We applied conventional incubation experiments combined with a 15 N-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°C, and chl a concentrations showed about 5-fold change (from 2.6 to 12 µg l ) 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.
KEY WORDS: DON · 15 N-tracer · PlanktonResale or republication not permitted without written consent of the publisher Aquat Microb Ecol 24: 99-107, 2001 by phytoplankton, micrograzers, and copepods at a subarctic embayment and (2) to discuss the role of DON in coastal marine nitrogen flux. Our results showed that the rates of DON release by micrograzers greatly exceeded those of the other planktonic assemblages, and the DON they supply in the upper layer would be important for relieving the short supply of dissolved inorganic nitrogen (DIN) available to primary producers.
MATERIALS AND METHODSAkkeshi Bay is located on the eastern Pacific coast of Hokkaido, Japan. All samplings were done at a fixed station (water depth 13 m: 43°01' N, 144°52' E) in the bay. Temperature and salinity were measured using a YSI salinometer (Model 33 S-C-T meter) at 1 m intervals from the surface to a depth of 12 m. In this study, we defined organisms that passed through a 94 µm net as microplankton. Because 91 to 100% of chlorophyll a (chl a) in whole seawater was contained in the < 94 µm fraction (Hasegawa unpubl.), our results represented the majority of phytoplankton activity. However, micrograzers' activity on nitrogen flux might be underestimated, since the general definition of micrograzer was < 200 µm (Sieburth et al. 1978).Incubation experiments (< 94 µm assemblage). Incubation experiments with size-fractionated seawater (< 94 µm) were carried out on 9 March, 25 May, 29 June, 24 August, and 10 November 1998. Conditions of incubation are shown in Table 1. NH 4 + regeneration rates were determined by the 15 N-isotope dilution method (Blackburn 1979, Caperon et al. 1979) and NH 4 + uptake rates were determined after . Further, DO 15 N release was measured on 25 May, 29 June and 24 August 1998 and expressed as percent extracellular release (PER: percentage of 15 N-labeled DON to 15 N-labeled DON plus 15 N-labeled particulate organic nitrogen [PON], Hasegawa et al. 2000c). Details of the methods used to measure DON release are described in Hasegawa et al. (2000a,c).The dilution method with 15 N-tracer. DON release rates mediated by micrograzers were estimated b...