Résumé
In marine ecosystems, pelagic copepods, chaetognaths and jellyfish play a key role in matter and energy flow. While copepods support most food webs and the biological pump of carbon into the deep ocean, chaetognaths and jellyfish may affect the strength of the top-down control upon plankton communities. In this study, we show that the main events in the long-term variability of these functional groups in the Northwestern Mediterranean were tightly linked to changes of climate forcing of the North Atlantic sector. Large-scale climate forcing has altered the pelagic food-web dynamics through changes in biological interactions, competition and predation, leading to substantial changes manifested as bursts or collapses in zooplankton populations, and consequently to a major change ca. 1987. These events become more frequent in the 1980s and the early 1990s in the studied zooplankton functional groups suggesting a shift in the functioning of the pelagic ecosystem. The environmental modifications and the results reported here are therefore, indicators of a regime change pointing to a more regeneration-dominated system in the study area. We suggest a chain of mechanisms, whereby climate variation has modified the long-term dynamics of pelagic copepods, chaetognaths and jellyfish in the Ligurian Sea.