Macrojellyfish blooms and mesozooplankton biomass: long-term study in the Gulf of Trieste (Adriatic Sea)

Jellyfish are considered important predators that play a key role in the organic matter cycling when they occur in blooms. Although their diet can vary spatially and temporally and be species-specific, mesozooplankton are generally recognised as important prey for various macrojellyfish. We present data from a long-term study (1974-2019) comparing the mesozooplankton biomass and blooms of four regularly occurring Scyphozoa species (Aurelia solida, Cotylorhiza tuberculata, Pelagia noctiluca, Rhizostoma pulmo) in the northern Adriatic, and since 2016, also of the invasive Ctenophora Mnemiopsis leidyi. The results showed large inter-annual variations in zooplankton biomass: annual dry mass geomean varied between 9.9 mg/m³ in 2016 and 49.9 mg/m³ in 1992, with 73% of the results below 20 mg/m³, while zooplankton carbon levels ranged between 0.2 and 22.7 mg/m³. In the period 1974-2019, there were years without massive jellyfish blooms and years in which several species occurred "en masse" in different seasons and/or together: A. solida in winter-spring, C. tuberculata in summer, R. pulmo in autumn-winter-spring and M. leidyi in summer-autumn. The Kruskal–Wallis test (p=0.0016) and Dunn's post-hoc multiple pairwise comparison test revealed significant differences in zooplankton biomass between years without blooms or single-species blooms of short duration and those with multi-species blooms with a cumulative duration of > 1 month.