Seasonal variability in phytoplankton carbon biomass and primary production, and their contribution to particulate carbon in the neritic area of Sagami Bay, Japan

抄録

Seasonal variations in environmental variables, chlorophyll a (Chl-a), particulate carbon and nitrogen (PC and PN, respectively), phytoplankton carbon biomass (Ph-C) and primary production were investigated at a neritic station in Sagami Bay, Kanagawa, from January 2008 to December 2013. Size-fractionated Ph-C was converted from cell volume by microscopic observation, adding valuable data for this area. During spring blooms, the micro-size fraction (>20 µm) comprised the majority of the total Chl-a and total Ph-C, whereas during other periods the pico- and nano-size fraction (<20 µm) comprised a larger proportion, indicating that phytoplankton standing crops were affected by sunlight conditions and physicochemical properties of the water. In February–March, phytoplankton biomass increased and formed the first peak of spring blooms under increasing sunlight intensities (>15.7 MJ m⁻² d⁻¹), high nutrient concentrations and balanced molar ratios. From the regression equations of size-fractionated Ph-C-Chl-a relationships, the mean Ph-C/Chl-a ratio was 5.3–7.7, 29.2–32.6 and 22.1–25.1 for the <20 µm, >20 µm and total fraction, respectively. The Ph-C/Chl-a ratio (1.8–128.8) was regulated by irradiance and nutrients. Growth rate (ca. 0–3.7 d⁻¹) was positively correlated with irradiance and assimilation number, and negatively with the Ph-C/Chl-a ratio. The depth-integrated primary production (DIPP) was 0.15–5.43 g C m⁻² d⁻¹. On the basis of the 0–50 m depth-integrated values, the total Ph-C and DIPP accounted for 1.3–34.4% and 1.3–30.9% d⁻¹ of PC, respectively, indicating that PC variations depended on the total Ph-C and DIPP.