The Scotsman Sat 9 Jun 2007

Tiny organisms bloom and die in the North Sea

PAUL TETT

MANY of the most important creatures in the sea can only be seen with a microscope, but they play a huge part in the health of our oceans. The "balance of organisms" in the tiny floating plants that make up phytoplankton is vital for a healthy marine food chain.

Recent studies by Napier University and others have analysed changes in North Sea phytoplankton since 1959, using samples taken by the Continuous Plankton Recorder (CPR) Survey founded by the far-sighted marine biologist Sir Alistair Hardy and, for a time, based in Edinburgh. After 20 years of downwards trend in this balance, there is evidence the North Sea is beginning to recover.

During spring and summer, the seas around Scotland are teeming with life - most of it invisible to the naked eye. But if you fill a glass with seawater and hold it up to sunlight, you should see dancing specks. Many of these specks are tiny floating plants - phytoplankton - food for tiny animals, which in turn are food for fish such as haddock.

Just as a well-balanced forest has a mixture of oak and birch, bluebell and bramble, phytoplankton should contain a mixture of species. Indeed, this is now a legal requirement - European Directives, implemented in Scots Law, prohibit "disturbance to the balance of organisms".

This balance can be hard to measure because, like bluebells, phytoplankton appear, bloom and then disappear with the seasons. Thus, part of the balance is the seasonal fluctuation.

The season usually commences with a "spring-bloom" of tiny brown plant cells called diatoms. On these feed little planktonic animals, which, in turn, are food for freshly hatched fish. The diatoms are followed by flagellates and dinoflagellates, which include a notorious species called Karenia mikimotoi, which sometimes becomes so abundant it discolours the sea. This is called a "red tide", and can result in the death of wild animals or farmed fish. A red tide occurred in Shetland a few years ago, from natural causes so far as we know.

So while nasty phytoplankters, like Karenia mikimotoi, are part of the natural order of things, a disturbance of the balance between diatoms and dinoflagellates is undesirable. So to keep an eye on this balance, we need to take regular samples.

We need to do this at least once a month over many years to track seasonal changes. UK environmental agencies, including the Scottish Environment Protection Agency, do this, but it generates long lists of names and numbers, and so the Department for Environment, Farming and Rural Affairs has funded a project to develop a tool called the Phytoplankton Community Index (PCI).

The PCI is a procedure for graphing and calculation that allows a single number to be pulled out of long statistical lists and compared with an earlier state of affairs in the plankton called a "reference condition". If there has been no change in the phytoplankton balance, the PCI would be given the number 1. If the number falls below 0.5, then there has been a big change.

Oceanography is a team game, and the team included Napier University; the Fisheries Research Service in Scotland (FRS); the Centre for Environment, Fisheries and Aquaculture Studies (CEFAS); and the Sir Alistair Hardy Centre for Ocean Study (SAHFOS) in England. We completed the work last year and have, since then, been applying the PCI to track long-term changes in the phytoplankton in the seas around the UK.

As mentioned, phytoplankton changes throughout the year, much like trees coming into leaf in spring but, as we've seen recently, trees are coming into leaf earlier each year as a result of global warming. This change has taken place over a human lifetime and, to observe similar changes in the sea, we need observations made over a similarly long period. Until recently, many marine scientists were discouraged from making repeated observations as it did not lead to immediate discoveries and was not seen as exciting science.

Awareness of climate change has altered this view and, since 1995, the FRS has taken weekly samples from the North Sea, though still not long enough to show changes. Thanks to Sir Alistair, we do have one long-term series of data. Sir Alistair invented the CPR, which is towed behind a ship, scooping plant and animal plankton samples up for later analysis. Si Alistair started his plankton survey from Hull during the 1930s. After the Second World War, the survey moved to Edinburgh, where it became part of the Scottish Marine Biological Association. In 1976, the survey moved to Plymouth where it flourishes as the SAHFOS.

Since 1957, the survey methods have been consistent and so we had 45 years of data to analyse. Starting in the last decade of the 1950s, which we took as the reference condition, we found the value of the PCI steadily decreased for 20 years, reaching a minimum in the late 1970s.

The change was greatest in the southern North Sea and least in the northern North Sea. This was not unexpected, as it is the southern part of the North Sea that is most impacted by human-produced nutrients and pollutants. Why has it changed in the northern North Sea? Possible causes include climate change and the effect of fisheries, which have taken too many of the large fish and so changed the rate of predation on the little animals that eat phytoplankton.

The good news is that we can see a recovery in most parts of the North Sea, a tendency for the PCI value to climb back towards 1. This is not to say there are not persistent alterations in the abundance of plankton in the North Sea - there are, and they have been shown by SAHFOS to be related to global warming. But it may be the marine ecosystem, relieved of some of the worst human interference, is regenerating a healthy balance of organisms, even if they are different species from those common during the 1950s.

• Professor Paul Tett is a biological oceanographer at the school of life sciences at Napier University.

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