The availability of nutrients in oligotrophic marine ecosystems is limited. Following jellyfish blooms, large die-off events result in the release of high amounts of nutrients to the water column and sediment. Our study assessed the decomposition effects of an infamous invasive jellyfish in the ultra-oligotrophic Eastern Mediterranean Sea. We found that jellyfish decomposition favored heterotrophic bacteria and altered biogeochemical fluxes, further impoverishing this nutrient-poor ecosystem.