Science Still Working To Unravel Future Of Plankton In Destabilized Oceans; Some Positives, But Long-Term Outlook Bleak

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Temperature has a huge impact on plankton, says Fabio Benedetti, an ecological data analyst at the University of Bern. His modelling, in agreement with other work, predicts that as waters warm, plankton will move polewards at a rate of about 21 miles per decade. “Some changes will be massive, like in the Arctic, where entire communities will be replaced by something new,” he says. Phytoplankton seem better able to cope with gradual warming than zooplankton, Benedetti says, so their habitat range and diversity looks set to increase in most regions. But that’s not necessarily good news: the most productive ocean regions, counterintuitively, have low diversity, Benedetti notes. And more diversity doesn’t necessarily mean higher numbers of plankton overall.

In the long term, researchers fear an overall decline in plankton numbers thanks to a dwindling supply of critical nutrients such as nitrogen and iron. Plankton typically rely on nutrients that have been mixed from the ocean bottom up to the surface. But melting polar ice and changing ocean currents are now increasing the density difference between top, fresh waters and deeper, saltier waters, making these layers ever-less-likely to mix. This increased “stratification,” as it is called, is expected to starve ever-more regions of the ocean. While some coastal regions are drowning in too much nitrogen, which runs off agricultural fields, much of the open ocean might become “more desert-like,” says Hutchins.

A factor working against this trend is pollution from both the burning of fossil fuels and from wildfires, which can carry nutrients far out into ocean waters. The wildly destructive Australian wildfire season of 2019-2020, for example, poured so much nutritional iron into the water that phytoplankton concentrations doubled between New Zealand and South America, creating an algae bloom bigger than Australia that lasted for months. The biggest surprise was how long the blooms lasted, says Jakob Weis at the University of Tasmania, Hobart, who studied the phenomenon. The amount of carbon soaked up by those organisms is thought to have roughly matched the carbon spewed by the fires — a helpful factor in the context of climate change. “We intentionally refrained from framing the blooms as ‘good news,’ given the devastating impact the wildfires had on the Australian public, wildlife, terrestrial ecosystems, and climate,” says Weis. But the plankton, at least, were “happy.”

Such nutrient influxes could be a big deal, says Hutchins, especially in the Southern Hemisphere where waters are particularly starved of iron. But the long-term impacts are entirely unclear. “We don’t really know if that’s going to balance out some of what’s not mixing up from below, or if it’s going to select for different communities, different species, because it’s chemically different [food],” says Hutchins. Such blooms won’t make a big dent in the big-picture fight against global warming, adds Ostle. “We’ve burned thousands of years of deposits of oil and gas. The phytoplankton blooms aren’t going to be able to cut it.”

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https://e360.yale.edu/features/plankton-climate-change