Abstract
Global changes have altered biogeography, phenology and abundance of marine populations, thereby promoting a reconfiguration of marine ecosystems’ functioning. Increasing jellyfish blooms warn of major changes not only in ecosystem structure but also in their services and ultimately in human welfare. Rhizostoma pulmo is a large native scyphomedusa of the southern European seas, and in the last decade has gained notoriety due to massive bloom events. Underlying factors driving this phenomenon remain so far elusive, mainly due to the lack of knowledge on the species’ ecology. The aim of this Ph.D. was to uncover drivers of the population dynamics and trophic ecology of R. pulmo through a multi-scale approach. To achieve it, three nested and complementary approaches were employed. The first one assessed, by means of data mining, environmental drivers shaping geographical patterns of bloom events on a long-term scale along the Mediterranean and Black Seas (7,359 records from 1875 to 2019). Secondly, the effect of biotic and abiotic factors on the dynamics of a R. pulmo population, and its trophic role along ontogeny were studied. To do so, an in-situ monitoring was performed during one year (2019) in Bages Sigean, a northwestern French Mediterranean lagoon. It is a natural mesocosm that offers the possibility to track the species on time, as all its life cycle occurs in the lagoon. For a deeper understanding of the processes behind the observed patterns, as a final step, these results were incorporated into a food web model. The 0D plankton food web model coupled low trophic level dynamics, based on a classical Nutrient Phytoplankton Zooplankton Detritus (NPZD) model, to copepods and a jellyfish model based on the pelagic life stages from ephyrae to large medusae.The study showed that R. pulmo blooms exhibited an enhanced magnitude and frequency in recent decades, concurrently with positive temperature anomalies. The biogeographical patterns of the species appear also to be shaped by latitudinal temperature gradients, as northern locations with colder waters showed less intense blooms than southern warmer locations, where the most intense bloom events were recorded. Results uncovered a significant effect of warmer springs on phenological changes, which boosted an earlier start and a longer duration of the medusae season. An intersite comparison revealed different environmental niches in three Mediterranean lagoons, suggesting the existence of a metapopulation. At a local scale, three cohorts were identified during one year, and mesozooplankton abundance appeared to drive the population dynamics in the lagoon, evidencing a bottom-up control. Results showed that the diet composition differs from the availability of prey in the environment with contrasting preferences along ontogeny. Calanoid and harpacticoid copepods were the most frequent prey and the major carbon contributors for young medusae (bell diameter <15 cm), whilst ciliates were the most frequent prey for large organisms (>15 cm). Model simulations emphasized a leading effect of temperature on the bloom timing, while bloom development was promoted by food availability. These results bring an integrated overview of how multiscale environmental signals shape the ecology and population dynamics of R. pulmo. With jellyfish's broad diversity of forms, life strategies and trophic roles, we encourage future research to apply similar strategies in other species, that will allow identifying which species will increase in the future, and where.