Impact of jellyfish and mussels on algal blooms caused by seasonal oxygen depletion and nutrient release from the sediment in a Danish fjord

The present case study in the heavily eutrophicated Skive Fjord (Denmark) provides an illustration of the potential links between primary production, oxygen deficiency, nutrients, mussels and jellyfish, and highlights the value of long-term monitoring and an experimental, integrative approach in the investigation of eutrophication processes. Skive Fjord suffers every summer from oxygen depletion in the near-bottom water causing large amounts of nutrients (phosphate and ammonia) to be released from the anoxic sediment. This subsequently stimulates a phytoplankton bloom, followed later on by an increase in the zooplankton. The surface chlorophyll a concentrations may become very high during periods with exceptionally severe oxygen depletion, and in certain years with mass occurrence of jellyfish (Aurelia aurita) peak concentrations as high as 60 to 80 μg chl a l− 1 have be measured in Skive Fjord because the jellyfish effectively eliminate the zooplankton-grazing impact on the phytoplankton bloom. Likewise, the grazing impact by dense populations of mussels (Mytilus edulis) can modify the phytoplankton biomass, especially in the near-bottom water. Here we combine available data on jellyfish and mussels with data on oxygen, nutrients, chlorophyll a, zooplankton, and other data from different studies conducted in Skive Fjord during the period 1996–2005. The study indicates that especially severe cases of oxygen depletion take place in years with mass occurrence of jellyfish because the blooming algae are not efficiently grazed, but settle to the bottom to be subsequently decomposed, leading to more severe oxygen depletion and killing of filter-feeding mussels, further escalation of the issue and severely impact of ecosystem services proved by the mussels.