Seasonal uptake and regeneration of inorganic nitrogen and phosphorus in a large oligotrophic lake: size-fractionation and antibiotic treatment

Uptake and regeneration of inorganic N and P in oligotrophic Flathead Lake (Montana) were measured with 15 N and 32 P incorporation and dilution experiments, six times over a seasonal cycle. The annual mean molar N P uptake ratio at ambient concentrations was 13 9 (range = 4 8–34.2); uptake of nitrate, ammonium and phosphate were always below saturation indicating both N and P deficiency Organisms >280 μm were responsible for 0–60% of ammonium and 0–40% of phosphate regeneration, 40–100% of the ammonium and 34–98% of phosphate regeneration occurred in the <3 μm fraction The <3 μm fraction accounted for 7–70% of the ammonium and 6–64% of the phosphate uptake. Results from antibiotic treatments indicated that both prokaryotic and eukaryotic ammonium uptake was important, and that eukaryotes accounted for 53–98% of the ammonium regeneration. During thermal stratification, heterotrophic ammonium and phosphate regeneration by organisms >3 μm supplied much of the inorganic N and P in the epilimnion. Estimated rates of allochthonous and diffusive (i.e ‘new’) ammonium, nitrate and phosphate input were <5% of biotic regeneration. These results suggests that (i) both N and P dynamics should be considered when examining nutrient regulation of primary productivity of oligotrophic lakes, (ii) bacteria probably compete with phytoplankton for both ammonium and phosphate, (iii) biotic regeneration is the main source of nutrients to the epilimnion during stratification, and (iv) crustacean zooplankton were relatively unimportant sources of regenerated ammonium and phosphate.