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