The ocean's oxygen levels may be taking a hit from the indirect impacts of zooplankton gobbling up tiny bits of plastic rather than more nutritious fare, perhaps exacerbating the effects of global warming.

Scientists used computer modeling to explore how reduced grazing from the creatures allowed marine algae to thrive and eventually die and decay, potentially decreasing the sea's oxygen levels by up to 10% in some places. The team reported the findings April 21 in Nature Communications.

"Zooplankton are part of the base of the ocean's food web, so if something affects them then it could have wide-ranging impacts on all ocean life and nutrient cycling," said Karin Kvale, a scientist at GNS Science in New Zealand and first author of the study.

Marine life depends on oxygen dissolved in the surrounding water, but the world's seas have lost about 2% of their oxygen content since the middle of the 20th century. This is partly because of climate change; warmer water holds less oxygen and leads to stratification, or separation of different levels of the ocean. This lack of mixing prevents oxygen from reaching deeper waters. 

Additionally, nutrient pollution from fertilizer, sewage and other human-related sources is feeding algal blooms. These excess algae, or phytoplankton, are ultimately broken down by microorganisms that consume oxygen during decomposition.

Among the consequences of deoxygenation are marine "dead zones" and die-offs of plants and animals; many large and commercially valuable fish such as tuna and swordfish are especially sensitive to low oxygen levels. Reduced oxygen levels can also affect ocean chemistry and cause greenhouse gases such as methane and nitrous oxide to form. 

Another serious problem facing the ocean is the millions of tons of plastic that makes its way into the water every year. Microplastics, which are fragments less than 5 millimeters in size, can be mistaken for food by zooplankton and other creatures. 

Kvale became interested in this phenomenon after seeing her children return home from the beach with pockets full of plastic rather than shells or rocks.

"There has been a lot of debate about whether plastics might have serious global effects on the base of the food web, because the concentrations of microplastics are generally pretty low," she said. "But what struck me about my kids' collecting habits was that they were concentrating plastic in their pockets because they liked it, so it doesn't actually matter if there isn't much plastic available — what matters is if the sea life wants it."

Scientists have observed zooplankton — a group that includes krill and other small drifters — eating microplastics. Kvale decided to investigate the potential implications of the animals having a predilection for plastic.

Previously, she and her colleagues used computer modeling to explore how microplastics become attached to organic particles and are eaten and then excreted by zooplankton. They estimated that hungry zooplankton could be the dominant explanation for so-called missing microplastic, which is "microplastic that is expected to be found at the ocean surface but which seems to be missing," Kvale said.

For the new paper, she and her colleagues used climate models to look at how zooplankton eating plastic affects how nutrients move through the ocean. The team relied on historical climate data from the 20th century and a "business-as-usual" climate scenario, in which greenhouse gas emissions are not curbed, that ran through the 21st century. They also drew upon previous estimates for the amount of plastic that enters the sea and its expected increase to determine the amount of microplastics that could replace proper food in the zooplanktons' diet.

When the zooplankton eat less of their normal food and nutrients are abundant, phytoplankton can take advantage of the situation and grow more.

"When they grow more, they die more, and their particles sink out of the surface ocean and rot as they settle," Kvale said. "This rotting consumes oxygen, and since there is more growth if the zooplankton aren't eating as much, there is more deoxygenation."

She and her colleagues estimated that this process would worsen deoxygenation in nutrient-rich tropical areas, the Southern Ocean and the North Pacific. They estimated that by 2020, this process could have decreased oxygen levels in the North Pacific by as much as 10% and accelerated global deoxygenation by an extra 0.2% to 0.5% relative to 1960 levels. By 2100, zooplankton snacking on microplastics could account for an additional 0.2% to 0.7% loss of global oceanic oxygen, the team concluded. 

The findings could help explain why climate models have typically underestimated the actual amount of deoxygenation that scientists observed in the ocean over recent decades.

"The model results demonstrate that even at low microplastic concentrations like what we seem to have in the ocean currently, there can be significant impacts on nutrient cycling and the base of the food web," Kvale said. "This implies that research into ocean change must be expanded to include direct pollution effects from factors such as plastic as well as indirect effects from climate change."

However, she cautions, the effect she and her team saw must still be confirmed in the real world.

"It's important to remember that it is only a preliminary model study from a research field still in its infancy — expect big changes to this initial estimate in the coming years," Kvale said. 

The study, "Zooplankton grazing of microplastic can accelerate global loss of ocean oxygen," published April 21 in Nature Communications, was authored by K. Kvale, GEOMAR Helmholtz Centre for Ocean Research and GNS Science; A. E. F. Prowe, C.-T. Chien and A. Oschlies, GEOMAR Helmholtz Centre for Ocean Research; and A. Landolfi, GEOMAR Helmholtz Centre for Ocean Research and ISMAR-CNR.