High-Resolution Vertical Observations of Phytoplankton Groups Derived From an in-situ Fluorometer in the East China Sea and Tsushima Strait

Qian Xu,Jutarak Luang-On,Takeshi Matsuno,Yuji Watanabe,Yongjiu Xu,Chiho Sukigara,Yuanli Zhu,Helga Do Rosario Gomes,Joji Ishizaka,Sinjae Yoo,Joaquim I Goes,Shengqiang Wang

doi:10.3389/fmars.2021.756180

Abstract

Vertical distribution of phytoplankton composition in the East China Sea (ECS) and Tsushima Strait (TS) was highly variable in the region where the Changjiang River diluted water (CDW), Kuroshio water (KW), and Tsushima water (TW) intersected. An in-situ multiple excitation fluorometer was used to obtain the high-resolution phytoplankton groups data from every meter of the water column. Sharp differences were noted in the distribution of phytoplankton groups in the CDW, KW, and TW. In the CDW, brown algae were generally present ~60% of all depths with exception of subsurface chlorophyll-a maximum (SCM), whereas cyanobacteria (&gt;40%) and green algae plus cryptophytes (&gt;40%) were found above and below the SCM, respectively. In TW, where chlorophyll a (CHL) was lower than in the CDW, brown algae predominated the water column (&gt;60%) and SCM (&gt;80%), except the surface layer where cyanobacteria dominated. In KW, a high fraction of cyanobacteria (&gt;40%) extended up to 40 m, while brown and green algae dominated (&gt;60%) the deeper waters below 40 m at western and eastern stations, respectively. These results can be further related to water property and nutrient concentration of the water masses in each region. This new data show that the in-situ multiple excitation fluorometer can be a powerful tool to estimate high-resolution vertical profiles of phytoplankton groups on a large scale in marine environments.

Highlights

Phytoplankton communities determine the structure and function of marine ecosystems and changes in phytoplankton community structure in response to physical and chemical environments that have direct effects on higher trophic levels and biogeochemical cycling in the ocean (Simpson and Sharples, 2012; Goes et al, 2014)
We used a newly developed multiple excitation fluorometer to estimate the vertical distribution of phytoplankton groups in the East China Sea (ECS) and Tsushima Strait (TS)
Compared with the historical approach of pigment biomarkers derived using high-performance liquid chromatography (HPLC) separation of phytoplankton pigments, the in-situ fluorometer provided a very large dataset of high resolution, vertical profiles of phytoplankton groups, which allowed us to infer their relationship with different water masses

Summary

Introduction

Phytoplankton communities determine the structure and function of marine ecosystems and changes in phytoplankton community structure in response to physical and chemical environments that have direct effects on higher trophic levels and biogeochemical cycling in the ocean (Simpson and Sharples, 2012; Goes et al, 2014). Satellite-based ocean color images reveal the large seasonal and spatial variability of chlorophyll-a (CHL) concentrations in the ECS in association with the movement of the CDW, with high phytoplankton concentrations (CHL > 60 mg m−3) observed in the Changjiang estuary and adjacent areas, but which is drastically reduced (

Results

Discussion

Conclusion