MESOCOSM STUDIES ON THE INFLUENCE OF PHOSPHATE ENRICHMENT ON AMMONIUM AND NITRATE FLUX IN AN OLIGOTROPHIC LAKE

The fate of (NH4+)-N-15 and (NO3-)-N-15 was followed in control and PO4(3-) enriched 1570 l mesocosms filled with epilimnetic water from an oligotrophic Rocky Mountain lake. Volumetric incorporation of (NH4+)-N-15 and (NO3-)-N-15 into phytoplankton and bacterioplankton (particulates between 280 and 0.7 mu-m), and crustacean zooplankton > 80 mu-m was enhanced by PO4(3-), but no increase in biomass specific rates of uptake by phytoplankton and bacteria occurred for either form of N-15. Dilution of both (NH4+)-N-15 and (NO3-)-N-15 by (NH4+)-N-14 and (NO3-)-N-14, respectively, was evident indicating regeneration of these nutrients, but regeneration rates were not effected by PO4(3-) enrichment. The results illustrate the strong trophic coupling between N dynamics and PO4(3-) enrichment in this system.