Phytoplankton—microscopic organisms that form the base of the ocean food web—are in decline across much of the North Atlantic, according to new research led by Dalhousie University in Canada.
Using six decades of data, the team found that populations of two major phytoplankton groups–– diatoms and dinoflagellates–– have been shrinking by up to 2% per year across most of the ocean.
Phytoplankton are microscopic, plant-like organisms that form the base of the marine food web. Through photosynthesis, phytoplankton consume carbon dioxide, much like land plants, and incorporate the carbon into their cells. Most of this carbon is released back into surface waters when phytoplankton are consumed or decompose, but some sinks into the ocean depths.
“This export process, known as the biological pump, is one way anthropogenic carbon in the atmosphere is stored in the deep ocean, which is one of the largest reservoirs of carbon,” says lead researcher Professor Andrew Irwin, a mathematician and expert in ocean biochemistry. “Diatoms are thought to be especially important to both processes.”
Climate change is expected to reduce primary productivity—the rate at which phytoplankton and other photosynthetic organisms convert carbon dioxide into organic matter—in many regions, particularly the highly productive North Atlantic, due to rising ocean temperatures, increased water column stratification, and declining nutrient availability.
“We wanted to see if there was evidence of this happening,” says Irwin. “There does seem to be evidence of a decrease in biomass, although there is some biogeographic complication. This mirrors an earlier result using independent data and methods”.
Using data from the Continuous Plankton Recorder, the researchers found that the total amount of diatoms and dinoflagellates has dropped by up to 2% per year across most of the North Atlantic, except along the eastern and western continental shelves.
The results also revealed a surprising shift in the composition of phytoplankton communities.
“The changing conditions in the North Atlantic are anticipated to lead to a shift away from diatoms towards dinoflagellates,” explains Irwin. “This is because in the present ocean dinoflagellates dominate relative to diatoms when water temperatures are warmer, have lower nutrient concentrations, and are more stratified. Diatoms are thought to be more efficient at export and in trophic transfer compared to dinoflagellates”.
But, to the team’s surprise, diatoms have actually become a larger share of the total phytoplankton community, increasing by 1–2% per year from 1960 to 2017 in most areas, except the Arctic.
“We did not find evidence of this anticipated shift over the past 60 years; in fact, we found mostly the opposite – a relative increase in diatom biomass compared to dinoflagellates,” says Irwin. “This shows us that anticipating changes to the marine phytoplankton community will be complex and not easily predicted from simple reasoning. Complex interactions among species and physical and or chemical forcing will ultimately determine the structure of future phytoplankton communities”.
These shifts could have wide-reaching effects on carbon cycling and marine food webs.
“Assuming the underlying mechanisms do not change, the next few decades could bring further decreases in diatom and dinoflagellate biomass, with a shift towards diatoms in much of the North Atlantic and a shift towards dinoflagellates in the Arctic,” the paper concludes. “These changes have likely had notable consequences for carbon export and the amount of biomass transferred up the food web”.
The results are published in PLOS One.
North Atlantic phytoplankton bloom
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