03-may be observed in the pH effect on inorganic carbon fixation. Titus (1977) found that photosynthesis of V. americana at a DIC level of 110 mg C02-litcr-1 clecreases sevcrely when pH changes from 7 to 8, while that of M. spicatunx decreases most between pII 8 and 9.Titus used buffcrcd water and adjusted DIC levels to determine the effect of DIC on photosynthesis of M. spicatum. In experirnents at pH 8 and five different levels of DIC between about 30 and 250 mg CO,.liter-I, he fit the data with nonlinear regression to the Michaelis-Menten relationship V = V,,,(DIC/k, + DIC) and found a k, value of 92 mg CO, * liter-'; i.e. photosynthesis (as measured by 14C fixation)would be half-saturated with DIC at a DIC level of 92 mg CO,* liter-', at pH 8.The experiments of Titus were conducted in the laboratory and we became interested in examining the hypothesis that results similar to his would be found for M. spicatum in different lake waters in which the DIC naturally ranges from low to high levels. In northern Italy several lakes contain M. spicatum and range 912
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