Half‐Saturation Constants for Uptake of Nitrate and Ammonia by Reservoir Plankton

Abstract

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.

Ecosystem functions and densities of contributing functional groups respond in a different way to chemical stress

“…Most notably, values of carbon uptake for strict autotrophs below 0.1 d À1 strikingly coincide with ammonia concentrations that were low enough to limit the growth of the strict autotrophs (<10 mg L À1 ; Supplemental Data, Fig. S6) [33]. For mixotrophs, the correspondence between nutrient concentrations and functional rates was less clear than for strictly autotrophic phytoplankton.…”

Inorganic nitrogen uptake by epilithic periphyton in a N‐deficient lake1

“…Halfsaturation constants for phytoplankton N assimilation in eutrophic+ lakes generally range from 15 to 300 pg N liter-' (e.g. Toetz et al 1973;Chan and Campbell 1978;Murphy and Brownlee 198 1). Values for both N03-and NH,+ uptake by freshwater and oceanic phytoplankton in oligotrophic, N-deficient systems are consistently low, < 15 pg N liter-l, and comparable to the reduced ambient substrate concentrations in these environments (MacIsaac and Dugdale 1969; Eppley et al 1969Eppley et al , 1973Eppley et al , 1977Axler et al 1982).…”

“…Lake Tahoe is warm monomictic and the summer thermocline is generally at h, 20 m. The circulation of NO,--enriched aphotic zone water into the euphotic zone (-O-105 m) is an important source of DIN for the spring increase in phytoplankton production; however, the vertical extent of mixing is not consistent on an annual basis (Paerl et al 1975). The littoral zone represents 19% of the lake surface area and it has been estimated that >60% of the primary production in this region can be contributed by epilithic periphyton (Loeb et al 1983). …”

Nitrogen metabolism in the brackish Lake Nakanoumi. IV. Seasonal variation of nitrate nitrogen.

“…In general, nitrogen uptake by natural phytoplankton is regulated by the environmental variables, such as water temperature (ToETZ et al, 1977 ;SEIKE et al, 1986a) and irradiance (EPPLEY et al, 1979 ;MITAMURA, 1986). The uptake rates of ammonium and nitrate by phytoplankton are lower in the dark than in the light, and the nitrate uptake especially seems to be greatly suppressed under dark conditions (CIIAN and CAMPBELL, 1978 ;HEALEY, 1973 ;MITAMURA and MATSUMOTO, 1981 ;SEIKE et at., 1986a) .…”