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and Atlantic compared to the eastern sides of the oceans. The boundary between the
warm- and cold-temperate regions is characterized by 16-29
C summer SSTs and
by 10-20
C winter SSTs. These extreme differences are attributed to the pro-
nounced compression of isotherms along the western part of the Atlantic and Pacific
in contrast to the eastern part of the oceans (see Figs 12.2, 12.3 in Muller et al.
2011
). When comparing both hemispheres pronounced differences become appar-
ent as well. The southern boundary of the Arctic region, for example, is described
by the 9-10
C summer isotherm, whereas the northern boundary of the Antarctic
region is the 4
C summer isotherm (Table
18.1
, Figs.
18.1a
and
18.2b
). The
situation in the southern hemisphere might be partially attributed to missing
continuous land masses in the circumpolar Southern Ocean. Thus, the limit between
the polar and cold-temperate coastal regions in the southern hemisphere is not
clearly defined. The largest differences between northern and southern hemispheri-
cal biogeographic regions become apparent at the boundary of the warm- and cold-
temperate regions at the western coasts of both the Atlantic and the Pacific. In the
N-Atlantic, this boundary is situated at the 27
C summer isotherm. In contrast, in
the S-Atlantic it is located at the 19
C summer isotherm (Table
18.1
, Figs.
18.1a
and
18.2a
). At the west coasts of the Pacific the differences are smaller (26
C
August isotherm in the north to 20
C February isotherm in the south).
18.3 Responses of Seaweeds to Temperature Changes
18.3.1 General Responses
Seaweeds can principally respond to environmental changes in four ways which all
may result in distributional and diversity changes: on short timescales, they can
acclimate. On medium to long timescales they either adapt to new conditions or are
able to slowly migrate keeping pace with the changing environmental pressure.
Species unable to acclimate, adapt, or disperse may be captured in isolated refugia
or become extinct. Acclimation to temperature stress in seaweeds has been mostly
studied on the level of photosynthesis or growth (e.g., Davison et al.
1991
;K
ubler
and Davison
1995
; Eggert et al.
2006
; see Chap.
3
by Eggert). Locally, the
acclimation potential of thermal traits can shape the vertical or seasonal distribution
pattern of species (Davison and Pearson
1996
; Ateweberhan et al.
2005
; Zardi et al.
2011
) and on a broader scale eurythermal species have a broader distribution range
than stenothermal species (see also Chap.
3
by Eggert). Hitherto, it is unknown
whether species with a broad acclimation potential also had or will have a faster
genetic adaptation potential—a fact which would help to explain historical or future
biogeographical processes.
Adaptive processes on the physiological and molecular level are explained in
detail in Chap.
3
by Eggert. On the organism level, the adaptation processes to
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