Declining nutrient availability and metal pollution in the Red Sea

Since the 1870s, element accumulation rates (EARs) in the South Red Sea have notably dropped, encompassing nutrient elements, hinting at a diminished nutrient influx from the Indian Ocean. Conversely, the North Red Sea's EARs have increased, signaling a boost in human-related coastal activities.
Declining nutrient availability and metal pollution in the Red Sea

In 2018, I was fortunate to commence my Ph.D. studies at King Abdullah University of Science and Technology (KAUST) in Saudi Arabia. At that time, my supervisor, Prof. Susana Agusti, was in the process of planning to measure element concentrations in two sediment cores collected from the South and North of the Saudi Arabian Red Sea in 2017. Considering my background in heavy metal pollution research, she deemed me a suitable candidate to assist in the element analysis at KAUST's Analytical Chemistry Core Lab. By December 2018, after approximately two months of diligent work, we completed the laboratory analysis. Subsequently, I created scatter plots (Figure 1) to depict the concentrations of 21 elements. These included nutrient elements such as total organic carbon (TOC), total nitrogen (TN), and phosphorus, as well as heavy metals—or trace metals—like chromium, iron, and nickel, in the sediment cores.

Figure. 1: Scatter plots of element concentrations in the Red Sea sediment cores.

Initially, we observed that the sediment cores from the South Red Sea had significantly higher element concentrations than the North. Yet, due to my limited understanding of the Red Sea's characteristics, I was struggled to explain these latitudinal gradients. This changed when Prof. Susana Agusti highlighted that Cadmium (Cd), though not widely recognized as a bioactive trace element, it can stimulate the growth of marine phytoplankton in zinc-depleted seawater. Upon reexamining my data, I noted a positive correlation between Cd levels and nutrient elements like TOC, TN, and P, but a negative correlation with latitude. This led to an epiphany: the elevated nutrient concentrations could stem from the horizontal intrusion of nutrient-rich waters from the Indian Ocean, driven by wind, through the Gulf of Aden. Importantly, we also detected significantly higher concentrations of heavy metals in the South Red Sea sediment cores. These findings suggest that the Indian Ocean acts not only as a nutrient repository but also as a significant source of trace metals for the Red Sea.

At that moment, I considered our findings intriguing, though not groundbreaking, so we prepared to submit our research to journals specializing in environmental studies. Then, fortuitously, Prof. Susana Agusti proposed calculating the element accumulation rates (EARs) in our sediment cores to gain a deeper understanding of the Red Sea's environmental evolution over recent centuries. Upon reviewing the scatter plots of the EARs, we were astonished to find a marked decline in the EARs in the South Red Sea, contrasted by an exponential increase in the North (Figure 2). Regrettably, I was unable to account for these temporal variations and thus set aside this manuscript for several years to concentrate on other aspects of my Ph.D. thesis.

Figure. 2: Scatter plots of element accumulation rates in the Red Sea sediment cores.

Subsequently, Prof. Susana Agusti and I collected and analyzed zooplankton from both open and coastal regions of the Red Sea, sediment cores from coral reefs, and leaves and sediments from mangroves and seagrasses. Through this extensive research, we became aware of the significant heavy metal pollution in the Red Sea, especially in the Northern region. This pollution was attributed to the increase in shipping activities since the Suez Canal's opening in 1869, coupled with the rapid industrialization along the Saudi Arabian Red Sea coast that followed the discovery of the first oil field in Saudi Arabia in 1938. The development of oil-related sectors—including oil refineries, petrochemical plants, accidental oil spills, and high-pollution industries such as cement factories and power plants burning crude oil as fuel—contributed to this environmental issue (Figure 3). Moreover, the semi-enclosed structure of the North Red Sea basin, the lack of river inputs, scant precipitation, and high seawater evaporation rates, all contribute to limiting water exchange and causing pollutants to persist in the aquatic environment. These conditions helped explain why the EARs in the North Red Sea sediment cores had increased exponentially over the past century.

Figure 3. Natural and Anthropogenic Sources of Elemental Pollution in the Red Sea.

Meanwhile, Prof. Susana Agusti highlighted the accelerated warming of the Red Sea and the global ocean since the Second Industrial Revolution (~1870). Motivated by this insight, we analyzed the Mg/Ca concentration ratios in the Red Sea sediment cores. According to thermodynamic principles, higher seawater temperatures promote the substitution of magnesium (Mg) for calcium (Ca) in carbonate sediments. We discovered that the Mg/Ca ratios in the South Red Sea were positively correlated with time, suggesting a significant rise in sea surface temperature since the 18th century. This finding is in accordance with previous studies indicating a global temperature increase since the conclusion of the Little Ice Age over the last 200 years. In the context of global warming, increased stratification in the oceans, exacerbated by warmer temperatures, could disrupt vertical mixing between the surface and the nutrient-rich depths, potentially reducing the influx of nutrients. Conversely, warming may also attenuate monsoon patterns over the Red Sea and Indian Ocean, affecting the horizontal transport of nutrient-rich waters into the Red Sea. This phenomenon could account for the observed decrease in EARs in the South Red Sea sediment cores.

Our research has thus highlighted a trifecta of marine challenges in the Red Sea: escalating thermal stress, nutrient depletion, and increasing elemental pollution, including heavy metals. We have named this triad of stressors the "Cai-Agusti Marine Crisis Conflux." This conflux, likely exacerbated by other environmental pressures, is contributing to the degradation of the Red Sea's marine ecosystems and could be indicative of a global trend. We advocate for immediate measures to mitigate global greenhouse gas emissions to address ocean warming. Moreover, we emphasize the necessity of adopting cleaner technologies to curtail pollutants from local human activities. Taking such proactive steps is essential for the conservation of the Red Sea’s biodiversity and for mitigating similar ecological challenges in marine environments worldwide.

Filled with a sense of accomplishment, we prepared the manuscript and submitted it to the Nature Portfolio. Fortunately, our manuscript was accepted and published by Communications Earth & Environment. This milestone would not have been possible without the continuous guidance of my mentor, Prof. Susana Agusti. Looking ahead, we plan to measure more EARs in sediments from various ocean currents around the world, to determine whether the Cai-Agusti Marine Crisis Conflux has became a global phenomenon.

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