Abstract
The relative importance of interannual environmental changes in shaping phytoplankton community structure remains unclear in the East China Sea (ECS), which is the largest marginal sea in the western North Pacific Ocean. Based on high-performance liquid chromatography (HPLC) analysis of a variety of photosynthetic pigments, we investigated spatiotemporal variations in the phytoplankton community in the northern East China Sea (NECS) from 2018 to 2020 to understand biomass and compositional responses to environmental conditions in a complex current system. Correlation heatmaps and generalized additive models (GAMs) were used to explore the variations in major phytoplankton groups and their relationships to temperature, salinity, depth, and nutrients. The results indicated that the phytoplankton community structure was significantly different between 2018/2019 and 2020. Under the conditions of high dissolved inorganic nitrogen (DIN) and low dissolved inorganic phosphate (DIP) concentrations in winter 2020, cryptophytes replaced diatoms as the dominant species and were observed for the first time in this study area. Dinoflagellates predominated throughout the euphotic zone in spring 2020, and statistical analysis showed that dinoflagellates tended to prevail at low DIP and high DIN conditions compared with diatoms. Summer was the only season with strong water stratification that reoccurred every year, and diatoms were mainly concentrated at nutrient-enriched 1% light depths in the summers of 2018/2019. However, a large amount of Changjiang River Diluted Water (CDW) from the surface and western part of the study area affected all sampling sites eastward to 126°E in 2020, allowing diatoms to be dominant at 100% and 30% light depths in the westernmost stations in the NECS. In autumn 2020, diatoms decreased significantly because of DIP-limited environmental conditions and were replaced by small phytoplankton. Under expected warming ocean scenarios with human-induced nutrient inputs, small phytoplankton will become dominant, while spring dinoflagellate blooms will occur more frequently in the NECS. International monitoring programs for marine ecosystems are currently needed in the ECS.
Highlights
Phytoplankton play an important role in both the global carbon cycle and marine ecosystems and are responsible for approximately 50% of global primary production today (Smith and Sakshaug, 1990; Sommer, 1994; Falkowski and Raven, 2013)
Compared with previous studies on the variations in phytoplankton community structure in this study area, (1) spatially, we studied three different light depths in the euphotic zone, (2) temporally, we studied all four seasons from 2018 to 2020, and (3) for different phytoplankton groups, we were not limited to the study of large phytoplankton and focused on cryptophytes, cyanobacteria, and prymnesiophytes, which were rarely considered in previous studies
The goal of this study was to (I) analyze the horizontal distributions of all environmental factors and chlorophyll-a concentrations based on seasonal observations from 2018 to 2020 in the Northern East China Sea (NECS), (II) find independent relationships between chlorophylla concentrations of all phytoplankton communities that were calculated by the CHEMTAX program and the environmental factors at all light depths using correlation heatmaps, and (III) establish generalized additive models (GAMs) for understanding the relative contributions of particular phytoplankton groups responding to the environmental factors
Summary
Phytoplankton play an important role in both the global carbon cycle and marine ecosystems and are responsible for approximately 50% of global primary production today (Smith and Sakshaug, 1990; Sommer, 1994; Falkowski and Raven, 2013). Global warming could cause a decrease in the abundance of the warm-temperate zooplankton species and could not be fully offset by warm-water species increasing in the same period, reducing the pressure on phytoplankton grazing which could destabilize the dynamics balance of zooplankton and phytoplankton (Cai and Tan, 2010). These could change result in frequent harmful algal blooms (HABs) formation and appearance in the ECS (Cai et al, 2016). Phytoplankton community structure variations in the Northern East China Sea (NECS) can be a useful indicator to detect potential responses of offshore ecosystems to anthropogenic changes in the ECS
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Free) 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
