Fig. 11. Phosphate concentration in North Sea water during the winter. The figures refer to
10 - 7 mol/I of phosphorus reported as phosphate; they are to be multiplied by 3.1 if values in
P.g/l are desired (McIntyre and Johnson 1975)
It will still be a number of years before scientists can determine whether an increase
in phytoplankton has also resulted, because marked differences from year to year
make it difficult to define trends statistically. It is an open question, then, whether
observed summer situations of oxygen deficiency in the nearbottom water are the
consequence of the eutrophication trend, and whether there is any effect on the
fisheries yield. Around 1.3 million t of fish were caught annually in the North Sea in
the 1930's, approximately 2.4 million t in the 1960's.
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