Seaweed Acclimation to Salinity and Desiccation Stress - Seaweed Biology - page 86

Fig. 5.1 Illustration of the abiotic factors influencing salt concentration resulting in horizontal and
vertical salinity gradients in various aquatic systems
largely depends on the rate and volume of freshwater runoff, and can be dramatic.
The water of the Amazon, for example, can be recognized at the surface of the open
Atlantic Ocean for approximately 300 km (Ryther et al. 1967 ). Since the geochem-
istry of the river's catchment area is typically highly variable as well, it strongly
affects besides the salt concentration also the salt composition in estuaries. In
addition, tidal flows, hydrological conditions, wind, precipitation, and evaporation
strongly influence salt concentration of the respective water bodies (Fig. 5.1 ).
Consequently, salinity is typically a local rather than a global parameter and can
be highly variable in coastal regions.
Since most seaweeds are sessile organisms that preferentially grow attached to
hard substrata such as rocks, gravel, or as epiphytes on salt marsh plants, on
mangroves, and on mussel colonies, they are mainly confronted with salinity
fluctuations and desiccation when exposed at low tides. At lowest water levels,
hyposaline conditions may be present due to the mixing of seawater with rain,
snow, or melt water, while hypersaline stress may occur due to evaporation during
high insolation in summer or freezing-out of freshwater in winter. In addition, in
estuaries and fjords which often exhibit extensive seaweed communities (Schramm
and Nienhuis 1996 ), rivers or freshwater runoff mix with seawater resulting in
diurnally and seasonally fluctuating salinity gradients. In Arctic waters, seaweed
species can be strongly affected by melt water influx and calving glaciers (Hanelt
et al. 2001 ; Karsten et al. 2003a ).
Most seaweeds such as kelps inhabit the sublittoral which is characterized by
quite stable environmental factors, and hence these more deepwater plants only