Biology Reference
In-Depth Information
E
k
as species growing higher up in the littoral. Also, within species
E
c
can decrease
as an acclimation to ambient light with water depth (G
´
mez et al.
1997
).
The ideal strategy for life in deep water is represented by crustose coralline
algae. They are well protected against grazing and can survive in spite of slow
growth. Their thallus structure represents a horizontal light receiver with none self-
shading by a single cell layer which enhances light absorption (Luning
1990
). In
dependence to the clarity of water and the annual sum of photosynthetic active
radiation impinging at the water surface, the lower depth limit of crustose coralline
algae shifts with lower latitude from several meters in cold temperate waters (e.g.,
15 m on Helgoland) to several hundred meters in tropical waters (e.g., 268 m
Bahamas, L
uning
1990
). The annual sumof impinging irradiancemust support at least
the annual need of energy for maintenance metabolism, measured by the maintenance
respiration rate, and guarantee a minimum of energy surplus for establishment of
growth and reproduction. Compared to cold-temperate regions, Caribbean algae are
able to survive in such extreme depths due to the higher solar irradiance, a 12 hour
period of day light and the clear water conditions (Jerlov type I) enables these
Caribbean algae to survive in such extreme depths. For algae with a more complex,
even erected thallus and the presence of nonphotosynthetic tissue as typically found in
kelps, the need for light energy increases and the algae have to grow in more shallow
waters as the amount of respiration and self-shading areas increases.
An ideal marker for macroalgal depth distribution seems to be the respective
stable carbon isotope composition. Rapid carbon assimilation under high photon
fluence rates leads to 13 C enrichments, probably due to extracellular and/or
intracellular isotopic disequilibria resulting in a trend toward more positive carbon
isotope values with increasing photon fluence rates (Wiencke and Fischer
1990
).
The pattern of isotope composition of algae grown at different depth was found
in sediment trap samples from the 2,000-m deep King George Basin off the
Antarctic Peninsula. It also revealed a strong contribution of seaweeds to the total
organic carbon pool of the deeper basin waters in spring and summer (Fischer and
Wiencke
1992
).
In conclusion, distribution of marine macrophytes to the lower light limit which
accommodates biomass production depends mainly on the minimal energy input
(Kirst and Wiencke
1995
). This is dependent on the annual fluence or minimum
light level occurring in the respective depth for the maintenance of existing plant
material and a surplus for growth and reproduction. Due to seasonal changes,
algae in low light habitats have to live for long periods each year at photon fluence
rates which do not cover their energy needs. Then, the photosynthetic activity in
high light periods of the year (e.g., summer season) needs to be high enough to
produce sufficient resources to endure periods with light conditions generally below
the compensation point (
E
c
). Light saturation of growth in seaweeds is fortunately
lower than those for photosynthesis (L
€
uning
1990
) so that algae can produce
enough reserve materials under favorable light conditions. Changes in the water
transparency may shift the lower light limit so that the algal distribution pattern
could be affected by anthropogenic changes of the light transmittance of the water
body.
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