Biology Reference
In-Depth Information
assessment agrees with the maximum depths found for the kelps from the Alboran
Sea and the Strait of Gibraltar. The lower limit of
L. ochroleuca
in the Strait of
Gibraltar (Jerlov's optical water oceanic II-III and coastal 1-2 types) is close to
25 m depth, but at Alboran Island (Jerlov's optical water oceanic I-II type) it
reaches 70-80 m depth (Flores-Moya
2004
; Templado et al.
2006
; Aguilar et al.
2010
). There is a report of possible heterotrophic growth of
L. ochroleuca
in deep
water from the Strait of Messina (Drew et al.
1982
), but data are inconclusive. The
most complete set of data about irradiance requirements was derived from a study
of the sporophyte of
P. purpurascens
. The compensation of dark respiration by
photosynthesis is achieved at 6.5
m
mol photons m
2
s
1
. This irradiance level
occurs at 35-40 m depth in summer, the lowest limit of the deepest populations in
the Strait of Gibraltar (Flores-Moya et al.
1995
). Similarly, the compensation
irradiance occurs at 85-100 m depth at Alboran Island (Templado et al.
2006
;
Aguilar et al.
2010
). This is the reason why this kelp and, in general, the four species
cited in this chapter must be recognized as “shade plants” (see also Chap.
1
by
Hanelt and Figueroa). Moreover, it can be hypothesized that the irradiance
requirements of the gametophytes are lower than for the sporophytes, although no
data are available.
Although the high PAR levels are usually considered to be responsible for
determination of the upper depth limit of sublittoral seaweeds (L
uning and Dring
1979
), the ultraviolet-B radiation (UV-B) also affects the germination of zoospores
and the growth of microscopic gametophytes of kelps. Thus, the capacity to tolerate
UV-B by zoospores and gametophytes of the four kelps in the Strait of Gibraltar is
correlated with those levels of solar UV-B that penetrate seawater column
(Fig.
15.3
; Wiencke et al.
2000
; Roleda et al.
2004
; see also Chap.
20
by Bischof
and Steinhoff). This could also be the reason for a deeper upper limit of kelps at
Alboran Island, because the water there is more transparent than in the Strait of
Gibraltar (see above).
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15.2.4 Temperature
In the deep waters where kelps occur in the Alboran Sea and the Strait of Gibraltar
(below 20 m depth), temperature never exceeds 15
C in summer. Thus, the thermal
regime is closer to those of cold temperate oceans than to warm temperate ones.
Moreover, this habitat is very constant in temperature. For instance, the temperature
in the Strait of Gibraltar at 35-45 m depth (where
P. purpurascens
form kelp beds)
ranges from 12.5 to 14.5
C during the year (Flores-Moya et al.
1993
; Flores-Moya
1997
).
The thermal limits for growth and reproduction of the four species are compiled
in Table
15.1
, but no data are available on the lethal limits. Moreover, data from
L. ochroleuca
and
S. polyschides
must be considered with caution since they
correspond to seaweed populations very distant from the Alboran Sea and the
occurrence of different thermal ecotypes in their distribution areas is probable.
