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 oligotrophicFlathead Lake (Montana) were measured with ¹⁵N and ³²P incorporationand dilution experiments, six times over a seasonal cycle. Theannual mean molar N P uptake ratio at ambient concentrationswas 13 9 (range = 4 8–34.2); uptake of nitrate, ammoniumand phosphate were always below saturation indicating both Nand P deficiency Organisms >280 µm were responsiblefor 0–60% of ammonium and 0–40% of phosphate regeneration,40–100% of the ammonium and 34–98% of phosphateregeneration occurred in the <3 µm fraction The <3µm fraction accounted for 7–70% of the ammoniumand 6–64% of the phosphate uptake. Results from antibiotictreatments indicated that both prokaryotic and eukaryotic ammoniumuptake 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 theepilimnion. 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 nutrientregulation of primary productivity of oligotrophic lakes, (ii)bacteria probably compete with phytoplankton for both ammoniumand phosphate, (iii) biotic regeneration is the main sourceof nutrients to the epilimnion during stratification, and (iv)crustacean zooplankton were relatively unimportant sources ofregenerated ammonium and phosphate.