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

Abstract

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 nanosize 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-2d-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-2d-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�’1of PC, respectively, indicating that PC variations depended on the total Ph-C and DIPP.