Biology Reference
In-Depth Information
rarely experience salinity and desiccation stress. One of these occasions, however,
is spring low tides where sublittoral seaweed species may be exposed for some time
to salinity changes and desiccation (L
uning
1990
). Besides the intertidal zone and
estuaries, there are other habitats occupied by seaweeds which are affected by
salinity changes or gradients (Fig.
5.1
). Rock pools at low tide often exhibit strong
salinity fluctuations due to precipitation or evaporation. Seaweeds living on tidal
flats (e.g., on mussel beds) or in interface ecosystems (salt marshes, mangroves) are
regularly affected by the tidal flows and hence to a combination of salinity and
desiccation stress. In mangroves epiphytic red algae exposed to the prop roots can
be observed completely covered with salt crystals. Semi-enclosed seas with only
small connections to the open oceans mostly exhibit strong horizontal salinity
gradients or hypersaline conditions depending on the hydrology of the catchment
area and precipitation. The best studied semi-enclosed system is the Baltic Sea with
its strong, but stable horizontal salinity gradient from almost freshwater conditions
in the Northeast to brackish/marine conditions in the western part. Since the largest
part of the Baltic Sea shows a salinity between 4 and 7
S
A
only, biodiversity of the
aquatic flora (incl. seaweeds) and fauna is generally strongly reduced, because this
salinity range (ß-mesohalinum 5-10
S
A
) is too low for marine species and too high
for freshwater organisms (Remane
1940
). In warm-temperate to subtropical coastal
regions such as South Australia, lagoons are typical shallow water systems that are
separated from the open ocean by some form of barrier (e.g., sand spit) resulting in
often hypersaline conditions because of the high prevailing evaporation. A typical
example is the Coorong lagoon in South Australia with salinities
€
100
S
A
, where
high abundances of charophyte algae can occur (Bisson and Kirst
1983
). Marine
and brackish waters are consequently almost infinitely variable in the amplitude and
frequency of their saline changes, which of course has various consequences for the
physiological performance of seaweeds.
>
5.2 Effects of Salinity Stress and Desiccation on the Physiology
of Seaweeds
Since decades, the effects of salinity and desiccation on the ecophysiological
performance of seaweeds have been described to outline the species-specific
width of tolerance, i.e., upper and lower limits for survival, as well as mechanisms
for acclimation (Kirst
1990
and references therein). Nevertheless, both salinity
stress and desiccation reflect two different forms of water deprivation. While
under salt stress conditions, seaweed cells are still in full contact to liquid water
of diminished water potential, desiccation leads to strong cellular dehydration.
Typical physiological parameters which have been studied under such stress
conditions include rate of survival, growth, photosynthesis, respiration, and
reproduction.
