Water Mass Control on Phytoplankton Spatiotemporal Variations in the Northeastern East China Sea and the Western Tsushima Strait Revealed by Lipid Biomarkers

Abstract

Continental margin ecosystems in the western North Pacific Ocean are subject to strong climate forcing and anthropogenic impacts. To evaluate mechanisms controlling phytoplankton biomass and community structure variations in marginal sea‐open ocean boundary regions, brassicasterol, dinosterol, and C37 alkenones were measured in suspended particles in summer and autumn from 2012 to 2013 in the northeastern East China Sea and the western Tsushima Strait. In summer, the concentrations of brassicasterol (40–1535 ng/L) and dinosterol (4.2–94 ng/L) were higher in the southwest of Cheju Island, while C37 alkenones (0–30 ng/L) were higher in the south of Cheju Island. In autumn, brassicasterol (12–106 ng/L), dinosterol (2.4–21 ng/L), and C37 alkenones (0.7–7.0 ng/L) were higher in the southwest of Cheju Island and the western Tsushima Strait, and higher C37 alkenones also occurred in the Okinawa Trough. Correlation analysis of biomarkers and environmental conditions (temperature, salinity, and inorganic nutrient concentrations) clearly demonstrated that phytoplankton biomass and community structure variations can be well elucidated by water masses as indexed by temperature and salinity. High nutrients from the Changjiang River were the main cause of high biomass in summer, while nutrients from subsurface water were likely the key factor regulating phytoplankton biomass in open ocean water stations in autumn. This study indicates that mechanisms controlling phytoplankton biomass in marginal sea‐open ocean boundary regions should be classified by various water masses with different nutrient concentrations, instead of by geography.

Regulation of Spatial Changes in Phytoplankton Community by Water Column Stability and Nutrients in the Southern Yellow Sea

“…Dinoflagellates (27%), cryptophytes (>20%), and haptophytes (>20%) contributed more to the total abundance in the YSBCW region, whereas diatoms accounted for ~60% in other regions, particularly affected by CDW (Figure ). This result coincided with previous studies in the SYS based on pigment signatures (Li et al, ; Liu, Huang, Huang, et al, ), lipid biomarker (Bi et al, ; Wu et al, ), and microscopic observation (Liu, Huang, Zhai, et al, ; Tian & Sun, ). However, early studies have observed higher dominance of cyanobacteria (mainly Synechococcus ) in YSBCW region than in coastal waters (Le et al, ; Li et al, ; Liu, Huang, Huang, et al, ), but the present study did not find the same result (Figure ).…”

“…However, small‐celled groups (e.g., cryptophytes and coccolithophores) are dominant in the stable oligotrophic YSBCW region because of their lower nutrient requirements and higher nutrient diffusion rates (Finkel et al, ; Irwin et al, ). Such spatial variability of different taxonomic groups due to the limitation of nutrients and light was reported from previous studies in the CJE and adjacent waters (Bi et al, ; Jiang et al, ; Zhu et al, ). Similar effect of large river discharge and associated nutrients on phytoplankton community was reported in plumes of Mississippi (Chakraborty & Lohrenz, ), Amazon (Goes et al, ), and Mackenzie Rivers (Ardyna et al, ).…”

“…Furthermore, the leading edge of TWCW intruded northward and shoreward farther near the CJE (Zhou et al, ) but was blocked by the increased buoyant CDW and integrated into YS Mixed Water, resulting in a stable halocline with a strong stratification (Figure k). Such an environment off the CJE was conducive to phytoplankton growth (Bi et al, ; Jiang et al, ; Zhu et al, ) and caused bloom (mainly Chaetoceros , Ceratium , and Distephanus ; Figure h). Consequently, phytoplankton abundance was considerably higher in CDW than in other water masses (Figure c).…”

“…Marine phytoplankton in coastal seas contributes essential roles in ecosystem functions and is highly sensitive to physicochemical variations, particularly nutrients, light availability, turbulence, and temperature (Carreto et al, ; Cloern, , ; Glibert, ; Irwin et al, ). Previous studies have evidently revealed the importance of physicochemical properties associated with water masses in shaping spatial distribution of phytoplankton community in several marginal seas (Ardyna et al, ; Bi et al, ; Chakraborty & Lohrenz, ; Goes et al, ).…”

“…YSBCW governs large‐scale deeper water (Zhang et al, ), which prevents the vertical transport of deeper nutrients because of a strong thermocline (Wei et al, ), resulting in low algal biomass and the domination of oligotrophic‐adapted groups (e.g., coccolithophores and picophytoplankton) in upper water (Le et al, ; Li et al, ; Liu, Huang, Huang, et al, ; Luan et al, ; Wu et al, ). By contrast, diatoms and dinoflagellates are dominant in eutrophic CDW and YSCCW and frequently bloom near the Changjiang Estuary (CJE; Bi et al, ; Jiang et al, ; Liu, Huang, Huang, et al, ; Liu, Huang, Huang, et al, ; Zhu et al, ). Therefore, the spatial distribution patterns of phytoplankton community in the SYS may be strongly structured by nutrients, water column stability (stratification), and turbidity (light) under water mass movement despite apparent seasonal variability.…”