Record-breaking ocean temperatures in the North Atlantic resulted in several intense marine heatwaves in the Northern Hemisphere last summer—but heat is only one of numerous stressors that challenge life under water.
While the ocean is becoming warmer, more acidic, and less oxygenated, how can we monitor and protect the marine biodiversity we depend upon?
In June 2023, the North Atlantic Ocean experienced record-breaking ocean temperatures, resulting in several unprecedented extreme marine heatwaves. Such periods of unusual heat can be detrimental to marine life, causing mass die-offs of fish and other organisms, disrupting fisheries, and spurring harmful algal blooms—but heat itself is not the only climate-linked stressor affecting marine biodiversity.
The “Three Horsemen” of Climate-Driven Biodiversity Loss
Professor Stephen Widdicombe from the Plymouth Marine Laboratory grew up in a coastal town, where he learned to dive at a young age. “The sea has always been part of my life, so I naturally got into the field of marine biological science,” he says. “I became especially interested in how human activities impact marine life, in particular through climate change. By carrying out experiments, I started testing hypotheses to better understand the responses of particular species and how they were impacted by potential future climate change conditions,” he adds.
Although the connection between greenhouse gas emissions and rising global temperatures is now widely understood in society, Prof. Widdicombe warns that heat is rarely the only stressor our planet’s marine ecosystems have to face. “Talking about rising temperatures is easy, but it is much harder to bring attention to the other ‘two horsemen’ of climate-driven biodiversity loss in the ocean,” he says. “You can put your feet in the water and feel that it’s warmer than usual, but ocean acidification and deoxygenation aren’t such tangibly understood and obvious concepts.”
Ocean acidification is characterized by a reduction in the pH levels of the ocean, driving the seawater to become more acidic than it used to be. Deoxygenation, on the other hand, refers to the reduction in oxygen levels within the ocean. Together with rising water temperatures, all three “horsemen” are directly or indirectly driven by increasing concentrations of greenhouse gasses in the atmosphere, and their absorption into the ocean. All three are also the major climate-driven stressors affecting marine biodiversity on a global scale.
Temperature, pH, and oxygen all influence the biological functioning and behavior of marine organisms. Prof. Widdicombe emphasizes that even slight changes in these three variables can have impacts that will be both local and global, chronic, and acute, affecting ecosystems and people: “Animals have evolved to thrive within specific temperature ranges, yet both increased acidity and oxygen depletion can shrink these comfort zones,” he says. “This can determine the animal’s success in competing for food, reproducing or evading a predator, ultimately having an impact on individual survival, but it can also impact animal populations on a larger scale.”
Bleached coral reef. (Image credit: The Ocean Agency, Ocean Image Bank)
Declines in the delicate yet biodiversity-rich coral reef ecosystems are an example of the impact these three stressors can have on marine life when working simultaneously. Warming temperatures are known to be the main cause of coral bleaching events in tropical waters, where the living components of corals die as a result of excessively high temperatures. Ocean acidification also reduces the corals’ ability to build calcium carbonate skeletons and grow, while decreasing oxygen levels are detrimental to reef organisms that rely on oxygen for life, just as humans do.
The effects of these climate-induced stressors extend far beyond tropical coral reefs. Other calcifying organisms, including cold-water coral, crustaceans, and shell-building mollusks like mussels, oysters, and clams, are increasingly vulnerable as their habitats become warmer, more acidic, and less oxygen-rich. This poses growing risks to a range of commercially valued species, such as oysters and crabs, and to locally important recreational and cultural species such as certain types of clams. “Protecting marine ecosystems as well as our own societies from these interconnected impacts requires an integrated, holistic approach,” says Prof. Widdicombe.
Oyster farming in France. (Image credit: Nicolas Job, Ocean Image Bank)
Knowledge for Managing Marine Ecosystems Under Multiple Stressors
An integrated approach to protecting marine ecosystems starts with reliable physical, chemical, and biological information coming from ocean observations. “If we don't know what's going on in terms of the speed and severity of changes that are happening in our ocean, we cannot hope to manage and protect marine ecosystems,” says Prof. Widdicombe, who also co-chairs the Global Ocean Acidification Observing Network (GOA-ON)—a collaborative international network that aims to detect and understand the drivers of ocean acidification, as well as the resulting impacts on marine ecosystems.
