MEPS 198:43-49 (2000)  -  doi:10.3354/meps198043

ABSTRACT: To evaluate the effect of differences in community structure on the process of dissolved organic nitrogen (DON) release in natural planktonic assemblages, DO¹⁵N release from size-fractionated (<20 and <94 µm) waters of Akkeshi Bay, eastern Hokkaido, Japan, was measured after the addition of ¹⁵NH₄⁺. Calculations of percent extracellular release (PER) based on particulate organic ¹⁵N nitrogen (PO¹⁵N) and DO¹⁵N accumulation were higher in the <20 µm fraction than in the <94 µm fractions in 2 out of 3 experiments; though the absolute magnitude of the release was higher in the larger size-fraction. Phytoplankton was primarily responsible for the DO¹⁵N release in the <20 µm fraction, which suggested more efficient DON release by smaller phytoplankton compared to larger ones. A passive permeation through the cell membrane is a likely explanation for the DON release observed in our study. PER (<94 µm) fell within the range of 2.7 to 4.9%, which is almost equal to, or somewhat lower than, the PER reported for carbon. In addition, the occurrence of tight coupling between DON release and its consumption by bacteria was suggested in a time-course experiment, showing efficient use of released nitrogenous compounds within the natural community.

KEY WORDS: DON · Phytoplankton · Size-fraction · ¹⁵N tracer