Recently, HU Qiwei, a doctoral student in our laboratory, published a paper "Effect of El Niño-related warming on phytoplankton's vertical distribution in the Arabian Sea" in the international journal Journal of Geophysical Research: Oceans, studying seasonal warming and the 2015/2016 El Niño event. On the impact on the vertical distribution of phytoplankton in the Arabian Sea, the corresponding author is researcher PAN Delu in our laboratory.
The Arabian Sea is one of the most productive and fastest-warming regions of the northern Indian Ocean. Studies have found that phytoplankton in the western Indian Ocean has declined by about 20% in the past 60 years due to rapid warming. However, the existing research is basically limited to the surface layer, and the impact of the significant sea surface warming trend on phytoplankton in the euphotic layer of this sea area is still unclear.
In this study, we used BGC-Argo data from 2015 to 2019 (Fig. 1) to analyze the effects of seawater warming caused by seasonal effects and the strong El Niño event in 2015/2016 on the euphotic layer (0- Effect of vertical distribution of phytoplankton within 100 m). The results show that phytoplankton in different depth layers have different responses to seawater warming (Figure 2). During spring (March-June) and the 2015/2016 El Niño warm event, phytoplankton biomass significantly decreased in the 0-50 m depth layer and increased significantly in the 50-100 m depth layer. In addition, due to the vertical expansion of the high temperature (>28°C) range and the joint limitation of nitrate and silicate, the formation of the subsurface chlorophyll maximum layer (SCML) in the study area was promoted, accompanied by SCML thickness becomes thinner and migrates downward. At the same time, we found that the 28°C isotherm on the vertical structure can approximately divide the vertical water body into upper and lower parts with completely opposite responses of phytoplankton to warming. In 0-50 m oligotrophic shallow seas, warming (>28°C) will lead to a decrease in phytoplankton due to a shallower mixed layer and reduced atmospheric deposition. However, for 50-100 m nutrient-rich deep sea areas, combined with enhanced light within a suitable temperature range (25.5-28.0 °C), warming of the seawater will promote the growth of phytoplankton. These results preliminarily reveal the impact of warming in the eastern Arabian Sea on subsurface phytoplankton, and help us more comprehensively understand the impact of global warming on tropical marine ecosystems.
Figure 1. (a) The Arabian Sea, the yellow trapezoidal area is the study area; (b) annual average sea surface Chla distribution in the study area; (c) annual average BGC-Agro Chla profile distribution in the study area; (d) Chla climate state in the study area Vertical distribution characteristics.
Figure 2. The response process mechanism of vertical distribution of phytoplankton in the study area to seasons and the 2015/2016 El Niño warm event.
Hu, Q., Chen, X., He, X., Bai, Y.,Gong, F., Zhu, Q., & Pan, D. (2021). Effect of El Niño-related warming on phytoplankton's vertical distribution in the Arabian Sea. Journal of Geophysical Research: Oceans, 126, e2021JC017882.
