For deep sea creatures, like this lithoid crab and the mussels it’s walking on, lower nutrient production at the ocean surface spells trouble. Photo via USGS.gov

Even the frigid depths of the world’s oceans aren’t immune to the effects of climate change. Up to 38 percent of northeast Atlantic deep sea marine life will disappear in the next century, according to a new analysis based on advanced climate models. Worldwide, around 5 percent of deep sea life is projected to die off due to changing environmental conditions.

“We were expecting some negative changes around the world, but the extent of changes, particularly in the North Atlantic, were staggering,” Dr. Daniel Jones, lead author of a study published in Global Change Biology, said in a release. “Globally we are talking about losses of marine life weighing more than every person on the planet put together.”

The authors say this study is the first time future losses have been quantified in the benthic zone, or layer of the water column that encompasses the seabed. In deeper waters, the benthic zone is also aphotic, which means the photosynthetic organisms that make up the base of the food chain can’t exist. Instead, organisms living in the benthic zone (known as benthos) rely on sinking nutrients that were produced in shallower waters.

Climate change has previously been modeled to decrease productivity at surface waters, whether through acidification, warming water temperatures, or decreased mixing of surface layers—which is key to moving nutrients through marine systems. And if surface waters become less productive, that means less biomass and nutrient load—measured here as particulate organic carbon (POC)—sinking down to deeper waters. The question is how much of an impact changes will have.

“There has been some speculation about climate change impacts on the seafloor, but we wanted to try and make numerical projections for these changes and estimate specifically where they would occur,” Jones said.

The Global Change Biology study is based on eight different climate models, all of which are used to project changes in how much POC will cycle down to ocean depths in the last decade of this century. Comparing that period, 2091-2100, to current data and models for the period of 2006-2015, the team found that POC flux will decline in most regions worldwide, with polar regions and shallow waters being the outliers. POC decline means less of the basic building blocks of life for benthos of all sizes.

As might be expected, the declines were highest for the deepest zones in the oceans, which are most isolated from surface production, and which partially explains why projections for the northeast Atlantic were so negative. As the authors write, “The projected changes in % biomass are greatest in the abyssal (>3000 m) and hadal zones (>6000 m) as a result of both higher relative changes in low-food conditions and the spatial co-occurrence of these areas with areas of change.”

The reduced nutrient load will also disproportionately affect larger organisms, who simply need more nutrients to survive. “The projected changes will result in a size-shift in global benthic biomass towards smaller organisms, particularly for the typically sediment-dwelling infaunal organisms (macro and meiofaunal),” the authors write, noting that such effects have been observed in the past when nutrient flux declined.

By quantifying projected change, the new study will hopefully provide good insight for developing mitigation plans and ocean policy, including for fishing; the researchers note that deep sea fishing will exacerbate the decline. It also stands as a reminder that ocean systems are highly connected. As we’ve seen in past studies, changes to one part of a marine ecosystem can have significant cascading effects throughout the rest of the system, and the same goes when looking at oceans at a more macro scale. 

@derektmead