Impact of ENSO events on phytoplankton over the Sulu Ridge

https://doi.org/10.1016/j.marenvres.2020.104934Get rights and content

Highlights

  • El Niño promotes Chla in the Sulu Ridge, whereas La Niña reduces Chla in winter.

  • Enhanced Chla during El Niño is mainly caused by Ekman pumping.

  • Reasons for the Chla reduction during La Niña differ for different seasons.

Abstract

Phytoplankton response to interannual climate variability has an important regulatory effect on the regional marine ecological environment and carbon cycle. In this study, we focused on the phytoplankton response in the upwelling region of the Sulu Ridge to the El Niño-Southern Oscillation (ENSO) based on monthly remote sensing chlorophyll-a concentration (Chl-a) and physical parameters from various sources from September 1997 to December 2017. We selected two El Niño events in 1997/1998 and 2015/2016 and two La Niña events in 1998/1999 and 2010/2011 to examine the response of Chl-a to ENSO events in this region. Results showed that El Niño and La Niña could enhance and inhibit the growth of phytoplankton in the Sulu Ridge in winter, respectively. For other seasons, the influence of ENSO on the Chl-a was inconsistent. Specifically, during El Niño events, the largest Chl-a increases occurred in winter, and the low sea surface temperature (SST) center appeared northwest of Sulu Ridge. The significant decrease of SST (~1.5 °C) during El Niño events in winter in the northeastern Sulu Ridge was mainly caused by the increase in Ekman transport (ET) and Ekman pumping velocity (EPV), which brought nutrient-rich subsurface water to the surface layer through the thin barrier layer and enhanced Chl-a. During La Niña events, the SST was higher (~0.8 °C) than the average and the high SST center generally appeared in the middle of the Sulu Ridge with the east-west direction in winter, which was resulted from the intensification of barrier layer thickness (BLT) and the decrease of ET, thus reducing the Chl-a. The different responses to El Niño and La Niña events indicate the high sensitivity of Chl-a in this region to the ENSO.

Introduction

Phytoplankton plays an important role in the marine carbon cycle and ecological change (Broecker and Peng, 1982; Sarmiento and Gruber, 2006). The main photosynthetic pigment contained in phytoplankton, chlorophyll-a concentration (Chl-a) is often used as an indicator of phytoplankton biomass (Tang et al., 1999). Therefore, spatial and temporal changes in Chl-a are crucial for studies on the marine ecological environment (Behrenfeld et al., 2001; Thomalla et al., 2011; Louw et al., 2016). Chl-a is highly sensitive to climate variations at different scales (Trenberth and Hoar, 1997; Philander and Fedorov, 2003; Mcphaden et al., 2006), including typhoons (Chen et al., 2013; He et al., 2014), precipitation (Taylor and Ferrari, 2011; Thompson et al., 2015), short-term climate variabilities (e.g., El Niño-Southern Oscillation (ENSO), Indian Ocean Dipole (IOD)) (He et al., 2013; Sarma et al., 2015; Vidya and Kurian, 2018), and long-term global warming (Kumar et al., 2009; Deng et al., 2016; Wagner et al., 2016). ENSO is an important coupled ocean-atmosphere phenomenon with considerable impact on Chl-a. Previous studies have shown that Chl-a in regional oceans varies greatly during ENSO events (Yoder and Kennelly, 2003; Loeb et al., 2009; Eichel et al., 2014; Zhang et al., 2018). For instance, the very strong El Niño event in 1997–1998 caused an abrupt decrease of Chl-a in the West Bank of Vancouver Island in 1997 (Harris et al., 2009); the 2015/2016 El Niño event resulted in a weakening of winter convection and mixing in the northeastern Arabian Sea, with a reduction in Chl-a of approximately 50% (Vidya and Kurian, 2018).

Upwelling bring low-temperature and high-nutrient water from deep layers to the surface, thus promoting the growth of phytoplankton (Alheit and Bakun, 2010; Finney et al., 2010; Zhao et al., 2018). However, ENSO can alter the intensity and direction of the wind field (Byju and Prasanna Kumar, 2011), leading to changes in upwelling intensity and barrier layer thickness (BLT) (Du et al., 2004; Wang and Liu, 2016), which can, in turn, affect phytoplankton biomass (Rao and Behera, 2005; McCreary et al., 2009; Dilmahamod et al., 2016). For example, in summer 1998 during a decaying period of a strong El Niño, upwelling along the east coast of Vietnam almost disappeared due to the weak southwest monsoon, causing the Chl-a to be lower than that in normal years. On the other hand, the upwelling intensity along the north coast of the South China Sea increased significantly due to the intensification of coastal wind stress, resulting in a higher Chl-a in the corresponding area than that in normal years (Jing et al., 2011).

The Sulu Ridge is part of the islands in Mindanao, Philippines, and is located between the Sulu Sea and Celebes Sea, southeast of the South China Sea (Fig. 1). Takeda et al. (2007) conducted a systematic analysis of field observations and seawater samples in November 2002 and found that the Chl-a was 5–10 times higher (>0.5 mg/m³) within the Sulu Ridge than that in the surrounding basins. Jing et al. (2012) revealed that the Sulu Ridge is a persistent upwelling area dominated by tides, which is also affected by local winds. Moreover, they reported that the Chl-a in the Sulu Ridge decreased significantly in summer 1998 and 2010 during decaying periods of El Niño events.

