The influence of the Atlantic and regional climate variability on the long-term changes in gelatinous carnivore populations in the northwestern Mediterranean

We investigated potential connections between climate and the population size of gelatinous carnivores in the northwestern Mediterranean. The interannual variability of largeand meso-scale climate factors was related to gelatinous carnivore dynamics by means of a multivariate regression model. Gelatinous carnivore population dynamics integrate the climate-related changes in the northwestern Mediterranean with close correlations between climate and the population size of the Calycophoran siphonophores Chelophyes appendiculata and Abylopsis tetragona and the Hydromedusae Rhopalonema velatum and Solmundella bitentaculata. We show that the sensitivity of particular gelatinous carnivore species to environmental forcing relates to their seasonal appearance and peak of abundance. Our results advocate the occurrence of short time windows, during which gelatinous carnivores appear more sensitive to environmental conditions, and favorable conditions may therefore substantially enhance their annual peaks. Furthermore, there were threshold values from which climate effects on gelatinous carnivores become noticeable, indicating that the climate–gelatinous carnivore relationship intensifies according to the strength of climate forcing. The possibility of using the North Atlantic climate variability for assessing and predicting interannual abundance changes of these organisms in the northwestern Mediterranean is considered. Pelagic gelatinous carnivores are of particular ecological importance in marine ecosystems because of their potential effect on the population size of mesozooplankton, including fish eggs and larvae (Purcell and Arai 2001). High abundances of these organisms may channel the energy flow away from fish and affect the fish : primary production ratio (Sommer and Stibor 2002). Several investigations have also emphasized the effects of gelatinous carnivores on fisheries. In British Columbia, the hydromedusa Aequorea victoria exerts a top-down control on the Pacific herring larvae (Clupea pallasi) (Purcell and Grover 1990); in the Bering Sea, high abundances of the scyphomedusa Chrysaora melanaster affect recruitment and population size of pollock (Theragra chalcogramma) (Brodeur et al. 2002); in the Baltic and North Seas, the scyphomedusa Aurelia aurita appears as a main predator of herring larvae (Clupea harengus) (Möller 1984; Lynam et al. 2005b). Also in the North Sea, other scyphomedusae, such as Cyanea capillata and Cyanea lamarckii, exert a top-down control on plaice (Pleuronectes platessa) and salmon (Salmo salar) larvae (Lynam et al. 2004, 2005); in the Black Sea, the collapse of stocks of anchovy (Engraulis encrasicolus), Mediterranean horse mackerel (Trachurus mediterraneus), and sprat (Clupeonella cultriventris) in the late 1980s has been linked to the introduction of the ctenophore Mnemiopsis leidyi (FAO 1997). Massive proliferations of gelatinous carnivores (e.g., medusae, ctenophores, and siphonophores) reported in some coastal areas and temperate seas in the last few decades may therefore be indicative of considerable changes in the functioning of marine ecosystems (Mills 2001; Purcell 2005; Hay 2006). Under climate warming scenarios, the recurrence of triggering factors for gelatinous outbreaks (e.g., warm waters and the subsequent enhancement of the water column stratification) makes ecologically plausible a higher predation or competition pressure upon fish stocks. A current challenge is, then, to identify how climate interacts with the population size of these organisms and to identify the potential for the consequent effects such interaction may have on the pelagic realm (Brodeur et al. 2002; Lynam et al. 2004). Retrospective investigations have proved very useful in exploring impending relationships between climate, hydrographic environment, and marine populations (Chiba et al. 2006). Particularly sensitive and observable indicators of 1 Corresponding author ([email protected]). This work is a contribution to the DFG priority program AQUASHIFT (the impact of climate variability on the aquatic ecosystems). Acknowledgments We thank the staff of the Laboratory of Oceanography of Villefranche and particularly Serge Dallot, whose efforts have made this long-term survey possible. We also thank Marina Chifflet for assistance in Ekman calculations and Frédéric Ibanez and Paul Nival for insightful discussion. Limnol. Oceanogr., 53(4), 2008, 1456–1467 E 2008, by the American Society of Limnology and Oceanography, Inc.