Jellyfish blooms have increased in magnitude in several locations around the world, including in fjords.While the factors that promote jellyfish blooms and the impacts of live blooms on marine ecosystems areoften investigated, the post-bloom effects from the sinking and accumulation of dead jellyfish at the seafloorremain poorly known. Here, we quantified the effect of jellyfish deposition on short-term benthic carboncycling dynamics in benthic cores taken from a cold and deep fjord environment. Respiration was measuredand 13C-labeled algae were used as a tracer to quantify how C-flow through the benthic food web wasaffected over 5 d in the presence and absence of jellyfish carcasses. Benthic respiration rates increased rapidly(within 2 h) in the jellyfish-amended cores, and were significantly higher than cores that were supplied withonly labeled phytodetritus between 17 h and 117 h. In the cores that were supplied with only labeled phytodetritus,macrofauna dominated algal-C uptake over the 5 d study. The addition of jellyfish caused a rapidand significant shift in C-uptake dynamics: macrofaunal C-uptake decreased while bacterial C-uptakeincreased relative to the cores supplied with only phytodetritus. Our results suggest that the addition of jellyfishdetritus to the seafloor can rapidly alter benthic biogeochemical cycling, and substantially modify C-flowthrough benthic communities. If our results are representative for other areas, they suggest that jellyfishblooms may have cascading effects for benthic ecosystem functions and services when blooms senesce, suchas enhanced bacterial metabolism and reduced energy transfer to upper trophic levels.
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