Sea surface temperatures for the North Sea for the period 1948 to 1983 do not show any clear trend but are rather dominated by apparently periodic fluctuations on relativelyshort time scales, at 9–11 years and, from February to June, also at 3–3·5 years. The short period events may be related to fluctuations on a similar time scale in the frequency of westerly weather over the United Kingdom and also to fluctuations in the abundance of the zooplankton. The relationship between temperature and plankton is negative, with the temperature leading the plankton by about a quarter of a wavelength. The problem of interpreting this relationship is investigated using a simple simulation model.