Evidence for potential top-down control of meso- and micro-copepods during jellyfish blooms revealed by eDNA metabarcoding

The limitation of traditional survey methods has catalyzed the rapid development and widespread application of eDNA metabarcoding. By collecting water samples from coastal waters off the Shandong Peninsula across the key blooming phase (May and August) of jellyfish, this study reveals the environmental drivers of jellyfish blooms and the potential top-down control exerted by jellyfish on meso- and micro-copepods. The results indicate that Aurelia coerulea is predominantly distributed in the northern Laizhou Bay, while Nemopilema nomurai is mainly found in the transition zone of Bohai and Yellow Seas. This distribution pattern may be attributed to different habitat selectivity during the polyp stage as well as environmental adaptability and ocean current effect during the medusa stage. The relative read abundance (RRA) of A. coerulea was significantly negatively correlated with that of meso- and micro-copepods, a pattern consistent with a potential top-down effect; however, under potentially phosphorus-limiting conditions, this inverse association did not cascade into an increase in phytoplankton biomass reflected by Chl a variation. Instead, elevated jellyfish RRA was associated with increased water turbidity. Our study provides a methodological paradigm for applying eDNA approach to elucidate the ecological consequences of regional jellyfish blooms, with future potential for scaling up to ecosystem-level investigations.