157
The North Sea Regime Shift
The North Sea regime shift involved different mechanisms
that induced changes which started at the bottom of the trophic chain and propagated up to higher trophic levels (Reid
et al. 2001; Beaugrand 2004; deYoung et al. 2008; Conversi
et al. 2010; Lynam et al. 2017). The North Sea regime shift
occurred during the 1980s and was mainly induced by a
combination of increased sea surface temperatures and
changes in hydro-climatic forces (Beaugrand 2004). Due to
the increase of sea surface temperature and changes in the
water inflows, phytoplankton biomass increased. As a consequence, the zooplankton assemblage, originally dominated
by cold waters species, e.g., Calanus finmarchicus, shifted to
an assemblage dominated by warmer water species, e.g.,
Calanus helgolandicus and gelatinous zooplankton such as
jellyfish (Reid et al. 2001; Beaugrand 2004; Möllmann and
Diekmann 2012). These changes in the zooplankton community, combined with hydro-climatic changes, propagated
to higher trophic levels. Changes in temperature and/or salinity led to an increase of flatfish biomass (Möllmann and
Diekmann 2012) while the decline of C. finmarchicus, which
is the preferred prey of gadoids and especially of cod larvae,
led to cod recruitment failures (Beaugrand et al. 2003;
Beaugrand 2004) enhancing the negative sea warming
effects. These changes in recruitment had a lagged impact on
the adult gadoids biomass that, already stressed by overfishing, started to decline inexorably at the end of the 1980s
(Hislop 1996). The changes in fish biomass and composition,
together with warmer temperatures, favored the emergence
of previously scarcely present species such as horse mackerel (Trachurus trachurus) and mackerel (Scomber scombrus), especially in the northern North Sea (Reid et al. 2001;
Beaugrand et al. 2003; Beaugrand 2004).
This regime shift, induced by bottom-up processes, was
more qualitative than quantitative in the sense that changes
in assemblage and not in total biomass of trophic levels
occurred (Beaugrand 2004). The dynamics of these changes
highlighted different response time patterns depending on
the organisms affected. Indeed, the phytoplankton and zooplankton communities, with their fast life cycles, responded
to climatic changes faster than the fish community. Spatial
patterns were also different: the coastal areas were less sensitive to change in hydrodynamic conditions, and the regime
shift was stronger in the northern North Sea (Reid et al.
2001; Beaugrand 2004; Möllmann and Diekmann 2012).
This regime shift completely changed the structure of the
North Sea fish community and led to the decline of various
commercial species like cod, while the abundance of other
species like flatfishes and mackerel increased, consequently
having impacts on fisheries (Reid et al. 2001).
Coral Reefs and Kelp Forests Transitions
Other examples of marine regime shifts are coral and kelps
transitions (Rocha et al. 2014b). For instance, the Caribbean
coral reefs were flourishing ecosystems providing many ecosystem services, sustaining large fish populations and associated human communities. The integrity of the reefs depended
on the presence of sea urchins and grazing fishes, which, by
eating the algae, maintained the coral reef structure. When
the populations of grazing fish started to decrease due to
overfishing, nothing seemed to change in the system. Indeed,
sea urchins were still able to regulate algae population
through predation, preserving the reef structure (Nyström
2006; Standish et al. 2014). However, the ability of the reef
to absorb disturbances was already eroded by overfishing,
when two concomitant and dramatic events occurred, leading to the total destruction of the reef (Mumby et al. 2007).
Sea-urchin populations quickly collapsed due to an illness
outbreak, while more nutrients, discarded from the islands,
were added to the system, causing rapid eutrophication. In a
short time, coral reefs were substituted by algae beds which
were not regulated by any top-down (sea urchin predation) or
bottom-up (limitation of nutrients) processes. This algaedominated system is now difficult to reverse due to the feedback mechanisms maintaining the system in its new status
(i.e., the number of new algae growing every year can impede
the reintroduction of corals, Mumby et al. 2007; Mumby
2009; Kates et al. 2012).
Similarly, kelp forests are highly diverse ecosystems
which can maintain flourishing fish populations and offer
many services for humans such as fisheries and cultural values (Steneck et al. 2013; Ling et al. 2015). Kelp forests are
mainly maintained by fish predation on sea urchins, which
controls sea urchin populations. In Australia, overharvesting
of predatory fish, coupled with diseases weakening the kelp,
led to a boom of the sea urchin population and a shift from
high biodiversity kelp forest to poorer urchin’s barren (Ling
et al. 2015). This state was then difficult to reverse due to
various feedback mechanisms such as the increase of juvenile urchin abundance and facilitation of juvenile survival,
but also because of the lack of efficient measures to recover
the stocks of the sea urchin’s predators (Ling et al. 2015). In
these two examples, the regime shifts were caused by multiple stressors which altered the regulation (top-down and/or
bottom-up) of previously highly productive ecosystems and
led to huge economic, social and ecological losses. Similarly
to the Atlantic cod example, management measures failed to
reverse these unexpected regime shifts due to feedback loop
mechanisms (Steneck et al. 2002; Ling et al. 2015).
