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During the winter months, the surface temperature gradient for the whole region trends roughly east–west. In the eastern North Sea, along the German and Danish coasts, mean winter surface temperature is typically less than 3  C. North–south isotherms extend almost to the middle of the North Sea but, near the east coasts of England and Scotland, they are sharply deflected to run east– west, influenced by warmer Atlantic water in the northern North Sea and warmer flows through the Dover Strait. However, from the Thames Estuary to Caithness, inshore waters have a mean winter temperature of only 5 or 6  C.

As day length and sea temperature rise in early spring, phytoplankton growth increases rapidly, developing a ‘bloom’, which over the north-west European shelf peaks between late April and May. This growth is fuelled by nutrient levels enhanced by winter stirring—the overturn of coastal waters by wind and storm, bringing inorganic nutrients and minerals to the sea surface. The phytoplankton bloom is followed by a bloom of grazing zooplankton, the peak of which coincides with the decline of the phytoplankton.

Ii. iii. Fig. 2 Go to 4 Thin, flat, leaf-like worms. Pastel-coloured or striped. Free living (Fig. 1) Flatworms (p. 133) Thin, flat, often with suckers or hooks. Parasitic, attached to or in fish or macroinvertebrates (Fig. 2) Flukes (Digenea, Monogenea), not treated further here Slender, soft, elongated worms, 5 mm to 10 m long. Often slimy. Head end may bear minute eyes; often brightly coloured (Fig. 3) Nemerteans (p. 141) A digenean fluke, Cryptocotyle. Fig. 1 A turbellarian, Prostheceraeus vittatus (Montagu).