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Another team of scientists, including Dr. Worm, mapped the diversity of marine life on a broad scale. One surprise finding was that while coastal marine species showed greater diversity at the equator, the diversity of oceanic species peaked in the mid-latitudes. That's unlike terrestrial diversity, which largely peaks at the tropics.

The researchers also analyzed possible links between the global distribution of 11,000 marine species—big and small—to such environmental factors as temperature, oxygen levels and habitat availability. For all species types, only one factor showed a consistent correlation to diversity: sea temperature.

When plant plankton plummet — like they do during El Nino climate cycles_ sea birds and marine mammals starve and die in huge numbers, experts said.

Worm said when the surface of the ocean gets warmer, the warm water at the top doesn't mix as easily with the cooler water below. That makes it tougher for the plant plankton which are light and often live near the ocean surface to get nutrients in deeper, cooler water. It also matches other global warming trends, with the biggest effects at the poles and around the equator.

Phytoplankton are microscopic marine organisms capable of photosynthesis, just like terrestrial plants. They float in the upper layers of the oceans, provide much of the oxygen we breathe and account for about half of the total organic matter on Earth. A 40 per cent decline would represent a massive change to the global biosphere."If this holds up, something really serious is underway and has been underway for decades. I've been trying to think of a biological change that's bigger than this and I can't think of one," said marine biologist Boris Worm of Canada's Dalhousie University in Halifax, Nova Scotia. He said: "If real, it means that the marine ecosystem today looks very different to what it was a few decades ago and a lot of this change is happening way out in the open, blue ocean where we cannot see it. I'm concerned about this finding."The researchers studied phytoplankton records going back to 1899 when the measure of how much of the green chlorophyll pigment of phytoplankton was present in the upper ocean was monitored regularly. The scientists analysed about half a million measurements taken over the past century in 10 ocean regions, as well as measurements recorded by satellite.They found that phytoplankton had declined significantly in all but two of the ocean regions at an average global rate of about 1 per cent per year, most of which since the mid 20th Century. They found that this decline correlated with a corresponding rise in sea-surface temperatures – although they cannot prove that warmer oceans caused the decline.The study, published in the journal Nature, is the first analysis of its kind and deliberately used data gathered over such a long period of time to eliminate the sort of natural fluctuations in phytoplankton that are known to occur from one decade to the next due to normal oscillations in ocean temperatures, Dr Worm said. "Phytoplankton are a critical part of our planetary life support system. They produce half of the oxygen we breathe, draw down surface CO2 and ultimately support all of our fishes." he said.But some scientists have warned that the Dalhousie University study may not present a realistic picture of the true state of marine plantlife given that phytoplankton is subject to wide, natural fluctuations."Its an important observation and it's consistent with other observations, but the overall trend can be overinterpreted because of the masking effect of natural variations," said Manuel Barange of the Plymouth Marine Laboratory and a phytoplankton expert.

However, the Dalhousie scientists behind the three-year study said they have taken the natural oscillations of ocean temperatures into account and the overall conclusion of a 40 per cent decline in phytoplankton over the past century still holds true. "Phytoplankton are the basis of life in the oceans and are essential in maintaining the health of the oceans so we should be concerned about its decline."It's a very robust finding and we're very confident of it," said Daniel Boyce, the lead author of the study."Phytoplankton is the fuel on which marine ecosystems run. A decline of phytoplankton affects everything up the food chain, including humans," Dr Boyce said.

Phytoplankton is affected by the amount of nutrients the well up from the bottom of the oceans. In the North Atlantic phytoplankton "blooms" naturally in spring and autumn when ocean storms bring nutrients to the surface. One effect of rising sea temperatures has been to make the water column of some regions nearer the equator more stratified, with warmer water sitting on colder layers of water, making it more difficult for nutrients to reach the phytoplankton at the sea surface.Warmer seas in tropical regions are also known to have a direct effect on limiting the growth of phytoplankton.