Effect of El Niño‐Related Warming on Phytoplankton's Vertical Distribution in the Arabian Sea
Qiwei Hu, Xiaoyan Chen, Xianqiang He, Yan Bai, Fang Gong, Qiankun Zhu, Delu Pan
Abstract
Abstract In this study, we analyzed the effect of warming in ecological responses due to seasonal cycle and 2015/2016 El Niño event on the vertical distribution of phytoplankton within the entire euphotic zone (0–100 m) in the eastern Arabian Sea using BGC‐Argo data during 2015–2019. We found that phytoplankton in different layers had different responses to warming. Phytoplankton biomass (defined as the integrated chlorophyll a concentration in this paper) decreased in 0–50 m layer while it increased in 50–100 m layer during spring (March‐June) and 2015/2016 El Niño event. At the same time, the vertical expansion of warming (temperature >28 °C), together with nitrate and silicate co‐limitation, triggered the formation of subsurface chlorophyll maximum layer, which became thinner and deeper during these warming periods. Results from the generalized additive models showed that the monthly 28 °C isotherm can be used as a first approximation to separate vertical water column where warming had opposite signed effects. For a 0–50 m layer of poor nutrients due to the mixed layer shoaling and reduced atmospheric deposition, a negative effect of warming on phytoplankton was seen if the temperature was higher than 28 °C. But, for a 50–100 m layer of rich nutrients, a positive effect of warming on phytoplankton was seen if the temperature was within the range of 25.5–28.0 °C and the light was enhanced. These results shed light on the effect of global warming on subsurface phytoplankton and may help research of the same topic regarding future warming.