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The kinetics of nitrate(N0, ) uptake, the maximum uptake velocity (V,) and the half-saturation constant (K,), were determined for 18 species of batch-cultured freshwater algae grown without nitrogen limitation. Values of K, ranged from 0.25 to 6.94 p M 1 for Chlorella pyrenoidosa Chick, and Navicula pelliculosa (Breb.) Hilse, respectively. Values of V, ranged from 0.51 to 5.07 p M 1-' h-I for Anabaena A7214 and Nitzschia W-32 O'Kelley, respectively. The mean positive values of K, for Chlorophyta, Cyanophyta and Chrysophyta were 1.89,3.67 and 4.07 p M 1-I, respectively. The mean values of V, for the same phyla were 1.6 1, 1.02 and 2.97 p M 1 h ' lo5 cells1, respectively. The ranges of these kinetic parameters encompass values of kinetic parameters for marine and freshwater species in batch culture, for freshwater algae grown in N-limited chemostats and for natural populations of freshwater phytoplankton. Thus, in spite of variability between species, uptake parameters for both marine and freshwater algae are identical.

The diurnal assimilation of NH, and NOa quantities of NOs and NH4 in natural waby communities of Ceratophyllum-periphyton ters by removing large quantities of these was studied in situ in a small lake. Plants nutrients from circulation. The work rewere exposed to "N-NH4 and 15N-N03 for 6 consecutive periods of 4-hr duration. The ported here uses uptake values obtained community assimilated NH, both during the during short periods to predict the daily day and at night. The rate of NO, assimila-

Observations were made in order to learn if the uptake of NH_4 and NO_3 by freshwater plankton can be described by the Michaelis—Menten expression. Uptake of NO_3 and NH_4 by reservoir plankton was estimated at 5 concentrations using (15)N Tracer techniques. A hyperbola results when the uptake velocity (v) of NO3 and NH4 is plotted against concentration (S). The S/v vs S transformation of the Michaelis—Nenten expression was used to estimate K_s. For a mixed population of blue—green algae in Lake Carl Blackwell, Oklahoma, K_s was about 43 mg NO_3 — N (m)(—3), when the initial concentration was 7.53 mg NO_3 — N (m)(—3). In Lake Keystone, where the initial concentration of NO_3 — N was 419.17 mg (m)(—3), enrichment with NO_3 increased v in a similar way, suggesting use of the Michaelis—Nenten model may not be realistic.

This paper describes the effect of total lake mixing with an axial flow (Garton) pump on the limnology and phytoplankton of two Oklahoma lakes .The Garton pump destratified Ham's Lake (4o ha) in 3 days . Except for one small isolated basin, Ham's Lake remained completely destratified for the rest of the summer . Algal biomass declined, numbers of species of green algae increased, but numbers of species of blue-green algae did not decrease as expected . After destratification, pH remained high (> 8), carbonate alkalinity was observed and reactive phosphate was undetectable .An axial flow pump increased the heat content of Arbuckle Lake (951 ha) and caused the lake to destratify about one month earlier than usual . Increasing the heat content of the lake did not affect the concentration of most water quality parameters or the biomass of algae .