A N-15 TRACER METHOD FOR THE MEASUREMENT OF DISSOLVED ORGANIC NITROGEN RELEASE BY PHYTOPLANKTON

We present a new method for separation of dissolved organic nitrogen (DON) from dissolved inorganic nitrogen (DIN) in marine samples which permits the measurement of (DON)-N-15 production during (DIN)-N-15 uptake and regeneration experiments. Ion retardation resin was used to separate (DIN)-N-15 from (DON)-N-15 and the isolated (DON)-N-15 was subsequently analyzed by mass spectrometry. Variation in (DON)-N-15 atom % enrichment in duplicate samples, determined with the ion retardation column method, was less than 4%. We also separated the low molecular weight (LMW) (DON)-N-15 from total (DON)-N-15 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 (DON)-N-15 release from both (NH4+)-N-15 and (NO3-)-N-15 uptake, and second, a series of short-term (0.5 h) release measurements from (NH4+)-N-15 uptake over the course of a day. In the former, total (DON)-N-15 release rates resulting from (NH4+)-N-15 uptake were several-fold higher than those resulting from (NO3-)-N-15 uptake due to extremely low rates of incorporation of (NO3-)-N-15 into cellular organic material. The release of LMW (DON)-N-15 resulting from (NH4+)-N-15 uptake decreased from 78 to 21% of total (DON)-N-15 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 experiment, LMW (DON)-N-15 release was 0 to 16% of total (DON)-N-15 release, which indicates that, in this case, also, processes other than direct release by phytoplankton were dominating the flux of DON from phytoplankton.