Seasonal and vertical variations in phytoplankton photosynthetic parameters and primary production in Suruga Bay, Japan

Seasonal and vertical variations in photosynthetic–irradiance ( P–E ) parameters and primary production were examined in offshore Suruga Bay, where both the subtropical warm Kuroshio Water and eutrophic coastal water were influential. The maximum light-utilization coefficient, α * , did not show significant variation between the surface and subsurface layers in both the warm and cold periods. Vertical differences in the maximum photosynthetic rate, P max ∗ , and the light-saturation parameter , E k , were small even during the warm period. Seasonal variations in the P–E parameters were not significantly related to dissolved inorganic nitrogen (DIN) at the surface layer. These results suggest that low-nutrient and high-light stresses were not serious at the surface layer, even in the stratified season, because of the continuous inflow of high-nutrient river water. Seasonal variations in P max ∗ and α * were suggested to be regulated by both the E k -dependent (photo-acclimation of phytoplankton) and E k -independent (response to nutrient and growth conditions) mechanisms from the correlation analyses. The subsurface chlorophyll maximum (SCM) formation was less obvious, and a major part of the primary production occurred in the upper half of the euphotic zone. The water-column primary production, ΣPP (total), tended to be overestimated by ignoring vertical variations in the P–E parameters and chlorophyll a , especially during the warm period. These features of primary production appeared to be similar to those reported in coastal waters rather than in subtropical waters, playing an essential role in the fishing ground formation in the bay.