Jellyfish-specific trophic footprint: Obelia sp. bloom reshapes plankton biomass flow in a coastal lagoon

Gelatinous zooplankton are often represented as a single functional group in ecosystem-wide frameworks and biogeochemical models, despite growing evidence that species-specific feeding traits and seasonal dynamics may lead to contrasting planktonic responses. Here, we compared plankton community structure and biomass flow across three different years, including one year characterized by a bloom of Obelia sp., a microphagous, filter-feeding hydromedusa, and two non-bloom years. During the bloom year, patterns in plankton biomass were consistent with an enhanced role of microbial pathways, with bacterial abundance positively associated with Obelia sp., potentially reflecting increased availability of jellyfish-derived dissolved organic matter. This was accompanied by transient increases in nanoflagellates and ciliates, compatible with a short-lived microbial cascade in which these groups may have contributed to prey availability for Obelia sp. At the same time, weak coupling between bacteria and their grazers during the bloom suggests that a substantial fraction of jelly-derived organic matter may have been processed through bacterial respiration rather than transferred efficiently to higher trophic levels. Following the bloom, the community exhibited fewer interactions among the groups assessed in this study, along with a poor recovery of mesozooplankton biomass in the subsequent year. Although the Obelia sp. bloom declined rapidly, lasting less than a month, its effects on microbial community structure through its selective feeding on motile protists were more persistent. Taken together, these observations are consistent with the hypothesis that intense Obelia sp. blooms can coincide with transient reorganization of planktonic trophic pathways, although broader inference is limited by the observational nature of the study and the occurrence of a single bloom event.