Abstract
Jellyfish are important components of marine ecosystems. Their spectacular blooms have severe ecological and socioeconomic impacts and are seemingly boosted by anthropogenic pressures on the marine environment. Concerns regarding increases in jellyfish blooms, at least in some areas of the world, call for a deeper understanding of their drivers. However, many jellyfish have complex life cycles, comprising both benthic and pelagic stages, which complicates the understanding of their blooms and predictions on their future evolution. Furthermore, the lack of knowledge regarding the fates of these large accumulations of biomass hampers the assessment of their impacts. With this regards, the semi-enclosed ecosystem of the Thau lagoon presents the rare particularity to harbour a complete resident population of the jellyfish Aurelia coerulea. Therefore, it offers the ideal background to study the multiple ecological processes affecting the dynamics of both its benthic and pelagic populations. This PhD built on this rare opportunity to precise the drivers and fates of the blooms of A. coerulea. This was accomplished in two steps. First, the benthic population dynamics in the lagoon was investigated and complemented with studies on its drivers and on the trophic ecology of both life stages over one year. To do so, in situ monitoring surveys and both stomach content and stable isotope analyses were employed. Second, the fate of A. coerulea biomass in the lagoon was assessed by investigating fish predation on its pelagic and benthic life stages and by studying the degradation of its medusae once dead on the seabed. This was performed by molecular analysis of fish gut contents and in situ experiments evaluating the decay rates of medusae and the potential role of the benthic fauna in their disappearance. The results obtained highlight a complex interaction of biotic and abiotic environmental parameters, which modulate bloom intensity by jointly influencing both the benthic and pelagic populations of A. coerulea. Temperature, salinity, food availability and predation appear to be the main drivers of the blooms of A. coerulea in Thau, with four critical periods, either boosting or lessening local bloom formation each year. Finally, the biomass produced by A. coerulea blooms has three main fates within the lagoon. The medusae can first be consumed alive by several pelagic predators like fish. Otherwise, when they die and sink to the seabed, some can be consumed by benthic scavengers like gastropods, but most are rapidly remineralised by the local microbial community. These findings shed light on the potential evolution of jellyfish blooms in the face of the ongoing anthropogenic forces on the marine environment, and on their impacts on coastal ecosystems functioning. However, they also highlight how intricate jellyfish blooms forecasting is and stress the need for similar comprehensive studies, not only for other jellyfish species but also in many other parts of the world.