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A parallel is shown to exist between the development of phytoplankton distributions and the establishment and breakdown of the seasonal thermocline, which in turn is determined largely by tidal streaming. The spring phytoplankton outburst was first observed as the thermocline became established in the area of weak tides south of Nymphe bank in the Northern Celtic Sea. In the summer, dense blooms of phytoplankton were found in the frontal regions between well-stratified shelf waters and more coastal areas where strong tides maintained well-mixed conditions. In the autumn the retreating thermocline was followed back to the area of weak tides south of Nymphe bank. Despite the greater availability of surface nutrients at this time, the increased mixed layer depth and the reduced light levels did not permit such high surface chlorophyll 'a' values.

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The edge of the Celtic Sea shelf is characterized during the summer by a band of cold water ( ca . 100 km broad), which is generally conspicuous in high resolution infrared images from satellites, particularly under high pressure atmospheric conditions with clear skies. Preliminary studies of mixing in this region were made in 1972, 1973 and 1974 and were followed by more detailed interdisciplinary studies in 1976, 1979 and 1980 relating phytoplankton growth to the ways in which turbulence in the environment controls the availability of nutrients and light energy. The results have shown the cooler water to be about 1-2 °C colder than the adjacent surface waters of the Celtic Sea and Atlantic Ocean. This cold band also exhibits higher than background surface values of inorganic nitrate and chlorophyll a . Although these values are generally low compared with the values that have been observed near the neighbouring shelf tidal fronts, the increased surface values along the shelf break in summer appear to be significant. The observed increases of chlorophyll a are thought to be related to physical processes associated with the slopes, ridges and canyons where enhanced mixing, particularly due to internal waves or upwelling, results in nutrient renewal and subsequent phytoplankton growth along the shelf-break region of the Celtic Sea.

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