MEPS 267:9-26 (2004)  -  doi:10.3354/meps267009

ABSTRACT: A seasonal bio-optical monitoring programme was carried out in the SW Finnish coastal zone, Baltic Sea. Photosynthetic performance, spectral absorption characteristics, and fast repetition-rate (FRR) parameters of the phytoplankton community, as well as the natural light regime, were monitored 22 times from April to November. Primary productivity (PP) was determined with in situ fixation of ¹⁴C, as well as with bio-optical models based on either variable fluorescence parameters, or P-E dependence, or a combination of the two. Compared to the ¹⁴C-based model, the bio-optical models overestimated near the surface and underestimated in deeper layers, since bio-optical estimates of PP tend to follow the vertical pattern of irradiance, while in situ PP does not. The areal PP of the ¹⁴C-based model was 85.9 g C m^-2 yr^-1, while the other models provided estimates 70 to 160% of this value. Seasonal variability in spectral light attenuation stressed the importance of spectral re scaling of the photoadaptive variables α (maximum light-utilisation coefficient) and σ_PSII¹ (functional absorption cross-section of Photosystem II [PSII]). In order to describe the availability of light for photosynthesis, we calculated spectral scaling factors as the ratio of the light source of the FRR fluorometer or the P-E incubator to the in situ light field. The values were always above unity, with a maximum level of 2 for the P-E data, and as high as 3 for the FRR data, and varied seasonally and vertically. The apparent spectral mismatch between the light generated by the FRR fluorometer and the natural light field of the Baltic Sea suggests that spectral correction procedures should routinely be included in the FRR measuring protocol in Case 2 waters. Degradation of the fluorescence signal by ambient red light interfered with the FRR measurements in near-surface layers. We term this phenomenon the Œred light effect¹.

KEY WORDS: Fast repetition rate fluorometry · FRRF · Primary productivity modelling · Spectral scaling · Bio-optics · Primary production · Phytoplankton · Baltic Sea