Regime Shifts – A Global Challenge for the Sustainable Use of Our Marine Resources
The North Sea Regime Shift
The North Sea regime shift involved different mechanisms
that induced changes which started at the bottom of the trophic chain and propagated up to higher trophic levels (Reid
et al. 2001; Beaugrand 2004; deYoung et al. 2008; Conversi
et al. 2010; Lynam et al. 2017). The North Sea regime shift
occurred during the 1980s and was mainly induced by a
combination of increased sea surface temperatures and
changes in hydro-climatic forces (Beaugrand 2004). Due to
the increase of sea surface temperature and changes in the
water inflows, phytoplankton biomass increased. As a consequence, the zooplankton assemblage, originally dominated
by cold waters species, e.g., Calanus finmarchicus, shifted to
an assemblage dominated by warmer water species, e.g.,
Calanus helgolandicus and gelatinous zooplankton such as
jellyfish (Reid et al. 2001; Beaugrand 2004; Möllmann and
Diekmann 2012). These changes in the zooplankton community, combined with hydro-climatic changes, propagated
to higher trophic levels. Changes in temperature and/or salinity led to an increase of flatfish biomass (Möllmann and
Diekmann 2012) while the decline of C. finmarchicus, which
is the preferred prey of gadoids and especially of cod larvae,
led to cod recruitment failures (Beaugrand et al. 2003;
Beaugrand 2004) enhancing the negative sea warming
effects. These changes in recruitment had a lagged impact on
the adult gadoids biomass that, already stressed by overfishing, started to decline inexorably at the end of the 1980s
(Hislop 1996). The changes in fish biomass and composition,
together with warmer temperatures, favored the emergence
of previously scarcely present species such as horse mackerel (Trachurus trachurus) and mackerel (Scomber scombrus), especially in the northern North Sea (Reid et al. 2001;
Beaugrand et al. 2003; Beaugrand 2004).
This regime shift, induced by bottom-up processes, was
more qualitative than quantitative in the sense that changes
in assemblage and not in total biomass of trophic levels
occurred (Beaugrand 2004). The dynamics of these changes
highlighted different response time patterns depending on
the organisms affected. Indeed, the phytoplankton and zooplankton communities, with their fast life cycles, responded
to climatic changes faster than the fish community. Spatial
patterns were also different: the coastal areas were less sensitive to change in hydrodynamic conditions, and the regime
shift was stronger in the northern North Sea (Reid et al.
2001; Beaugrand 2004; Möllmann and Diekmann 2012).
This regime shift completely changed the structure of the
North Sea fish community and led to the decline of various
commercial species like cod, while the abundance of other
species like flatfishes and mackerel increased, consequently
having impacts on fisheries (Reid et al. 2001).
Coral Reefs and Kelp Forests Transitions
Other examples of marine regime shifts are coral and kelps
transitions (Rocha et al. 2014b). For instance, the Caribbean
coral reefs were flourishing ecosystems providing many ecosystem services, sustaining large fish populations and associated human communities. The integrity of the reefs depended
on the presence of sea urchins and grazing fishes, which, by
eating the algae, maintained the coral reef structure. When
the populations of grazing fish started to decrease due to
overfishing, nothing seemed to change in the system. Indeed,
sea urchins were still able to regulate algae population
through predation, preserving the reef structure (Nyström
2006; Standish et al. 2014). However, the ability of the reef
to absorb disturbances was already eroded by overfishing,
when two concomitant and dramatic events occurred, leading to the total destruction of the reef (Mumby et al. 2007).
Sea-urchin populations quickly collapsed due to an illness
outbreak, while more nutrients, discarded from the islands,
were added to the system, causing rapid eutrophication. In a
short time, coral reefs were substituted by algae beds which
were not regulated by any top-down (sea urchin predation) or
bottom-up (limitation of nutrients) processes. This algaedominated system is now difficult to reverse due to the feedback mechanisms maintaining the system in its new status
(i.e., the number of new algae growing every year can impede
the reintroduction of corals, Mumby et al. 2007; Mumby
2009; Kates et al. 2012).
Similarly, kelp forests are highly diverse ecosystems
which can maintain flourishing fish populations and offer
many services for humans such as fisheries and cultural values (Steneck et al. 2013; Ling et al. 2015). Kelp forests are
mainly maintained by fish predation on sea urchins, which
controls sea urchin populations. In Australia, overharvesting
of predatory fish, coupled with diseases weakening the kelp,
led to a boom of the sea urchin population and a shift from
high biodiversity kelp forest to poorer urchin’s barren (Ling
et al. 2015). This state was then difficult to reverse due to
various feedback mechanisms such as the increase of juvenile urchin abundance and facilitation of juvenile survival,
but also because of the lack of efficient measures to recover
the stocks of the sea urchin’s predators (Ling et al. 2015). In
these two examples, the regime shifts were caused by multiple stressors which altered the regulation (top-down and/or
bottom-up) of previously highly productive ecosystems and
led to huge economic, social and ecological losses. Similarly
to the Atlantic cod example, management measures failed to
reverse these unexpected regime shifts due to feedback loop
mechanisms (Steneck et al. 2002; Ling et al. 2015).
Regime Shifts – A Global Challenge for the Sustainable Use of Our Marine Resources