Potential impact of the extensive oil spill on primary productivity in the Red Sea waters

Highlights

• A significant increase in Chlorophyll-a (Chl-a) along the track of oil spill in Red Sea waters.

• Formation of Chl-a bloom is influenced by hydrocarbons released from oil spill.

• Both wind forcing and Ekman pumping velocity support the upwelling of nutrient-rich waters.

Abstract

Red Sea is one of the world's major strategic shipping routes for crude oil transportation. Accidental spills of crude oil into the environmentally sensitive marine environments are known to impact the marine life. However, studies on direct assessment of oil spill on marine ecosystem, in particular on the primary productivity and bio-optical parameters in the Red Sea waters were poorly constrained owing to lack of sustained observations. Thus, in this study, an attempt has been made to use high spatial and temporal resolution of Moderate Resolution Imaging Spectroradiometer (MODIS) observations to monitor the oil spill explosion occurred on October 11, 2019, and its impact on the chlorophyll -a (Chl-a) concentrations. We noticed a significant increase in concentrations of Chl-a on the following day of oil spill. These changes are accounted for and are influenced by hydrocarbons along the track of oil spill. Our findings suggest that both wind speed and high Ekman pumping velocity support the upwelling of nutrient-rich water from subsurface, which facilitated the growth of phytoplankton and thus the high Chl-a in the oil spill areas.

Introduction

Oil spill in oceanic environment is extremely dangerous to the marine and coastal ecosystem, as they pose serious threat to the marine population, contaminating beaches, estuaries, coral reefs, and mangroves (Escobar, 2019; Magris and Giarrizzo, 2020). Natural hydrocarbon seeps, deep water drilling explosions, pipeline break, accidental spill of crude or refined oil from tanker ships, and collisions during transportation are the major sources of oil pollution in the marine environment (Jernelöv, 2010; Fingas, 2016). Presence of oil in water can hinder marine phytoplankton from photosynthesis, thereby reducing the dissolved oxygen content, changing the water temperature, and other constituent particles (MacDonald et al., 2002; Yang et al., 2020). Physical and chemical disturbance of oil in seas not only pollute the ecosystem, but also contaminating the marine food chain in the euphotic zone (Graham et al., 2010). In addition, oil spill ruins industrial tourism and fishing that lead to affect millions in economic losses (Loureiro et al., 2006; English et al., 2018).

Large oil spills are often resulting in dangerous disasters. For example, the 2010 Deepwater Horizon ‘British Petroleum’ oil spill caused by a drilling rig explosion was considered a mega-disaster by environmentalists (Beyer et al., 2016; Romero et al., 2017). The explosion that happened on April 10, 2010 was the largest oil spill in U.S. history, and leaked about 5 million barrels of oil into the Gulf coast (Liu et al., 2011a), killing several endangered species such as whales, turtles, and migratory birds. Another infamous case is the ‘Prestige’ tanker accident in 2002 which spilled over 60,000 MT of oil into Finisterra Cape, causing pollution of 1300 km of the Galician coast, causing a total of 557 million euros in financial losses apart from various environmental issues (Loureiro et al., 2006). The detection and monitoring of oil spill and slicks tracks, clean up the oil, and measure the impacts of pollution are therefore important factors for analyzing the marine pollutants in the coastal environment (Al-Ruzouq et al., 2020). Driving factors such as the wind speed and surface currents are resulting in the shape and thickness of oil spills into patches and spread up to hundred meters in width and kilometers long. For instance, a large anticyclonic eddy broke the loop current system in Gulf of Mexico, thus helped retain the surface oil in the northeastern Gulf and kept an oil-free environment for most of Florida's coastal waters during the Deepwater Horizon Oil Spill Event (Liu et al., 2011b). The spills are dissipating gradually through physical and chemical processes such as weathering, spreading, dissolution, flocculation, and evaporation over subsequent period of time (Garcia-Pineda et al., 2010).

