Influence of decomposing jellyfish on the sediment oxygen demand and nutrient dynamics

Jellyfish populations can grow rapidly to attain large biomasses and therefore can represent significant stocks of carbon and nitrogen in the ecosystem. Blooms are also generally short-lived, lasting for just weeks or months, after which time the population can decline rapidly, sink to the bottom and decompose. The influence of decomposing jellyfish (Catostylus mosaicus, Scyphozoa) on benthic dissolved oxygen and nutrient fluxes was examined in a mesocosm experiment at Smiths Lake, a coastal lagoon in New South Wales, Australia. Sediment (10 l) was placed in each of 10 mesocosms (50 x 40 cm, 30 cm deep and 60 l volume) which were supplied a continuous flow of fresh lagoon water. One jellyfish (1.6 kg wet weight or 25 g C m(-2)) was added to each of five mesocosms, with the remaining five mesocosms serving as controls. Exchanges of dissolved oxygen, organic and inorganic nutrients between the benthos and water column were measured 14 times over a period of nine days. The addition of dead jellyfish tissue to the mesocosm sediments initially resulted in an efflux of phosphate, dissolved organic nitrogen and dissolved organic phosphorus to the water column. Dissolved organic nitrogen and dissolved organic phosphorus effluxed at rates more than 8 and 25 times greater than those measured in control mesocosms, respectively. This was probably due to the intracellular nutrients leaching from the jellyfish tissues. As decomposition proceeded, a large quantity of ammonium was released to the water column and sediment oxygen demand increased, indicating bacterial decomposition was dominant. Overall the addition of dead jellyfish caused a 454% increase in ammonium efflux and 209% increase in sediment oxygen demand over the 9-day experiment relative to the controls. The decomposition of large numbers of jellyfish after major bloom events could be a significant source of nutrients and, depending on the system, could have a major impact on primary production. Moreover, depending on the degree of mixing in the water column, decaying jellyfish may also contribute to bottom water hypoxia.