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 −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.

Annual dynamics of eukaryotic and bacterial communities revealed by 18S and 16S rRNA metabarcoding in the coastal ecosystem of Sagami Bay, Japan

“…Cyanobacteria were relatively abundant in clusters 2 and 3-1, accounting for up to 61% and DBRDA ordination revealed that cyanobacteria (Synechococcus_ CC9902) were highly correlated with water temperature. The sporadic increase of cyanobacteria from summer to autumn is consistent with previous studies in the same Sagami Bay area (Mitbavkar et al 2009;Ara et al 2019). Cyanobacterial abundance and growth rate are dependent principally on the temperature (Chen and Liu 2010;Chen et al 2014;Wang et al 2020).…”

Observation of a coccolithophore <i>Gephyrocapsa oceanica</i> bloom in the temperate coastal waters of Sagami Bay, Japan

“…In particular, nitrate and phosphate were three and two times higher, respectively (Table 1). These nutrient conditions are favorable for phytoplankton growth, and microplankton such as diatom and dinoflagellate blooms have been previously reported in Sagami Bay (Satoh et al 2000, Ara et al 2019. Ogura & Sato (2001) concluded that the nitrate and nitrite concentrations at the time of the coccolithophore bloom in May 1995 were higher than May 1994, when dinoflagellates and diatoms were the dominant bloom-forming groups.…”

“…The study supports the hypothesis that G. huxleyi is a good competitor at low nitrogen concentrations, since these conditions limit diatom growth. In Sagami Bay, microplankton (mainly diatoms) caused an annual spring bloom from February to May (2008May ( -2013, when nutrient concentrations declined considerably (Ara et al 2019). Thus, although nutrients tend to deplete in Sagami Bay in May after spring blooms, our results indicated that the concentrations of nitrate and nitrite were relatively high in May 2020.…”

“…Sagami Bay is a typical temperate area that has been well-studied from an oceanographical viewpoint (e.g., Fujiki et al 2003, Miyaguchi et al 2006, Beak et al 2008, Tsuchiya et al 2013, Kuwahara et al 2015, Sugai et al 2016. Although annual diatom blooms and dinoflagellate blooms are known to occur in spring and summer, respectively (Satoh et al 2000, Beak et al 2009, Ara et al 2019, coccolithophore blooms are unique phenomena.…”