Seasonal and year-on-year variations in physicochemical properties (i.e. temperature, salinity, dissolved inorganic nutrient concentration), chlorophyll a (Chl-a) concentration, Chl-a size composition and abundance of microphytoplankton (Ͼ63 mm) assemblages were investigated at a neritic survey station in Sagami Bay, Kanagawa, Japan, from January 2001 to December 2009. These abiotic/biotic variables varied seasonally in an essentially similar way during the 9 year period. During spring blooms (February-May), the micro-size fraction (Ͼ20 µm) comprised a greater proportion of the total Chl-a, whereas during other periods the pico-and nano-size fraction (Ͻ20 µm) comprised a large portion. Larger diatoms (e.g. Eucampia zodiacus, Coscinodiscus spp.), which dominated the microphytoplankton during the initial-mid stage of spring blooms, were substituted by smaller ones (e.g. Chaetoceros spp., Pseudo-nitzchia pungens, Skeletonema spp.) during the final stage of spring blooms, and then these smaller diatoms continued to be dominant in summer. Dinoflagellates (e.g. Ceratium fusus, C. furca) increased their population densities after the decline of spring diatom blooms, maintained their abundance in spring-summer and became sporadically dominant in the microphytoplankton in summer. The deficiencies in concentration and molar ratio of Si, P and Si-P together in seawater in spring, especially in the photic zone, induce the final stage of spring blooms and lead to the variations in Chl-a concentration, Chl-a size composition and microphytoplankton abundance and species (size) composition. The year-onyear variations in Chl-a and abundance of microphytoplankton assemblages are correlated weakly with the temporal variations in physicochemical properties in relation to water conditions.

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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 −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.