ABSTRACT: To estimate the release of DON (dissolved organic nitrogen) by freshwater planktonic algae, we modified the Bronk & Glibert method used for marine planktonic algae, by adding NaCl (3.5%) to the freshwater samples and using ion retardation resin columns to separate DON from inorganic nitrogen in the solutions. Clear separation and high recoveries of amino acids indicate that the ion retardation resin column method can be applied to freshwater samples. We derived equations for the calculation of DON release and net DIN (dissolved inorganic nitrogen) assimilation which enable the estimation of the TNR (total nitrogen release) and NNA (net nitrogen assimilation). Further, we derived equations to estimate RANR (recently assimilated nitrogen release; release of DON produced from DIN assimilated during ¹⁵N incubation), and PANR (previously assimilated nitrogen release; release of DON produced before the incubation). Using the modified method and equations, we examined time courses of DON release from the cyanobacterium Microcystis novacekii. Both TNR and NNA increased with time during ¹⁵N incubation, with more than 91% of the TNR consisting of RANR.

KEY WORDS: Ammonium · Nitrogen uptake · Dissolved organic nitrogen release · Freshwater planktonic algae · Microcystis novacekii · Tracer method

Published in AME Vol. 16, No. 3 (1999) on February 24
ISSN: 0948-3055. Copyright © Inter-Research, Oldendorf/Luhe, 1999