Are marine ecosystems headed toward a new productivity regime?

July 16, 2015

Phytoplankton have been projected to produce less organic material as the oceans' temperatures rise—with carry-on effects for higher levels of the food web. Based on new climate model simulations, a team of scientists from Sydney and Kiel suggests now that this assumption might be misleading. According to the researchers, ocean productivity might be pushed into a completely new regime in the more distant future.

Human-induced dioxide (CO2) emissions to the atmosphere are projected to rise to up to 30 gigatonnes of carbon per year by 2100, assuming a "business-as-usual" scenario. As a result, global mean temperatures are projected to increase by almost five degrees Celsius. According to a team of scientists from the Climate Change Research Centre at the University of New South Wales (UNSW), Australia, the Australian Research Council's Centre of Excellence for Climate System Science and GEOMAR Helmholtz Centre for Ocean Research Kiel, these changes might have the power to shift the ocean into a fundamentally different type of production regime. Although simulations with more complex models need to be integrated and analysed, the study published in the current issue of the Environmental Research Letters demonstrates that predictions made for the next 100 years might not be valid in the more distant future.

"We integrated a 600-year simulation starting in the year 1800 and followed the IPCC's Representative Concentration Pathway RCP 8,5", Dr. Karin Kvale, modeller at UNSW and GEOMAR explains. The three slightly different models run by the Australian and German scientists first showed a decline in . The slowdown is due to the fact that warming waters are also stratified more strongly and less mixing can take place. If less water from the deep reaches the sunlit top layer, fewer nutrients are available for phytoplankton and primary production - the production of from inorganic carbon for example through photosynthesis - decreases. This short-term result was in line with our current understanding of near-future shifts in productivity.

But starting about the year 2000, rising water temperatures cause respiration rates to pick up. "Rates of heterotrophic consumption, such as from bacteria, metabolic processes and from plankton that live on organic matter from other organisms, increase faster than rates of primary production", Dr. Kvale summarizes. "Eventually, this unbalanced ratio pushes global primary production away from being driven by the physical limitation of access to newly upwelled nutrients from deeper ocean layers. The new regime is essentially driven by the biology itself." In a future ocean system with more heterotrophy, carbon and nutrients will be recycled more effectively near the surface than they are now, and less carbon will be exported and stored in the deep. This will also have implications for the ocean's ability to absorb from the atmosphere and to mitigate the effects of global change.

Currently, the models do not include the possible effects of decreasing calcification and the greater susceptibility of the aragonite form of calcium carbonate to dissolution on carbon export. Either of these phenomena would both hasten and magnify the transition to more heterotrophy by further reducing carbon export from the surface. Therefore, more complex models need to be developed and integrated to better understand the potential long-term changes and possible tipping points that could occur in a high-CO2 world, the scientists emphasize. "Our study indicates possible surprise changes in the ocean's more distant future and we think that it is important to consider longer-term predictions", Dr. Kvale states. "But many uncertainties have to be discussed and analysed - both concerning the drivers of this massive change and its possible effects."

Explore further: Southern Ocean is less efficient at exporting carbon than thought

More information: Kvale, K.F., Meissner, K.J., Keller, D.P., 2015: Potential increasing dominance of heterotrophy in the global ocean. Environmental Research Letters,10, DOI: 10.1088/1748-9326/10/7/074009

Related Stories

Solving corrosive ocean mystery reveals future climate

May 11, 2015

Around 55 million years ago, an abrupt global warming event triggered a highly corrosive deep-water current through the North Atlantic Ocean. The current's origin puzzled scientists for a decade, but an international team ...

Analyzing ocean mixing reveals insight on climate

June 24, 2015

Scientists at Los Alamos National Laboratory have developed a computer model that clarifies the complex processes driving ocean mixing in the vast eddies that swirl across hundreds of miles of open ocean.

Ocean warming leads to stronger precipitation extremes

July 13, 2015

Due to climate change, not only atmospheric, but also oceanic, temperatures are rising. A study published in the international journal Nature Geoscience led by scientists at the GEOMAR Helmholtz Centre for Ocean Research ...

Recommended for you

Climate change made Harvey rainfall 15 percent more intense

December 14, 2017

A team of scientists from World Weather Attribution, including researchers from Rice University and other institutions in the United States and Europe, have found that human-caused climate change made the record rainfall ...

East Antarctic Ice Sheet has history of instability

December 13, 2017

The East Antarctic Ice Sheet locks away enough water to raise sea level an estimated 53 meters (174 feet), more than any other ice sheet on the planet. It's also thought to be among the most stable, not gaining or losing ...

Hydraulic fracturing negatively impacts infant health

December 13, 2017

From North Dakota to Ohio to Pennsylvania, hydraulic fracturing, also known as fracking, has transformed small towns into energy powerhouses. While some see the new energy boom as benefiting the local economy and decreasing ...

0 comments

Please sign in to add a comment. Registration is free, and takes less than a minute. Read more

Click here to reset your password.
Sign in to get notified via email when new comments are made.