MEPS 179:123-134 (1999)  -  doi:10.3354/meps179123

ABSTRACT: A new scheme has been recently proposed by Tanaka et al. (1997; Aquat Microb Ecol 13:249-256) to illustrate the temporal variation in the numerical relationship between heterotrophic nanoflagellates (HNF) and bacteria on both short-term and seasonal scales. To clarify this scheme, seasonal variations in abundance of HNF and bacteria, together with environmental variables, were monitored at 1 to 3 d intervals in Onagawa Bay on the northeastern Pacific coast of Japan. In addition, bacterial growth rates were also measured weekly or bi-monthly. As expected from the marked seasonality of environmental variables, bacterial growth rate showed distinct seasonal variation, being higher in warmer seasons and vice versa. The seasonal variation of bacterial abundance in nature, however, was only 1 order of magnitude, while HNF abundance showed marked seasonal changes. On shorter temporal scales, peaks of bacterial abundance were usually followed by increases in HNF abundance with a lag of 2 to 7 d, and bacterial and HNF abundances changed with 2 to 14 d and 3 to 17 d periods, respectively, indicating so-called predator-prey oscillations. These values generally agree with previously reported values, even though there were temporal and spatial differences. These predator-prey oscillations were confined to a particular region in phase space during a period of ca 1 mo and showed a sequential movement in a anticlockwise and/or clockwise direction. Such movement was termed the predator-prey eddy. On annual scales, the eddy's position and magnitude were different between seasons, but the eddy was confined to a vertically elongated elliptical region in phase space which was similar to findings in our previous study. These results support our contention, namely, that the predator-prey eddy of the HNF-bacteria system always exists and continuously migrates over a certain region of phase space on an annual basis.

KEY WORDS: Heterotrophic nanoflagellates · Marine bacteria · Short-term fluctuations · Predator-prey eddy