Release of dissolved organic nitrogen by a planktonic community in Akkeshi Bay

We applied conventional incubation experiments combined with a N-15-tracer technique to evaluate the relative contributions of phytoplankton, micrograzers and copepods to the release of dissolved organic nitrogen (DON) or dissolved nitrogen (DN) in coastal waters from Akkeshi Bay, eastern Hokkaido, Japan. From March to November, water temperature ranged from -1 to 17 degreesC, and chi a concentrations showed about 5-fold change (from 2.6 to 12 mug l-(-1)). The calculated rates of DON release by phytoplankton (18 to 28 nmol l(-1) d(-1)) were comparable to those of DN release by copepods (18 to 67 nmol l(-1) d(-1)). However, the DON release rates by micrograzers (140 to 350 nmol l(-1) d(-1)) greatly exceeded those of the other planktonic assemblages. DON release by micrograzers was well coupled to DON uptake by bacteria, indicating that there was a significant nitrogen flux from phytoplankton to bacteria via micrograzers, DON supply in the upper layer would activate microbial food webs and consequently relieve the short supply of dissolved inorganic nitrogen for phytoplankton during the stratified season.