Marine Protected Areas (MPAs) are now considered one of the best measures to protect marine biodiversity, enshrined through the recently adopted United Nations High Seas Treaty and the global ambition to protect 30% of Earth’s land and ocean area. Establishing protected areas in the ocean reduces the impacts on marine ecosystems from human activities such as fishing, this way increasing their ability to withstand climate-driven stressors. But Prof. Widdicombe highlights that the application of such measures must be supported by a continuous flow of ocean data: “We can create MPAs now based on where we think the biodiversity hotspots are, but unless we understand how fast climate change is progressing in the ocean and directly observe marine life to document the impact of this change, how can we ensure these MPAs will be effective at increasing ecosystem resilience and protecting biodiversity?”
Essential ocean data is still lacking in many key areas. The Global Ocean Observing System’s (GOOS) Expert Panel on Biology and Ecosystems estimates that only about 7% of the ocean is regularly observed in a sustained way, and coastal zones are particularly under-observed in this regard. “We first engage with the ocean through the coasts, and the amount of data available from coastal systems is still way below what we actually need to support effective decision making,” says Prof. Widdicombe.
Sustained coastal observations and open access to the information they produce are especially important for countries such as Small Island Developing States, which are not only the first to experience the negative impacts of climate change, but whose economies and people are most dependent on the ocean. Coincidentally, ocean observing infrastructure in these areas is often more limited, even though these countries normally have especially large and biodiversity-rich ocean territories under their national jurisdiction. A significant part of global marine biodiversity is found in these waters and helping SIDS to improve their ocean observing capacity is thus a critical step in preventing a catastrophic global biodiversity loss. Through ocean observing and information delivery services, timely and effective measures can be ensured to protect vulnerable and globally valuable ecosystems and the people that depend on them.
A fisherman in Fiji. (Image credit: Tom Vierus, Ocean Image Bank)
Adding the Missing Pieces to the Observing System
Central to building an efficient and integrated global observing system for marine life is a set of biology and ecosystem focused Essential Ocean Variables (EOVs), defined under GOOS. This set of 12 EOVs was established as a framework for observing different types of organisms—from seagrass to seabirds and microbes to marine mammals—in order to provide information on biodiversity and the state of marine ecosystems. Communities of scientists collecting these observations can share them worldwide through the Ocean Biodiversity Information System (OBIS).
International cooperation combined with investment in national systems is key to expanding ocean observations efficiently and ensuring maximum utility of the collected data in serving societal needs. “GOOS is the key international effort to coordinate and improve ocean observing capacity across the intertwined physical, biogeochemical, biological and ecosystem processes around the world,” says Dr. Emma Heslop, GOOS Program Specialist at UNESCO. “We need to observe in order to adapt to climate change in the ocean, and GOOS is here to ensure that we help develop observing capacity and efficient systems to deliver information to those who need it,” she adds.
“The EOVs have been defined in such a way that they can be implemented broadly and in a sustained manner in many different places,” says Dr. Karen Evans, co-chair of the GOOS Biology and Ecosystems Expert Panel. Ideally, the observations of biology and ecosystem EOVs are linked to physical and chemical variables, enabling a better understanding of how marine life responds to changing environmental conditions.
Researcher carrying out seagrass monitoring. (Image credit: GOOS)
Addressing the Root Causes Behind Marine Biodiversity Loss
“Our first and foremost goal should always be reducing greenhouse gas emissions.” says Prof. Widdicombe, ‘’but measures such as MPAs and government biodiversity policies are the steps that will enable society and the ecosystems we depend upon to become more resilient to climate change.”
Aside from the “three horsemen”, marine life around the globe already faces an intricate web of various other human-induced stressors, from pollution to overfishing. Reducing these alongside the climate strategies is also as important, says Prof. Widdicombe: “If we can help reduce other stressors, we provide the system with greater resilience and resistance to climate stress.”
Amid these challenges, ocean observations offer vital information about the state of our marine ecosystems as well as the intricate dynamics of our ocean. They empower us to comprehend the interaction of multiple stressors on marine life, enabling informed conservation strategies and adaptive management. As we stand at a critical crossroads, the link between robust observations and effective biodiversity protection is paramount to preserve the marine life we depend on for generations to come.
