From fish to jellyfish in the eutrophicated Limfjorden (Denmark)

The heavily eutrophicated Limfjorden (Denmark)
provides a good illustration of the value of long-term monitoring,
especially if this is combined with an experimental,
interdisciplinary research approach. Here, we first give a short
overview of the environmental status of Limfjorden, including
the historical development of nutrient overloading and subsequent
oxygen depletion in near-bottom water, and how the
annual landings of edible bottom-dwelling fish species
(plaice, flounder, eel and others) caught in Limfjorden have
decreased from about 2,500 t in the early 1920s to only about
20 t in recent years where the fish have been replaced by an
increasing number of especially the moon jellyfish, Aurelia
aurita, which mainly preys on zooplankton. Next, we evaluate
the ecological consequences of the present high number of
jellyfish, based on data from recent years’ research on the
abundance of jellyfish, their population dynamics and predation
impact. In Limfjorden, the benthic polyp stage of A.
aurita ensures a large number of small ephyrae in the early
spring and subsequently a large population of adult medusae
that control the zooplankton during summer and autumn. The
holopelagic invasive ctenophore Mnemiopsis leidyi, which
was observed in Limfjorden for the first time in 2007, is a
second carnivore adding additional predation pressure of the
indigenous A. aurita so that copepods and other mesozooplankton
organisms may be virtually absent, as observed in
2008 and 2009 where ciliates made up a substantial part of the
zooplankton biomass. Marine environmental management
programmes should be aware of the increasing importance
of both indigenous and new invasive jellyfish species that may
show mass occurrence in especially eutrophicated and overfished
areas