Growing global demand for oil and gas, as well as exploration and production operations into coastal and offshore areas increases the frequency of accidental oil discharge into the marine environment (Harfoot et al., 2018). About 13% of oil spill accidents and pollution around the world are caused during the transportation phase (Fingas and Brown, 2014). Red Sea is one of the most important strategic shipping routes for oil transportation because of the large production of crude oil in the vicinity areas. Increased oil production, refining and transport in these areas have resulted in frequent oil spills and chronic pollution in the Red Sea environment (Kostianaia et al., 2020). In ecological terms, Red Sea is an environmentally sensitive area with extensive areas of shallow, fringing coral reefs and sand shelves, and any oil slicks may adversely affect its coastline, marine resources, and the coral reefs. A large number of cases of oil pollution in the Red Sea region were reported since the 1970s. Yet the studies that have focused on responses of primary productivity to the oil spill in short and – long term evaluation is still lacking.

Typically, water quality parameters and their relationships are monitored by point observations which provide detailed information on the area of interest with accurate results. However, such measurements have the limitations of temporal and spatial coverage, as well as restricted by different weather conditions, time and cost. On the other hand, observations from remote sensing techniques are very helpful for mapping of oil spills and chlorophyll (Chl-a), a proxy for the phytoplankton biomass in the oceans (Sravanthi et al., 2017; Tang et al., 2019). Satellite remote sensing data have the advantage of providing continuous coverage of synoptic images in a short interval of time (Singha et al., 2013; Ali et al., 2015; Nukapothula et al., 2018). Previous studies have utilized both the optical and microwave remote sensing sensors for the detection and mapping of oil spills (Lennon et al., 2006; Adamo et al., 2009; Fingas and Brown, 2011; Chaturvedi et al., 2019). While airborne sensors have high spectral and spatial coverage; however, these images are expensive. On the other hand, coarse spatial and medium sun-synchronous resolution optical remote sensing satellites can have limited utility of technology (Hu et al., 2011). Both airborne and satellite remote sensing sensors are essential to select cloud free conditions and efficient atmospheric conditions for mapping appropriate estimations of oil spill evolution in time (Espedal and Johannessen 2000). Microwave remote sensing, the SAR technology has proven for mapping oil spills and are not constrained by cloud coverage with higher sensitivity to oil spills (Prastyani et al., 2018; Fiscella et al., 2000). During the Exxon Valdez oil spill in 1989, a single image of cloud free data was used to locate, contain, and recover free oil on the open water (Noerager and Goodman, 1991). Although microwave remote sensing is popular for locating oil spill, but their longer revisit time (e.g., freely available Sentinel-1) does not cover the variability of oil spills and primary productions.

Fortunately, the number of optical remote sensing sensors in orbit around the Earth system observations such as moderate imaging spectraoradiometer (MODIS) is usually available with medium spatial resolution and shorter revisit interval of time i.e., daily data products can provide the information and monitoring on both oil spill features (Hu et al., 2003; Lacava et al., 2017) and bio-physical parameters (Sravanthi et al., 2013; Kim et al., 2017; Lei et al., 2020). The effect on oil spill on chlorophyll-a (Chl-a) have been studied previously to understand the impacts of oil spills (D'souza et al., 2016). The Chl-a concentrations are used as a proxy for estimations and mapping of phytoplankton biomass in marine environments (McKee et al., 2007). While the impacts of oil spills on the overall marine environment are discussed widely, but primary productivity, in particular phytoplankton population dynamics has been a subject of debate for some time (Li et al., 2019). Contrasting results in phytoplankton response to oil spills are reported by researchers (Lee et al., 2009; D'souza et al., 2016).

The aim of this study was to evaluate whether the Chl-a concentration was impacted by the oil spills in the Red Sea region (Fig. 1). We used the case of an Iranian oil tank disaster that happened on October 11, 2019, about 95 km off the coast of Jeddah, Saudi Arabia to understand the variability of aforementioned bio-physical properties and their relationship with the oil spill disaster. Firstly, we detected the oil spill path using MODIS Terra and Sentinel-1 imagery. Then the long-term variability and spatiotemporal variations of the MODIS Chl-a were evaluated. Finally, the detection of oil spill path from MODIS imagery on board Terra satellite and its impact of Chl-a and sea surface temperature (SST) are discussed. The wind speed and direction data products were used to understand the patterns of oil presence and pathways are inferred from satellite imagery.