So far, limited studies have been conducted on the effects of the ENSO on the biomass of phytoplankton in the Sulu Ridge. The occurrence of a strong El Niño event during 2015/2016 provided a good opportunity to study the impact of ENSO events on phytoplankton in the Sulu Ridge. In this study, satellite-derived long-term Chl-a and physical parameters from various sources from September 1997 to December 2017 were used to analyze the responses of the Chl-a in the Sulu Ridge to the ENSO events.

Section snippets

Study area

This study focuses on the Sulu Ridge (also known as the Sulu Archipelago) as the oblique rectangle in Fig. 1, which consists of volcanoes and coral islands and is 150–200 km wide, extending from the southwestern part of Mindanao to the northeastern part of Borneo (Rangin and Silver, 1990). The climate of the adjacent waters is dominated by seasonal monsoons, with the dry season occurring from January to April, and the rainy season occurring from May to October (Kudrass et al., 1986).

Data

The level-3

Spatiotemporal variation of Chl-a

The seasonal climatological data for Chl-a, which were constructed by averaging all the seasonal data from autumn 1997 to summer 2017, are shown in Fig. 3. We defined spring from March to May, summer from June to August, autumn from September to November, and winter from December to February (of the following year). The Chl-a in most regions of the Sulu Ridge was always higher than that in the Sulu Sea and Celebes Sea in any season, which is mainly induced by the persistent upwelling in the

Response of Chl-a changes to strong El Niño events

Generally, nutrients rather than light are the dominant controlling factor that limits phytoplankton growth in tropical oceans (Chen et al., 2013). Previous research has indicated that the seasonal surface current in the Philippine Archipelago (including Sulu Ridge) showed strong changes or reversals from January 2004 to May 2008 (Han et al., 2009). Furthermore, in winter (summer), the cyclonic (anticyclonic) circulation in the Sulu Sea and the seasonal countercurrent in the archipelago are

Conclusions

Based on monthly remote sensing Chl-a and multiple physical parameters from various sources from September 1997 to December 2017, we investigated the response of phytoplankton to ENSO events in the upwelling region of the Sulu Ridge. Two El Niño events in 1997/1998 and 2015/2016 and two La Niña events in 1998/1999 and 2010/2011 were selected. Our results showed that El Niño could enhance the growth of phytoplankton while La Niña could inhibit the growth of phytoplankton in the Sulu Ridge in

CRediT authorship contribution statement

Xiaoyan Dang: Writing - original draft. Xiaoyan Chen: Writing - original draft, Writing - review & editing. Yan Bai: Conceptualization, Methodology, Writing - original draft, Writing - review & editing. Xianqiang He: Conceptualization, Methodology, Writing - original draft, Writing - review & editing. Chen-Tung Arthur Chen: Conceptualization, Writing - original draft, Writing - review & editing. Teng Li: Writing - original draft, Methodology, Writing - review & editing. Delu Pan:

Declaration of competing interest

The authors declared no interest conflicts.

Acknowledgments

This research was supported by the National Key Research and Development Program of China (Grant #2019YFD0901402), the Southern Marine Science and Engineering Guangdong Laboratory (Zhanjiang)(Zhanjiang Bay Laboratory) (Grant #ZJW-2019-08), the National Natural Science Foundation of China (Grants #41676172, #41676170, #41825014), the Public Science and Technology Research Funds Projects for Ocean Research (Grant #201505003), and the Global Change and Air-Sea Interaction Project of China (Grants #

References (49)

  • S.P. Kumar et al.

    Response of the Arabian Sea to global warming and associated regional climate shift

    Mar. Environ. Res.

    (2009)
  • D.C. Louw et al.

    Seasonal and interannual phytoplankton dynamics and forcing mechanisms in the Northern Benguela upwelling system

    J. Mar. Syst.

    (2016)
  • S.A. Rao et al.

    Subsurface influence on SST in the tropical Indian Ocean: structure and interannual variability

    Dynam. Atmos. Oceans

    (2005)
  • V.V.S.S. Sarma et al.

    Impact of river discharge on the coastal water pH and pCO2 levels during the Indian Ocean Dipole (IOD) years in the western Bay of Bengal

    Continent. Shelf Res.

    (2015)
  • S. Takeda et al.

    Biological and chemical characteristics of high-chlorophyll, low-temperature water observed near the Sulu Archipelago

    Deep Sea Rese. Pt II

    (2007)
  • P.A. Thompson et al.

    Precipitation as a driver of phytoplankton ecology in coastal waters: a climatic perspective

    Estuarine, Estuar. Coast. Shelf S.

    (2015)
  • P.J. Vidya et al.

    Impact of 2015–2016 ENSO on the winter bloom and associated phytoplankton community shift in the northeastern Arabian Sea

    J. Mar. Syst.

    (2018)
  • H. Wagner et al.

    Title: freshwater phytoplankton responses to global warming

    J. Plant Physiol.

    (2016)
  • J. Wang et al.

    An analysis of the characteristics of chlorophyll in the Sulu Sea

    J. Mar. Syst.

    (2006)
  • M. Zhang et al.

    Spatiotemporal evolution of the chlorophyll a trend in the north atlantic ocean

    Sci. Total Environ.

    (2018)
  • M.J. Behrenfeld et al.

    Biospheric primary production during an ENSO transition

    Science

    (2001)
  • W.S. Broecker et al.

    Tracers in the Sea

    (1982)
  • P.C. Chu et al.

    Evidence of a barrier layer in the Sulu and Celebes seas

    J. Phys. Oceanogr.

    (2002)
  • Y. Du et al.

    Seasonal variation of the barrier layer in the south China Sea and its relationship to the sea surface flux

    J. Atmos. Sci.

    (2004)
  • Cited by (0)

    View full text