Anomalous seasonal cycles of decapod crustacean larvae in the North Sea plankton in an abnormally warm year

Abstract

In 1989 the seasonal cycles of most common decapod larvae in Continuous Plankton Recorder samples taken in the North Sea were significantly different from those described for 1947–1951 and for 1981–1983. In all cases where the differences were significant the larvae occurred earlier in 1989 than in previous years. Zoeae of Atelecyclus rotundatus were found in the Southern Bight in the spring. The winter of 1988–1989 was exceptionally mild with air temperatures over Great Britain 2–3 °C above average and positive temperature anomalies persisted through the year. The sea surface temperatures in the North Sea were also mainly higher than usual. The events were treated as a “natural experiment” to examine effects of the high temperatures on seasonal cycles of decapod larval production and the implications of these effects for marine communities with reference to predicted global warming. The relationships between timing of occurrence of the larvae in the plankton and parameters of sea temperatures were generally consistent with previous results from studies of geographical variations, and possible causal mechanisms are discussed in the light of the 1989 data.