How does warming, acidic ocean affect marine life? Study suggests commercial fish population likely to dwindle

Unabated climate change could jeopardize the future of some marine inhabitants. According to new research, as oceans continue to warm and turn acidic, fish and other invertebrates are unlikely to cope well, causing them to crumble. Algae and other primary producers, on the other hand, are likely to flourish.

Organisms eat one another and pass on energy, from producers to consumers such as small fish and then to larger predators. Climate change could destabilize this marine food web by allowing certain species to dominate the oceans while sending others towards depletion. The experts arrived at these results after creating an experimental set up called mesocosms. It recreates the natural environment of marine organisms. The experiment tested species when temperatures are raised to the ones predicted by 2021. 

"When we started thinking about this experiment in 2014, most studies were still being done in small aquaria," Dr Ivan Nagelkerken, from the University of Adelaide’s Environment Institute, told MEA WorldWide (MEAWW). "We realized that species interactions and the role of natural habitats were critical components of how species respond or adjust to climate change. Hence, we decided to set up large mesocosms experiments that harbored dozens of species," he added.

Nagelkerken and his team used 1,800-liter mesocosms containing marine life such as cyanobacteria, algae, shrimps, crabs, mollusks, sponges, and fish. One experiment mimicked the current conditions seen along the South Australian coast. The others tested the impacts of warming temperature, rising carbon dioxide levels, and the combined effects of the two on these organisms. The team increased the temperatures over six months until it mirrored temperatures expected at the end of the century. 

The outcomes of the experiments are "worrying," the researcher said. While algae and cyanobacteria grew abundantly, the population of primary consumers such as small fish and other invertebrates dwindled. The former thrived because carbon dioxide is a nutrient, Nagelkerken explained. Excessive build-up of carbon dioxide in the oceans turns water acidic. Changing conditions did nothing to alter the food choices of these species. "This rigidity led to some food sources dominating in their abundance (such as turf algae and cyanobacteria) without herbivores being able to consume them quickly enough (or at all)," Nagelkerken explained. Even as algae flourished,  herbivores could not take advantage of that.

It could have ripple effects on the top predators such as killer whales or great white sharks. Their energy demands could shoot up due to warming temperatures. But these carnivores are unlikely to meet these demands due to dwindling prey numbers. "This could eventually lead to their demise -- either [through] decreased abundance or smaller body sizes," he explained. The destabilized food web will also affect humans as they rely on fish for nutrients.
 
However, there is a possibility that some species might learn to cope with the changing conditions. "We still know relatively little about how species may genetically adapt to climate change,  but some might," Nagelkerken noted. "Destabilization of marine food webs can only be mitigated if further concerted action is taken to reduce greenhouse gas emissions," the researchers wrote in their study.  Dr. Nagelkerken highlighted one issue with the study: scale. "Even though our mesocosms were large (1,800 l each), they are still very small compared to the ocean," he added.

The perspective is published in Science.