Effects of Miniaturization of the Summer Phytoplankton Community on the Marine Ecosystem in the Northern East China Sea

Su-Yeon Moon,Man-Ho Yoo,Kyung-Woo Park,Hyun-Ju Oh,Seok-Hyun Youn

doi:10.3390/jmse10030315

Abstract

After the construction of the Three Gorges Dam (Changjiang River), the northern East China Sea has been exposed to major environmental changes in the summer due to climate change and freshwater control. However, little is known regarding phytoplankton in this area. Here, we investigated differences in the summer phytoplankton-community structure as a consequence of marine-environment changes from 2016 to 2020. In the 2000s, the key dominant species in the summer phytoplankton community in the northern East China Sea were diatoms and dinoflagellates. In this study, however, nanoflagellates of ≤20 µm were identified as the dominant species throughout the survey period, with abundances ranging from 43.1 to 69.7%. This change in the phytoplankton-community structure may be ascribed to low nutrient concentrations in the area, especially phosphate, which was below the detection limit, seriously hampering phytoplankton growth. The relative contribution of picophytoplankton to the total chlorophyll a biomass was highest in the surface mixed layer with low nutrient concentrations. Spatially, higher percentages were observed along the east-side stations than the west-side stations, where nutrient concentrations were relatively high. Conclusively, decreased nutrients led to phytoplankton miniaturization. Accordingly, as the dominance of picophytoplankton increases, energy transfer is expected to decrease at the upper trophic level.

Highlights

The northern East China Sea is exposed to various currents depending on the season and exhibits singular seasonal fluctuations [1]
It is known to be a highly valuable fishing ground because of its high primary productivity [2,3,4]. In particular, it shows diverse water mass characteristics, with the surface layer affected by the freshwater flowing in from the coastal areas of China, the bottom layer affected by the cold deep water of the Yellow Sea, and the eastern part affected by the high temperature and salinity of the Kuroshio water
As a major nutrient source [8], the coastal waters of China that are adjacent to the Changjiang are rich in nutrients, with high primary productivity [9,10,11] and characteristics of an estuary dominated by Chinese coastal waters

Summary

Introduction

The northern East China Sea is exposed to various currents depending on the season and exhibits singular seasonal fluctuations [1]. It was reported that the primary productivity decreased by approximately 86% in waters adjacent to the Changjiang River after the construction of the Three Gorges Dam and that the phytoplankton-community structure in the East China Sea changed [34,35]. Phytoplankton communities in marine ecosystems are sensitive to environmental changes, resulting in noticeable changes in community compositions and standing stocks following changes in physical- and chemicalenvironmental factors. The results of this study and those of previous studies were compared to confirm the change in the marine environment in the northern East China Sea due to the effect of constructing the Three Gorges Dam and the effects on the pelagic ecosystems according to the changes in the phytoplankton community structure were investigated

Cruises and Sampling

Dissolved Inorganic Nutrients

Phytoplankton Abundances and Dominant Species

Picophytoplankton

Chl-a Size Fractionation

Data Analyses

Physical Environments

Dissolved Inorganic Nutrient Concentrations

Vertical

Phytoplankton Abundance and Dominant Species

Dominant phytoplankton taxa in the northern

Picophytoplankton Cell Abundances