Spatiotemporal Variation in Phytoplankton Community Driven by Environmental Factors in the Northern East China Sea

Abstract

The East China Sea (ECS) is the largest marginal sea in the northern western Pacific Ocean. In comparison to various physical studies, little information on the seasonal patterns in community structure of phytoplankton is currently available. Based on high performance liquid chromatography (HPLC) pigment analysis, spatiotemporal variations in phytoplankton community compositions were investigated in the northern ECS. Water temperature and salinity generally decreased toward the western part of the study area but warmer conditions in August led to strong vertical stratification of the water column. In general, major inorganic nutrient concentrations were considerably higher in the western part with a shallow water depth, and consistent with previous results, had no discernable vertical pattern during our observation period except in August. This study also revealed PO4-limited environmental conditions in May and August. The monthly averaged integral chlorophyll-a concentration varied seasonally, highest (35.2 ± 20.22 mg m−2) in May and lowest (5.2 ± 2.54 mg m−2) in February. No distinct vertical differences in phytoplankton community compositions were observed for all the sampling seasons except in August when cyanobacteria predominated in the nutrient-deficient surface layer and diatoms prevailed at deep layer. Canonical correlation analysis results revealed that nutrient distribution and the water temperature were the major drivers of the vertical distribution of phytoplankton communities in August. Spatially, a noticeable difference in phytoplankton community structure between the eastern and western parts was observed in November with diatom domination in the western part and cyanobacteria domination in the eastern part, which were significantly (p < 0.01) correlated with water temperature, salinity, light conditions, and nutrient concentrations. Overall, the two major phytoplankton groups were diatoms (32.0%) and cyanobacteria (20.6%) in the northern ECS and the two groups were negatively correlated, which holds a significant ecological meaning under expected warming ocean conditions.

Interannual Variation in Phytoplankton Community Driven by Environmental Factors in the Northern East China Sea

“…Cryptophytes contribute to total chlorophyll-a concentrations and are mainly distributed in coastal areas of the ECS (Gao and Song, 2005). However, in recent studies, cryptophytes were found in the entire ECS (Liu et al, 2016) and were confirmed as one of the main contributors in autumn and winter 2018 (Kim et al, 2020). In this study, we first observed that cryptophytes replaced diatoms in the NECS and were the dominant species in autumn and winter 2020.…”

“…These water masses provide most of the nutrients for the growth of phytoplankton in the NECS (Furuya et al, 2003;Chen, 2009).Therefore, separate analyses after dividing water masses according to temperature and salinity, have become mainstream in previous studies in the ECS Yoon et al, 2015;Liu et al, 2018;Xu et al, 2019). This approach led to ideal results for short-term seasonal variations in phytoplankton community structure (Liu et al, 2016;Kim et al, 2020) and the long-term interannual variations in summer when the water mass was well stratified (Xu et al, 2019). However, the different water mass-based analyses were not applicable to this study because in all of the seasons, except summer, the water column was mixed during this study period from 2018 to 2020.…”

“…to the laboratory immediately after the cruise. Acetone-extracted phytoplankton pigments were separated and characterized by the method described in Kim et al (2009Kim et al ( , 2020 and Kang et al (2018). The pigment analysis was completed within 1 month after the cruise.…”

“…Then, 60 completed runs were conducted one more time (S = 0.4), and the average value of the smallest RMS error's 6 ratio matrices from the result was used for phytoplankton group calculation (Wright and Jeffrey, 2006). As the seasonal and annual variation in phytoplankton communities in the East China Sea is considerable (Xu et al, 2019;Kim et al, 2020), CHEMTAX was run on each dataset according to the different seasons and years for our samples to obtain the 'best' ratio matrix.…”

“…Previous studies on the phytoplankton community in the ECS have focused on the Changjiang (Yangtze) River estuary and adjacent waters (Furuya et al, 2003;Zhou et al, 2008;Guo C. et al, 2014;Liu et al, 2016) but have been limited in the NECS. Several previous studies on phytoplankton community structure in the NECS focused primarily on short-term seasonal variations (Yoon, 2003;Park et al, 2008;Kim et al, 2020) or long-term interannual variations in summer when water mass stratification is well established (Xu et al, 2019). To date, no studies have been performed on the interannual variations in phytoplankton communities encompassing the entire euphotic zone in all seasons in the NECS.…”