Research suggests that the amount of phytoplankton found in the top layers of the ocean has declined markedly over the last century.
Scientists wrote in the journal Nature that the decline appears to be linked to rising water temperatures.
The researchers looked at records of the transparency of sea water, which is affected by the plants.
The decline could be ecologically significant as plankton sit at the base of marine food chains.
This is the first study that has attempted a comprehensive global look a plankton changes over such a long time scale.
“What we think is happening is that the oceans are becoming more stratified as the water warms,” said research leader Daniel Boyce from Dalhousie University in Halifax, Nova Scotia, Canada.
“The plants need sunlight from above and nutrients from below; and as it becomes more stratified, that limits the availability of nutrients,” he told BBC News.
Phytoplankton are eaten by zooplankton, which are prey for small fish and other animals.
The first reliable system for measuring the transparency of sea water was developed by astronomer and Jesuit priest Pietro Angelo Secchi.
Secchi was asked by the Pope in 1865 to measure the clarity of water in the Mediterranean Sea for the Papal navy. He then developed the “Secchi disk,” which is lowered into the sea until its white color disappears from view.
A variety of substances in the water affects it transparency, but one of the main ones is the concentration of chlorophyll, which is the green pigment that is key to photosynthesis in plants at sea and on land.
Secchi disk measurements around the world have been augmented by shipboard analysis of water samples, and more recently by satellite measurements of ocean color.
The final tally included 445,237 data points from the Secchi disk spanning the period 1899-2008.
“This study took three years, and we spent lots of time going through the data checking that there wasn’t any ‘garbage’ in there,” Boyce told BBC.
“The data is good in the northern hemisphere and it gets better in recent times, but it’s more patchy in the southern hemisphere – the Southern Ocean, the southern Indian Ocean, and so on.”
The researchers have used higher quality data to calculate that since 1950, the world has seen a phytoplankton decline of about 40 percent.
The decline has been seen throughout most of the world, except the Indian Ocean. There are also phytoplankton increases in coastal zones where fertilizer run-off from agricultural land is increasing nutrient supplies.
However the pattern is far from steady. There are strong variations spanning a few years or a few decades.
Many of these variations are correlated with natural cycles of temperature seen in the oceans, such as the El Nino Southern Oscillation (ENSO), the North Atlantic Oscillation and the Arctic Oscillation.
The warmer ends of these cycles coincide with a reduction in plankton growth, while abidance in the colder phase is higher.
Carl-Gustaf Lundin, head of the marine program at the International Union for the Conservation of nature (IUCN), said that there could be other factors involved, such as the huge expansion in open-ocean fishing that has taken place over the century.
“Logically you would expect that as fishing has gone up, the amount of zooplankton would have risen – and that should have led to a decline in phytoplankton,” he told BBC News.
“So there’s something about fishing that hasn’t been factored into this analysis.”
He said the method of dividing oceans into grids like the Dalhousie researchers used did not permit scrutiny of areas where this might be particularly important, like the upwelling in the Eastern Pacific that supports the Peruvian anchovy fishers.
The team said that if the trend is real, it could also accelerate warming.
Photosynthesis by phytoplankton removes carbon dioxide from the air and produces oxygen.
Scientists have already said that the waters appear to be absorbing less CO2 in several parts of the world, notably in the Southern Ocean. However, this is principally thought to be due to changes in wind patterns.
“Phytoplankton… produce half of the oxygen we breathe, draw down surface CO2, and ultimately support all of our fisheries,” said Boris Worm, another member of the Dalhousie team.
“An ocean with less phytoplankton will function differently.”
The overall decline in phytoplankton might be expected to continue if the planet continues to warm in line with projections of computer models of climate.
However, Daniel Boyce said that it was not certain.
“It’s tempting to say there will be further declines, but on the other hand there could be other drivers of change, so I don’t think that saying ‘temperature rise brings a phytoplankton decline’ is the end of the picture,” he said.
Lundin told BBC that the implications could be significant.
“If in fact productivity is going down so much, the implication would be that less carbon capture and storage is happening in the open ocean,” he said.
“So that’s a service that humanity is getting for free that it will lose; and there would also be an impact on fish, with less fish in the oceans over time.”
Image Caption: Phytoplankton are the foundation of the oceanic food chain. Credit: NOAA MESA Project
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