Copepods Cope with Acidification

At Earth’s northern latitudes, animals have been forced to evolve evolutionary solutions to cope with cold temperatures. Grizzly bears hibernate, minimizing their energy use during the coldest months. The famously massive walrus maximizes its size to store as much fat as possible for the spare winter. But what is a zooplankton no larger than one’s little finger nail to do?

Copepods are a tiny crustacean vital to Arctic marine ecosystems. According to Senior Research Scientist David Fields, their answer to the challenges of cold is complex and beautiful, and it involves special, densely packed fats and slow metabolic rates that burn this precious energy slowly.

“Fat is the currency of all high latitude ecosystems,” Fields said. “Everything runs on fat.”

Copepods’ ability to produce these fats, however, may be compromised by global climate change. Fields compared the survival strategies of three cold-climate copepod species and learned how they will likely respond as climate changes, causing northern waters to warm and become more acidic.

“Factors like shifting temperature and acidity are added stressors,” Fields said. “We're trying to understand what the net effects of these more pronounced climate change variables are, and what it’s going to do to their populations.”

Since the Industrial Revolution, global carbon dioxide production has skyrocketed. Some of the additional carbon reaches the atmosphere, causing an increase in global temperatures. However, the majority dissolves into the ocean.

As the ocean absorbs this excess carbon, it becomes more acidic—a process that is happening faster at polar latitudes. Many marine species will be compromised in a more acidic ocean, and biologists have long worried about copepods, which are ubiquitous across the global ocean and an important food source for fish and zooplankton. Damage to their populations could ramify up the food chain, threatening entire ecosystems.

In his experiments, Fields found hope for the future—the more acidic conditions predicted to occur in the next century did not have negative effects on the Arctic and subarctic copepod species he studied.

“This is great news,” Fields said. “We should be dancing in the streets that acidification isn’t going to have a major effect on these keystone species.”

However, he found that another aspect of global climate change will dramatically impact copepods: temperature. Fields found that copepods that grow in the warming waters of the Arctic and along the coast of Maine develop faster and are physically smaller at maturity. Smaller females produce fewer eggs and store less precious fat, threatening their survival.

“This is an effect with long-term implications,” Fields said. “And it’s not only limited to copepods—we’re seeing these small adult sizes across a number of crustacean and zooplankton species.”

Growing in warmer water also had complex metabolic effects for the copepods. Their important role in global carbon cycling means that changes to copepod metabolism could scale up to impacts on Earth’s climate.

Just like humans, copepods use the energy from the food they eat to carry out a number of vital processes. They must grow, process carbon through respiration, produce eggs for reproduction, and generate waste.

Fields found that living in warmer water sped up the copepods’ metabolism, which made them run through their energy stores more quickly. Without being able to eat more, the copepods had less carbon to grow, produce eggs, and pass as waste. That waste is significant to global carbon cycling: as their fecal pellets fall into the deep ocean, the carbon they contain is sequestered away from the atmosphere.

“The global population of copepods processes so much carbon that these higher respiration and lower carbon sequestration rates matter on a global scale,” Fields said. “What happens at these northern latitudes will cause fundamental climate shifts with human impacts.”