MEPS 273:17-30 (2004)  -  doi:10.3354/meps273017

ABSTRACT: In order to apply the natural fluorescence method for the long-term monitoring of chl a and primary production, the seasonal variation in the light absorption coefficient of phytoplankton (a* _ph), maximum quantum yield ratio of photosynthesis to fluorescence [(φ_c/φ_f)_max], and its light saturation index (k_cf) were examined. The mooring of a natural fluorescence sensor was conducted during winter to early spring (January to April 1998), late spring (May 2000) and summer (July to August 2000) at a station in Otsuchi Bay on the north east coast of Japan. The bio-optical parameters, a* _ph, (φ_c/φ_f)_max and k_cf, were measured by water sampling and incubation experiments twice a week. The temporal variation in a* _ph was expressed as an exponential of chl a. This relationship varied seasonally. The variations in (φ_c/φ_f)_max were related to chl a during winter to early spring, and to chl a and temperature during late spring and summer. The value of k_cf was relatively low and stable during the winter and early spring; it reached higher values during late spring to summer, when temperature and light intensities were also higher. Based on these relationships, the long-term monitoring of chl a and primary production from natural fluorescence was realized in coastal water where environmental conditions and phytoplankton abundance fluctuated markedly. By developing algorithms in various areas and seasons, as the present study has shown, the natural fluorescence method is expected to be useful for continuous monitoring of primary production, with a high spatial resolution by a mooring web of sensors, even in coastal waters.

KEY WORDS: Natural fluorescence · Primary production · Chl a · Light absorption of phytoplankton · Coastal waters