Biology Reference
In-Depth Information
measured at higher temperatures. Moreover, the temperature optima for photosyn-
thesis in the few tested Antarctic species are well below values determined in
temperate species (G
´
mez et al.
2011
). Likewise, the optimum temperature for
photosynthesis in zoospores of
Alaria esculenta
,
Laminaria digitata
, and
Saccharina latissima
from Arctic Spitsbergen is with 7-13
C relatively low
(Roleda
2009
). In contrast, the optima for respiration are clearly located at higher
temperatures. Photosynthesis:respiration (P:R) ratios in Antarctic species are
highest at the lowest tested temperature (0
C) and decrease with increasing
temperatures due to different
Q
10
values for photosynthesis (1.4-3.5) and respira-
tion (2.5-5.1; G´mez et al.
2011
). The high P:R ratios at low temperatures explain
the high growth rates of polar species at low temperatures, which in the end
determine the geographic distribution.
The northern distribution of endemic Antarctic species is often limited by the
temperature demands for growth. Endemic Antarctic Desmarestiales for example
occur only south of the Antarctic Polar Front in areas with maximum temperatures
5
C allowing sufficient growth of their sporophytes (Wiencke et al.
1994
;M
uller
et al.
2011
; see also Chap.
18
by Bartsch et al.). The southern distribution of Arctic-
North Atlantic species is often limited both by the USTs and the upper limit of
gametogenesis (van den Hoek
1982a
,
b
). In the West Atlantic distribution limits are
determined by lethal, high summer temperatures, whereas in the East Atlantic they
are determined by high winter temperatures inhibiting reproduction. Examples for
species from this group are
Laminaria digitata
,
Chorda filum
, and
Halosiphon
tomentosus
.
During the ice ages, both the Arctic and the Antarctic were not hospitable for
seaweeds. In the southern hemisphere, sub-Antarctic islands and the southern tip of
South America have probably served as refugia. Migration of species from
Antarctica to South America and vice versa probably took place along the Scotia
Arc (Wiencke et al.
1994
). In the northern hemisphere, the distribution area of
Atlantic seaweeds was extremely reduced during the ice ages. In the north, their
distribution was limited by the glaciers and in the south by the 10-15
C winter
isotherm, their southern reproduction boundary. This most probably explains the
present depauperate flora in the North West Atlantic. A comparable situation did
not exist in the Pacific, probably a major reason for the richness of the cold North
Pacific (Wiencke et al.
1994
). The temperature decrease during the ice ages allowed
polar species to extend their distribution limits toward the equator. Some species
even crossed the equator during the maximum of the last glaciations (see Chap.
18
by Bartsch et al.; Wiencke et al.
1994
).
€
13.3.4 Effect of Salinity, Temperature, and Desiccation
on Supra- and Eulittoral Seaweeds
Information on osmotic acclimation of supra- and eulittoral seaweeds during salin-
ity changes is available only for green algae from the Antarctic. Eulittoral species
survive salinities between 7 and 102 psu and grow between 7 and 68 psu.
