Navigating the seas of change: the divergent impact of ocean acidification and warming on marine trophic levels

Butterfly fish are an example of an herbivorous species often mapped in the lower trophic levels. Scientists have been observing how this trophic level might be the one most heavily affected by climate change and ocean warming (Image Source: “Butterfly fishes” by Consuelo Puchades, licensed under CC BY-NC-ND 2.0 DEED).

Everything gets energy from somewhere. Plants make their own energy from sunlight, but all other life on earth needs to eat to get energy. In the ocean, for example, tiny photosynthetic phytoplankton are food for krill, which are then food for whales. These all constitute different trophic levels. Trophic levels are essentially different tiers or levels within a food chain or food web. Think of them as steps in a ladder, each representing a stage of energy transfer within an ecosystem.

At the very bottom are the primary producers, which are usually plants or algae that convert sunlight into energy through photosynthesis. They’re the foundation of every ecosystem, transforming sunlight and nutrients into organic matter. Next up are the primary consumers, also known as herbivores, which eat the primary producers. They’re like the first level of consumers in the food chain, munching on plants to get their energy. Moving up, we have secondary consumers, which are typically carnivores or omnivores. They feed on the primary consumers, gaining energy from the plants indirectly through the herbivores they eat. Then there are tertiary consumers, which are predators that eat other predators. They’re often at the top of the food chain in many ecosystems, feasting on secondary consumers. It is important to note, though, that not every ecosystem will have organisms for every single trophic level described, but trophic levels are an important tool to better understand how much energy goes into maintaining an ecosystem.

Each trophic level represents a transfer of energy from one organism to another, and as you move up the levels, there’s typically less energy available because some is lost on each step. Trophic levels help us understand the flow of energy and nutrients within ecosystems and how different species depend on each other for survival. But our ecosystems are ever-changing. Global climate change and ocean acidification are real and tangible pressures that marine ecosystems are facing—and that’s why Dr. Nan Hu and their collaborators embarked in a meta-analysis to define how the effects of ocean acidification and warming would affect different marine trophic levels. My question to you would be, would you rather be predator or prey in a warming and acidic ocean?

Sharks are a perfect example of how complex trophic levels can be. Depending on the ecosystem you’re trying to understand they can be considered secondary consumers (preying on herbivorous fish) or tertiary consumers (preying on rays and sea turtles). Sometimes they are the top of the food web, but sometimes they are not (as some orcas may prey on sharks). (Image Source: “Sharks” by Klaus Stiefel, licensed under CC BY-NC 2.0 DEED).

How Warming and Acidification Are Changing Our Oceans

Ocean acidification and warming can have profound effects on marine ecosystems, impacting everything from individual species to entire habitats. As carbon dioxide from the atmosphere dissolves into the ocean, forming carbonic acid, seawater becomes more and more acidic. Acidic levels can inhibit the ability of marine organisms, particularly those with calcium carbonate shells or skeletons like corals, mollusks, and some plankton, to build and maintain their structures. Acidification can also disrupt the reproductive and developmental processes of marine species, affecting their survival and population dynamics.

Rising global temperatures also cause the ocean to absorb more heat, leading to ocean warming. Warming waters can directly stress marine organisms, especially those with narrow temperature tolerances. It can disrupt their metabolism, growth rates, reproduction, and distribution patterns.

Coral bleaching is a significant consequence of ocean warming and acidification, where corals expel the symbiotic algae living in their tissues due to stress from high temperatures. This results in the loss of the vibrant colors associated with healthy corals and can lead to coral death if prolonged. Changes in ocean temperature can also affect the distribution and abundance of species, as they may shift their ranges in response to changing conditions, making it harder to protect marine species as ocean conditions change.

All in all, ocean acidification and warming are two interconnected consequences of human-induced climate change that pose significant threats to marine ecosystems. Mitigating these impacts requires global efforts to reduce greenhouse gas emissions and implement measures to protect and restore marine habitats and biodiversity locally.

Coral bleaching is one of the gravest effects of ocean warming and acidification (Image Source: “Bleached coral” by Oregon State University, licensed under CC BY-SA 2.0).

Would You Rather Be Predator or Prey in a Warming and Acidic Ocean?

Dr. Hu and their collaborators assembled a dataset including 486 observations from 162 studies to determine if marine species from different trophic levels are affected differently by ocean warming, ocean acidification, or the combined effect of both changes. For this, they observed trophic levels as primary producers, herbivores, meso-predators (predators that are both predators and prey), and top-predators.

Their findings revealed herbivores as the most susceptible and vulnerable to these changes, while primary producers and diverse predators exhibited greater resilience. Interestingly, the combined effect of acidification and warming did not significantly alter impacts across trophic levels. Some primary producers even thrived in acidic conditions, albeit negatively affected when coupled with warmer temperatures. Herbivores experienced milder effects under combined stressors, suggesting warmer waters might mitigate acidification’s impact on them. Predators, however, showed no statistical difference in response to either stressor or their combination.

Overall, this underscores the importance of understanding how different factors affect marine life and how this is important for conservation efforts, especially as the oceans face more challenges due to climate change… Oh, and that if you had to choose between being a predator or prey in this changing environment, you should choose predator every single time.

Andrea Valcarcel, Envirobites, 1 May 2024. Press release.

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