Predator-prey eddy in heterotrophic nanoflagellate-bacteria relationships in a coastal marine environment: a new scheme for predator-prey associations

Seasonal and short-term variations in abundance of bacteria and heterotrophic nanoflagellates (HNF) together with environmental variables including chlorophyll a (chl a) were monitored in Onagawa Bay on the northeastern Pacific coast of Japan. Bacterial abundance varied within a narrow range throughout the year, while HNF abundance showed marked seasonal changes. Shortterm observations revealed that peaks of bacterial abundance were usually followed by increases in HNF abundance with a lag of 2 to 6 d and both abundances changed cyclically with a 4 to 10 d period, indicating so-called predator-prey oscillations. These predator-prey oscillations appeared as circular orbits, or 'predator-prey eddies', in phase space during a period shorter than ca 1 mo. Considering the seasonal and short-term trends, we propose a new schematic diagram to describe seasonal changes in this predator-prey relationship. The relationship appears as a HNF-bacterial eddy during the shorter period, while the location and magnitude of the eddy in phase space differ with the seasons. Therefore, the annual cycle of the HNF-bacteria relationship is formed by a continual migration of the predator-prey eddy in phase space. A possible relationship between HNF and chl a is also discussed.