Impact of ENSO events on phytoplankton over the Sulu Ridge
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)
- et al.
Phytoplankton assemblages respond differently to climate warming and eutrophication: a case study from Pyhäjärvi and Taihu
J. Great Lake. Res.
(2016) - et al.
Population synchronies within and between ocean basins: apparent teleconnections and implications as to physical–biological linkage mechanisms
J. Mar. Syst.
(2010) - et al.
Physical and biological response of the Arabian Sea to tropical cyclone Phyan and its implications
Mar. Environ. Res.
(2011) - et al.
Episodic phytoplankton bloom events in the Bay of Bengal triggered by multiple forcings
Deep Sea Res. Pt I
(2013) - et al.
Chlorophyll-a variability in the Seychelles-Chagos Thermocline Ridge: analysis of a coupled biophysical model
J. Mar. Syst.
(2016) - et al.
Carbonate parameters in high and low productivity areas of the Sulu Sea, Philippines
Mar. Chem.
(2017) - et al.
Paleoecological studies on variability in marine fish populations: a long-term perspective on the impacts of climatic change on marine ecosystems
J. Mar. Syst.
(2010) - et al.
Seasonal surface ocean circulation and dynamics in the Philippine Archipelago region during 2004–2008
Dynam. Atmos. Oceans
(2009) - et al.
Nutrient and phytoplankton dynamics off the west coast of Vancouver Island during the 1997/98 ENSO event
Deep-Sea Res. Pt II
(2009) - et al.
Mesozoic and cainozoic rocks dredged from the south China sea (reed bank area) and Sulu Sea and their significance for plate-tectonic reconstructions
Mar. Petrol. Geol.
(1986)