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Marine Ecology Progress Series

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

Release of dissolved organic nitrogen by size-fractionated natural planktonic assemblages in coastal waters

Toru Hasegawa1,*, Isao Koike1, Hiroshi Mukai2

1Ocean Research Institute, University of Tokyo, 1-15-1, Minamidai, Nakano, Tokyo 164-8639, Japan
2Akkeshi Marine Biological Station, Hokkaido University Akkeshi, Hokkaido 088-1113, Japan

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

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