Abstract
The ecosystems of coastal and enclosed seas are under increasing
anthropogenic pressure worldwide, with Chesapeake Bay, the Gulf of Mexico and
the Black and Baltic Seas as well known examples. We use an ecosystem model
(Ecopath with Ecosim, EwE) to show that reduced top-down control (seal
predation) and increased bottom-up forcing (eutrophication) can largely explain
the historical dynamics of the main fish stocks (cod, herring and sprat) in the
Baltic Sea between 1900 and 1980. Based on these results and the historical fish
stock development we identify two major ecological transitions. A shift from seal
to cod domination was caused by a virtual elimination of Marine mammals
followed by a shift from an oligotrophic to a eutrophic state. A third shift from
cod to clupeid domination in the late 1980s had previously been explained by
overfishing of cod and climatic changes. We propose that the shift from an
oligotrophic to a eutrophic state represents a true regime shift with a stabilizing
mechanism for a hysteresis phenomenon. There are also mechanisms that
could stabilize the shift from a cod to a clupeid dominated ecosystem, but there
are no indications that the ecosystem has been pushed that far yet. We argue
that the shifts in the Baltic Sea are a consequence of human impacts, although
variations in climate may have influenced their timing, magnitude and persistence.
Key words:
eutrophication;
fishing;
marine mammal;
predation;
regime shift;
trophic cascade.
http://dx.doi.org/10.1007/s10021-007-9069-0
Notes
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- Atlantic Grey Seal (Halichoerus grypus)
- Ringed Seal (Phoca hispida)
- Common Seal (Phoca vitulina)
- Harbour Porpoise (Phocoena phocoena)
- Clupeid (Clupeidae)
- Cod (Gadus morhua)
- eutrophication
- fishing
- predation
- regime shift
- trophic cascades
Österblom, H.,
Hansson, S.,
Larsson, U.,
Hjerne, O.,
Wulff, F.,
Elmgren, R.,
Folke, C.,
- Baltic Sea
- Ecosystems
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