A 15N tracer method for the measurement of dissolved organic nitrogen release by phytoplankton

Abstract

We present a new method for separation of dissolved organic nitrogen (DON) from dissolved inorganic nitrogen (DIN) in marlne samples which permlts the measurement of DO'*N production during DII5N uptake and regeneration experiments. Ion retardat~on resin was used to separate DI1'N from DOI'N and the isolated D0I5N was subsequently analyzed by mass spectrometry Variat~on in D0I5N atom % enrichment in duplicate samples, determined w ~ t h the ion retardation column method, was less than 40 . We also separated the low n~olecular weight (LMW) from total D015N using ultrafiltration and found the ratio of these variables to be a useful index in determining what release processes were likely occurring in a given sample. Examples are presented from 2 types of experiments conducted in Chesapeake Bay waters: first, a 6 h time-course of D015N release from both NH,' and I5NO3uptake, and second, a series of short-term (0.5 h) release measurements from 15NH,* uptake over the course of a day. In the former, total DO1% release rates resulting from 15NH,* uptake were several-fold higher than those resulting from IsNO3uptake due to extremely low rates of incorporat~on of NO? into cellular organic material. The release of LMW DOI'N resulting from I5NH,+ uptake decreased from 78 to 21 % of total D0I5N release during the 6 h incubation which suggests that processes other than direct release by phytoplankton (e.g. sloppy feeding, cell lysis) likely became more important as the incubation progressed In the latter expenment, LMW DO'% release was 0 to 16 % of total D0I5N release, which indicates that, in this case, also, processes other than direct release by phytoplankton were dominating the flux of DON from phytoplankton.