Biology Reference
In-Depth Information
of 0.79
C in the intertidal of Monterey Bay California between 1931 and 1996 led
to a significant increase in southern species and a decrease of northern species
(Sagarin et al.
1999
). In the northern Baltic, another monitoring study of long-
lasting warming in the sea (up to 10
C) diminished ice cover and thereby increased
light availability in winter. The situation also caused major changes in the quanti-
tative composition of species over the seasonal cycle. While growth of
cyanobacteria was promoted and red and brown algae decreased in abundance or
disappeared over summer, the latter had a prolonged growth season in autumn and
winter due to “better” winter temperatures. Generally, a species-specific response
was evident (Snoeijs and Prentice
1989
). In future, we expect similar transitional
changes in rocky shore communities along all warm- and cold-temperate shorelines
possibly subjected to change according to our model results (Figs.
18.1
and
18.2
).
18.3.4.3 Tropical Regions
Many coastal hard-bottom tropical and subtropical regions are characterized by
coral reefs which also inherit a high seaweed species richness (Diaz-Pulido et al.
2007
). The abundance of macroalgae in reefs has been thought to be generally low
and controlled by grazing pressure of herbivorous fish (e.g., Wanders
1977
;
Carpenter
1986
; Hay
1997
, see Chap.
16
by Mejia et al.). Only in recent years, it
was realized that tropical reefs are also algal reefs and a high coverage of
macroalgae among corals and natural variability of seaweed abundance on coral
reefs is not necessarily indicative of environmental degradation (Vroom et al.
2006
,
2010
; Vroom and Timmers
2009
). Coral-algal interactions are manifold and it is
known that algae may inhibit or kill corals (e.g., Titlyanov et al.
2007
; Rasher et al.
2011
) and vice versa dead corals may negatively influence macroalgal growth (Liu
et al.
2009
).
As tropical corals and seaweeds are currently living near to their lethal limit, a
slight temperature increase of 1-2
C above the mean summer temperatures as
predicted for the end of the twentieth century (Fig.
18.3
) may already lead to
catastrophic events. Coral reefs worldwide have faced severe damage by periodic
heat waves especially through extreme ENSO activities since the 1980s inducing
so-called coral bleaching events which involve the loss of the symbiotic
zooxanthellae after thermal stress (Jokiel and Coles
1990
). Baker et al. (
2008
)
describe in their extensive review all facets of this phenomenon. There is a
correlation between coral bleaching with maximum monthly SSTs (Manzello
et al.
2007
). Temperature thresholds for coral bleaching are not uniform but site
specific and range from 27.5 to 32
C (Baker et al.
2008
). Thereby, they are
generally above current mean tropical summer SST of 27-29
C (Fig.
18.2a
;M
uller
et al.
2009
, Appendix Figs. 1 and 2), but this will change in future when this region
will experience an unprecedented warming (Solomon et al.
2007
) with annual mean
SSTs of 30-31
C over wide areas (Fig.
18.2b
). A possible acclimation of corals to
increased temperatures has been observed in the Great Barrier Reef as threshold
temperatures increased over time (Berkelmans
2009
) and up to now no coral
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