Ecosystem responses to anthropogenic and natural forcing over the last 100 years in the coastal areas of the East China Sea

Abstract

Ecosystem changes of the coastal areas of the East China Sea (ECS) during the past 30 years have been characterized by increased productivity and increased occurrences of dinoflagellate red tides; both were commonly attributed to be caused by anthropogenic input of nutrients. However, climate-driven regime shifts were found in the Pacific, but they have not been observed in the ECS owing to the lack of continuous high resolution data. Hence, it is still a challenge to distinguish the influences of anthropogenic activities and climate changes on ECS shelf ecosystem. We compare phytoplankton biomarker (brassicasterol, dinosterol, and alkenones) data of five box cores from the coastal area of the ECS, which afford phytoplankton productivity and community structure records at decadal-scale resolution for the last 100 years. Phytoplankton productivity in all cores increased during the past 50 years, but community structure changes showed different trends. Relative diatom contribution revealed a decreasing trend during the past ca. 50 years in cores DH5-1 and CJ43 collected near the Changjiang Estuary, which were likely caused by increased N/Si and N/P ratios, providing conditions more favorable for dinoflagellates growth. In contrast, the relative diatom contribution increased in cores 32, 34, and DH6-3 further away and south of the Changjiang Estuary, suggesting limited influence of the Changjiang water. Instead, diatom increasing trend is likely caused by increased supply of phosphorus and silicate, as a result of larger Kuroshio intrusion flux and the corresponding strengthened coastal upwelling. Therefore, both anthropogenic activities and climate-driven coastal upwelling have contributed to increased productivities, but these two forcing mechanisms have resulted in contrasting community structure changes at different sites in the coastal area of the ECS.

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

“…Lipid biomarkers have provided a powerful approach to study past and present Earth ecosystems based on their unique qualities (Bianchi & Canuel, ). Lipid biomarkers can provide source‐specific information about ecosystem changes, and they have been used for the paleoreconstruction of phytoplankton productivity and community structure in both shelf sea and open ocean environments (Schubert et al, ; Xing et al, ; M. X. Zhao et al, ). Also, in modern ecosystems, lipid biomarkers as a semiquantitative approach have been used to reflect phytoplankton productivity (or biomass) and community composition (Hernandez et al, ; Li et al, ; Wu et al, ).…”

“…Lipid biomarkers have provided a powerful approach to study past and present Earth ecosystems based on their unique qualities (Bianchi & Canuel, 2011). Lipid biomarkers can provide source-specific information about ecosystem changes, and they have been used for the paleoreconstruction of phytoplankton productivity and community structure in both shelf sea and open ocean environments (Schubert et al, 1998;Xing et al, 2016;M. X. Zhao et al, 2006).…”

Air-sea interactive forcing on phytoplankton productivity and community structure changes in the East China Sea during the Holocene

“…A recent study in the ECS also validated the applicability of brassicasterol, dinosterol, and C 37 alkenones as proxies of productivity and community structure of the three phytoplankton taxa: diatoms, dinoflagellates, and coccolithophores (Wu et al, 2016a). Therefore, community structures with relative contributions of diatoms, dinoflagellates and coccolithophores to phytoplankton productivity can be represented by brassicasterol/Sum (B/Sum, Sum = ∑brassicasterol +dinosterol+alkenones), dinosterol/Sum (D/Sum) and alkenones/ Sum (A/Sum), respectively (Schubert et al, 1998;Xing et al, 2016). Brassicasterol/dinosterol ratio (B/D) is used to indicate the relative contribution of diatoms compared to dinoflagellates (Duan et al, 2014;Xing et al, 2016).…”

“…Therefore, community structures with relative contributions of diatoms, dinoflagellates and coccolithophores to phytoplankton productivity can be represented by brassicasterol/Sum (B/Sum, Sum = ∑brassicasterol +dinosterol+alkenones), dinosterol/Sum (D/Sum) and alkenones/ Sum (A/Sum), respectively (Schubert et al, 1998;Xing et al, 2016). Brassicasterol/dinosterol ratio (B/D) is used to indicate the relative contribution of diatoms compared to dinoflagellates (Duan et al, 2014;Xing et al, 2016). In the ECS, water mass properties (temperature, salinity, and inorganic nutrient) were important factors controlling phytoplankton biomass and community structure spatial variations (Wang and Cheng, 1988;Bi et al, 2018).…”

“…Thus the intensified circulation system of the YSWC and the YSCC can induce the upwelling of nutrients to surface layer (Li et al, 2014 and references therein), although the enhanced EAWM somehow might weaken the strength of the KC (Jian et al, 2000). Biomarker records reveal that the ecosystem change has been driven by both natural (Pacific Decadal Oscillation) and anthropogenic forcing (terrestrial nutrient) over the last 100 years in the coastal areas of the ECS (Xing et al, 2016). However, lack of long-term and spatially-resolved records of phytoplankton productivity and community structure in the region hinders the understanding of ecosystem variation and its forcing mechanisms.…”