Biology Reference
In-Depth Information
depth gradients in the ocean (Kitching
1941
; Saffo
1987
). For example, Novaczek
(
1984
) observed that the lower depth limit of
Eckonia radiata
off the coast of New
Zealand was set by minimum light requirements of ca 40 mol photons m
2
d
1
.
Conversely, Graham (
1996
) and Fejtek et al. (
2011
) found that the shallow limits of
Macrocystis pyrifera
and
Pelagophycus porra
(respectively) were set, at least in
part, by high irradiance (PAR) effects on their microscopic gametophytes. Further,
high UV-B irradiance appears to be especially important in reducing spore survival
and consequently inhibiting algal recruitment in shallow water (e.g., Wiencke et al.
Bischof and Steinhoff). With regard to spectral quality, Luning (
1981
) examined
the effects of blue light on sexual reproduction in gametophytes of
Saccharina
latissima
near Helgoland (North Sea) and observed that egg release by the
gametophytes was reduced by 50% when they were held under only 1.4
mol
m
photons m
2
s
1
of blue light (
449 nm) for 45 min. However, blue light may
also have positive effects, such as enhancing nitrate (e.g., Aparicio et al.
1976
) and
carbon (e.g., Schmid and Dring
1996
) uptake and storage as seen in some green and
brown algae, respectively. While irradiance and spectral quality may be important
in controlling recruitment, growth, survival, and reproduction in many algae,
photoperiod may be as important in controlling the timing of their growth and
reproduction. For example, L
l
¼
uning and Kadel
(
1993
) observed that new frond formation in several brown algae is regulated by
changes in photoperiod, resulting in circannual rhythms and synchronized seasonal
growth. Further, Edwards (
1998
) observed that recruitment in the brown alga
Desmarestia ligulata
in central California, USA, was closely tied to increase in
day length during the early spring, but this was limited to areas where the dominant
kelp canopies had been removed either experimentally or by winter storms. In
summary, light quantity and quality appear important to macroalgal physiology and
thus factors that reduce access to light may be integral in establishing spatial and
temporal patterns in their distribution and abundance, especially in the ocean where
irradiance rapidly diminishes with increasing depth.
In addition to natural attenuation of light in the ocean (e.g., Kirk
1992
), light is
absorbed and/or scattered by the macroalgae themselves, placing them in both
direct and indirect competition with each other. As a result, many species of
macroalgae have developed functional morphology forms that allow them to
more effectively capture light for photosynthesis in their respective habitats
(Vadas and Steneck
1988
). These morphologies range from single cells to multi-
cellular parenchymatous thalli with complex tissue differentiation. Within this later
group, many species have evolved morphologies that elevate their photosynthetic
blades above their competitors. For example, Kitching (
1941
) observed that on the
shores of Carsaig Island, Scotland, the dominant macroalga
Laminaria digitata
possesses erect nonflexible stipes that hold its blades above the substrate in order to
more effectively capture light when occurring in the subtidal where light can be
limiting, but flexible stipes that allow its blades to lay prostrate in the water when
occurring in the intertidal where light was otherwise abundant but desiccation may
be an issue. Similarly, many subtidal kelps produce canopies that are either buoyed
uning (
1986
,
1990
,
1994
) and L
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