Climate variability across three distinct spatial scales and the interannual changes of planktonic copepods and jellyfish were studied from November 1966 to December 1993. The results obtained identified a cascade of links between the large-scale climate pattern playing out in the North Atlantic and the local climate variability governing the Ligurian Sea, northwestern Mediterranean. The chain of events appeared driven by the long-term temperature anomalies that in turn played a key role in the relationship between copepods and jellyfish. Indeed, the increase in jellyfish outbreaks during the 1980s was largely favored by high positive anomalies observed in climate at large (North Atlantic), regional (Ligurian Sea), and local (Villefranche Bay) spatial scales and in water temperature. Linked to jellyfish abundance, the dynamics of the relationship between copepods and jellyfish showed a strong modification during the middle-late 1980s, which highlight the top-down effect exerted by jellyfish on copepods. The warming temperatures and the observed ecological changes, high abundance of jellyfish, and marked drop in the abundance of copepods are discussed in the framework of a potential change in the functioning of the planktonic ecosystem indicative of a more regeneration-dominated production system in the Ligurian Sea. Overall, the cascade of links identified by these results should be considered and integrated into the assessment and modeling studies of pelagic ecosystem and biogeochemical fluxes in the northwestern Mediterranean Sea.

From measurements of the distances between sctules (mesh size) on the maxilla of Acartia clazcsi, a spectrum of filtration efficiencies for small particles has been established. The slope of the spectrum decreases from young stages (copepodite I) to adults. The average size of particle that can be collected increases slightly from 3 p in CI to 7 ,U in adult stages, whereas the size for which the cfficicncy is 100% increases sharply from 5 u to 12 p.Two expcrimcnts were done to show the reality of the spectrum of filtration efficiency, A model of growth of particles and grazing by copepods, using the initial experimental conditions and the efficiency spectrum, was used to simulate the evolution of particles in the two experiments. The fit of the model to the experimental data is good for experiment I ( dark ) and the first 4 clays of cxpcriment II (natural daylight). The results of expcriment II show that processes other than grazing become important in a long term experiment.

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.

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