Scientists discover a new and simple way to classify marine biomes

Scientists at Washington State University have developed a new way to classify different ocean environments, shedding new light on how marine biomes are defined and changed by nature and humans.

Just published in Global ecology and biogeography, a research by Alli Cramer, a 2020 doctorate from the WSU School of the Environment, now at the University of California Santa Cruz, and WSU Professor Stephen Katz revealed a new approach that sorts biomes according to their potential life support and stability of the sea floor.

Cramer and Katz looked at more than 130 studies to evaluate variables such as light, depth and nutrients in seven biomes that incorporate dozens of environments, including coral reefs, algae beds, ocean ice, and deep abyssal plains.

By analyzing the data inductively, instead of proceeding from an initial hypothesis, they found that biomes were ordered more clearly by two strong variables: gross primary production, a measure of energy in the food web; and the mobility of the substrate, or the movement and composition of the ocean floor.

“This means that energy flow and mobility are common organizational forces in a wide variety of marine ecosystems,” said Cramer. “Despite their differences, coral reefs and deep-sea deserts respond to the same processes.”

While biomes on earth have long been defined by climate, marine biomes have eluded a clear categorization.

“The oceans are one big black box,” Katz said. “Scientists have traditionally considered depth, temperature and light to be important. But we’ve found they don’t capture every community. The energy economy of the sea works in ways other than sunlight.”

As a PhD student, Cramer began developing a more effective way to solve marine biomes.

After analyzing many variables, “there were really only two that end up revealing the grand pattern,” Katz said.

Gross primary production measures the energy flowing through a marine community, whether it is powered by sunlight, the recycled “brown food web” of the depths, or by chemicals flowing from hydrothermal vents. Coral reefs, sea ice and mangrove swamps have high primary production, while deep, muddy abyssal plains are low-productivity marine deserts.

The other strong variable, substrate mobility, has classified biomes based on the nature of their lower layer: what it is made of and how much it is moved and agitated by waves and currents. A mostly stable sandy bottom defines a biome other than one that is constantly in motion.

“These two axes are important forces in determining ecosystems in the ocean and in driving their formation,” Katz said.

“One of the novelties of this classification system is that it is simple, so simple that no one has bothered,” he added. “When we told our colleagues about it, they were surprised that no one had tried it before.”

The new method could help scientists, fisheries managers and conservationists reconsider the richness and diversity of ocean biomes, as well as the value of high-productivity regions affected by humans.

“Previous work looked at the marine environment on an ecosystem-by-ecosystem basis,” Cramer said. “By combining data from many ecosystems, we found the common thread that binds them together. This allows us to see the ocean in new ways and highlights some key points where our actions can alter ecosystem function.”