[A-CG39] Coastal Ecosystems - 1. Water Cycle and Land-Ocean Interactions

[ACG39-P01] Seasonal variation in the primary production of phytoplankton community in Wakasa Bay (Miyazu Bay)

Mao Kuragano¹, Toshimi Nakajima², *Ryo Sugimoto¹ (1.Faculty of Marine Biosciences, Fukui Prefectural University, 2.Atmosphere and Ocean Research Institute, The University of Tokyo)

Keywords:Primary production, Nutrient, Phytoplankton, Coastal sea

Trophic transfer of energy and essential elements from phytoplankton biomass supports marine organisms of higher trophic levels, including shellfish aquaculture. External nutrient supply is crucial for phytoplankton primary production. Submarine groundwater discharge (SGD) as well as riverine and oceanic nutrients are now recognized as one of the most important nutrient sources for phytoplankton production in coastal ecosystems. In this study, we evaluated seasonal variation in the primary productivity of phytoplankton and the effects of environmental factors on it at a shellfish aquaculture farm in Miyazu Bay, by using pulse amplitude modulation (PAM) fluorescence methodology, groundwater geochemical tracers, and traditional environmental assessment tools. Vertically integrated values of chlorophyll-a (iChl-a) and primary production (iPP), which was estimated by electron transport rate (ETR), were higher during the winter when nutrient supply from offshore water was dominant. However, Chl-a specific PP (iPP/iChl-a) and the ratio of ETR to maximum ETR showed that weaker solar radiation suppressed the photosynthetic activity of phytoplankton. Although the iChl-a and iPP decreased in the summer, photosynthetic activity was higher than in winter due to a stable nutrient supply from groundwater and favorable light conditions. Size fractionation revealed that smaller phytoplankton (<20um) showed more active photosynthesis than the larger ones (20-100um) in many months, suggesting their crucial role in primary production at the study site. The predominance of small phytoplankton in Miyazu Bay may also be explained by low nutrient levels and Chl-a concentrations. The study provides significant implications that the dominance of small, actively photosynthetic phytoplankton contributes significantly to the initial survival of cultured shellfish in the bay.