Nutrients, particularly nitrogen and phosphorus, play a major role in stimulating primary production, which in turn supports ecosystem productivity by providing organic matter to the marine food chain. While flowing towards higher trophic levels, organic matter is partially remineralised by heterotrophic microbes supplying nutrients back to primary producers. In coastal areas, jellyfish blooms represent a significant but largely overlooked source of organic matter that can impact microbial communities and ecosystem productivity via processes such as the excretion of dissolved inorganic nutrients (ammonium and phosphate) and dissolved organic matter (mucus). The overall objective of this thesis was to investigate the combined role of jellyfish and microbes in nutrient cycling and the consequences for ecosystem productivity, using a combination of incubations experiments and analytical measurements. I show evidence of microbes living in association with jellyfish thriving by oxidizing up to one-third of the ammonia excreted by their host to nitrite and nitrate. The results showed that the jellyfish-associated release of nitrogen can provide more than 100% of the nitrogen required for primary production and revealed a new pathway for pelagic nitrification. I also highlight the macromolecular and elemental similarity between the jellyfish body and mucus compositions and provide biochemical ratios to support the integration of jellyfish into trophic and biogeochemical models. To further investigate the jellyfish mucus, I finally show that jellyfish mucus is a source of organic and inorganic nutrients that is quickly utilised by microbes albeit at a low growth efficiency. These results suggest that jellyfish blooms and their associated microbes can locally support primary production while inducing changes in nutrient stoichiometry and microbial community composition. Overall, this thesis provides data and equations particularly suitable for the integration of jellyfish bloom populations into marine ecosystem models, which is essential to better understand and quantify their effects on the structure and function of coastal ecosystems.