Seasonal and year-on-year variability in chlorophyll a and microphytoplankton assemblages for 9 years (2001–2009) in the neritic area of Sagami Bay, Japan

Abstract

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 (>63 μm) 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 (>20 µm) comprised a greater proportion of the total Chl-a, whereas during other periods the pico- and nano-size fraction (<20 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-on-year variations in Chl-a and abundance of microphytoplankton assemblages are correlated weakly with the temporal variations in physicochemical properties in relation to water conditions.