Ocean greenery under warming stress
A century of phytoplankton decline suggests that ocean ecosystems are in peril.
Marine phytoplankton — the vast range of tiny algae species accounting for roughly half of Earth's total photosynthetic biomass — have declined substantially in the world's oceans over the past century, researchers report in Nature1 this week. The findings add to concerns that climate change is dangerously altering marine ecosystems.
Phytoplankton are the basis of the entire marine food chain, and have an important role in the global carbon cycle. Through photosynthesis, they produce around half of the oxygen in Earth's atmosphere and drive the 'biological pump' that fixes 100 million tonnes of atmospheric carbon dioxide a day into organic material, which then sinks to the ocean floor when the phytoplankton die, or are grazed and digested.
Phytoplankton activity fluctuates widely according to season and location, making long-term monitoring of trends difficult. An earlier study2, based on satellite observations of ocean colour, suggested a link between climate variability and ocean productivity, but this was limited to observations from 1997 to 2006. Boris Worm, a marine biologist at Dalhousie University in Halifax, Canada, and his team have now combined satellite-derived observations of phytoplankton with historical shipboard measurements stretching back to the pioneering days of oceanography.
The research reveals an unsettling centennial downwards trend, superimposed on shorter-term variability. The scientists found that the average global phytoplankton concentration in the upper ocean currently declines by around 1% per year. Since 1950 alone, algal biomass decreased by around 40%, probably in response to ocean warming — and the decline has gathered pace in recent years.
"Clearly, 40% is a huge number," says Paul Falkowski, an oceanographer at Rutgers University in New Brunswick, New Jersey. "This implies that the entire ocean system is out of steady state, slowing down."
"This is severely disquieting," adds Victor Smetacek, a marine biologist at the Alfred Wegener Institute of Polar and Marine Research in Bremerhaven, Germany. "One must really digest the very magnitude of this decline and its possible implications."
Worm and his colleagues spent three years unearthing, filtering and analysing available data on ocean transparency and chlorophyll concentration — common 'proxies' for phytoplankton abundance. After removing data on shallow coastal waters and any obviously erroneous — that is, biologically impossible — observations, the data set still included some 450,000 globally distributed measurements collected between 1899 and 2008.
Since 1899, ocean transparency has been measured using a simple device called a 'Secchi disk' after the Italian astronomer who invented it in 1865. The disk is lowered into the sea and a depth measurement is taken at the point where observers lose sight of it. Using optical equations, the researchers compared Secchi depth measurements of ocean transparency to measurements of chlorophyll concentrations at research sites and within phytoplankton, and to satellite observations of ocean colour.
The combined data suggest that phytoplankton biomass has decreased in eight of the ten ocean regions measured, with the largest rates of decline in the South and Equatorial Atlantic, the Arctic and the Southern Ocean. Only in the Indian Ocean has phytoplankton biomass increased — slightly in the north and more markedly in the south — since 1899.
"We've looked at the data from all different angles, local to global, to make sure we're not producing any statistical artefacts," says Worm. "We're very confident that the overall result is robust."
"The study adds to a growing body of global ocean research, all evidencing a fundamentally common result: the net effect of a warming ocean surface is a reduction in phytoplankton surface chlorophyll concentration," says Michael Behrenfeld, a marine ecologist at Oregon State University in Corvallis.
Although the effect of reduced phytoplankton growth on atmospheric CO2 concentrations is relatively small, the marine food web and fisheries could be badly affected, says Falkowski. "We're squeezing big open-ocean fish like tuna and swordfish from both ends," he says. "We're overfishing the oceans for sure. Now we see there is pressure from the bottom of the food chain as well."
In most regions tested, the phytoplankton decline seems to be the result of a 0.5–1.0 °C warming of the upper ocean over the past century. The warming leads to enhanced vertical 'stratification' of ocean layers, thus limiting the supply of nutrients from deeper waters to the surface.
But ocean warming does not explain reduced productivity in regions, including the Arctic Ocean, where algal growth is mainly constrained by sunlight. So scientists must try to find out what other drivers, such as changes in wind and ocean circulation, might force the decline, says Falkowski.
Reduced phytoplankton growth, says Worm, adds a new dimension — comparable to coral bleaching, overfishing and acidification — to the problems of global change in the ocean. "We don't know what happened before 1899, and we're not sure about what will happen in the future," he says. "But we absolutely need to monitor this worrying trend and watch how it is unfolding."
- Boyce, D., Lewis, M. & Worm, B. Nature 466, 591-596 (2010).
- Behrenfeld, M. et al. Nature 444, 752-755 (2006).
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