Invasive upside-down jellyfish tolerate organic eutrophication and warming

The upside-down jellyfish Cassiopea is a successful invasive organism, due to high tolerance of different environmental conditions. So far, some single local or global factor experiments have been conducted to assess the effect on the ecophysiology of Cassiopea, but information about combined factor effects is scarce. Therefore, we performed a 42-d manipulation experiment exposing Cassiopea sp. to a local (organic eutrophication) and global factor (warming) simultaneously, i.e., four different dissolved organic carbon (DOC) concentrations [3 (Control), 10 (Low), 20 (Medium) and 40 (High) mg L−1)], followed by a stepwise water temperature increase from 26 to 32 °C, in which we assessed four ecophysiological parameters: respiration, net photosynthesis, gross photosynthesis, and pumping rate. Our results show a steep increase in respiration and simultaneous decline in gross photosynthesis in response to Medium and High DOC enrichment causing medusae in the respective treatments to become net heterotrophic over time. This was accompanied by increases in pumping rates, likely to optimize gas exchange and potentially increase nutrient availability by advection of nutrients trapped in underlying sediments. No effect of warming was found for any of the assessed parameters. The observed transition from auto- to heterotrophy under high DOC concentrations together with thermal tolerance of Cassiopea sp. aids to understand the current invasive success of members of the genus Cassiopea as nonnative organisms and may shed light on the future of Cassiopea sp. as potential native invaders.