Boom and bust population dynamics of the helmet jellyfish Periphylla periphylla: Implications on carbon fluxes in marine ecosystems

Jellyfish blooms occur resulting from redistribution/aggregation events or peaks in population growth. Such blooms can affect ecosystem structure and stability due to the role of jellyfish as top predators of fish larvae and eggs and as competitors of fish preying on the same zooplankton resources. The factors leading to bloom formation have received considerable attention in the past while the factors causing blooms to collapse are less studied so far. However, a better understanding on bloom dynamics is crucial to allow estimates on vertical carbon transport, turnover and flux processes.

In this study, the helmet jellyfish Periphylla periphylla served as a model organism representing a species that causes mass occurrences in several Norwegian fjords thus stimulating debates on potential regime shifts, changes in ecosystem stability and socioeconomic implications.

Here, we used 15-years of trawl data (2006-2015; 2018-2021) complemented with state-of-the art imaging approaches using a remotely operated vehicle (ROV) to study the boom and bust population dynamics of P. periphylla in different areas and seasons in Trondheimsfjorden, Norway. The successful bloom formation of P. periphylla could be attributed to its longevity, dispersal, population connectivity, holoplanktonic life cycle and the lack of natural predators while parasites were identified as potential bloom-controllers thus eventually causing blooms to collapse. The described patterns in jellyfish population dynamics can have substantial effects on the marine carbon cycle by enhancing carbon sequestrations and a subsequent vertical transport to deeper layers of the ocean.