Environmental Engineering Reference
In-Depth Information
Fig. 4.24
Biomass of
seaweeds (in gm m
−
2
)in
western and eastern sectors
of Indian Sundarbans
2500
2000
1500
Western sector
Eastern sector
1000
500
0
Enteromorpha
intestinalis
Ulva lactuca
Catenella repens
Seaweeds are noted for their capacity to utilize
dissolved inorganic carbon (DIC) from the sur-
rounding sea water (Smith and Bidwell
1989
;
Raven and Johnston
1991
) for photosynthesis and
growth, resulting in a decrease in the DIC con-
centration of sea water and a drop in the partial
pressure of carbon dioxide (pCO
2
) below atmo-
spheric levels. The morphology and biomass of
seaweeds are also regulated by physical condi-
tions of the surrounding aquatic phase. Differ-
ences in morphology occur along wave exposure
gradients in a variety of seaweeds. Narrower
blades with several splits along their length typify
Kelps from wave-exposed areas (
antactica
which both occurs in the sublittoral of
central and southern Chile.
Lessonia
has a strong,
stiff stipe that bends with the
ow. In contrast, the
elastic stipe and stretchy blade of
fl
Durvillea
allows
it to align with the
ow completely. Flexible stipes
that can be bent parallel with water movements
reduce the stress on the thallus and result in the
plant being closer to the rock where movement is
less. These structures respond best to the chaotic
multidirectional water movements typical of
exposed shores. In the North Atlantic,
fl
exible
elastic stipes are found in species from wave-
exposed sites, such as
fl
Alaria esculenta
, while
Laminaria hy-
perborea, L. digitala, Sacchoriza polyschides,
Hedophullum sessile
Laminaria digitata
exible stipe than
the deeper and hence less wave-beaten
has a more
fl
L. hyper-
), whereas plants with
broader blades with fewer or no divisions occur in
more sheltered conditions, even within the same
species. Strong water movement imposes various
stresses on the thallus: tension, shear forces,
bending and twisting. Narrow,
borea.
L. hyperborea
is more important in maximizing access to light
than in resisting wave action.
While avoiding dislodgement by wave
movement is a major priority for rocky shore
species, all seaweeds require some degree of
water movement to break down the boundary
layer around the thallus. Materials have to diffuse
in and out of the plant
Presumably, the stiff stipe of
at blades, espe-
cially if divided like streamers, provide a highly
streamlined shape differences appear to be phe-
notypic growth responses. For instance,
fl
Lami-
through this layer of
naria sacharina
can be induced to take on a more
streamlined shape with a narrow blade and greater
elongation rates simply by attaching weights to
the extreme end of the blade. Similarly trans-
planted
slowly moving
uid, so that the thicker the
boundary layers, the slower the uptake of mate-
rials. In still water, the boundary layer can be
several millimetres thick. Many species have a
surface of spiny outgrowths (
fl
sp. has been shown to change from
a broad-fronded sheltered form to an elongated
type characteristic of exposed shores (Denny
1988
; Norton
1991
). Different seaweeds may cope
with the same wave conditions in different ways,
as shown by
Alaria
Macrocystis pyrif-
era
), wavy margins, (
Laminaria saccharina
)or
even holes or undulations in the blade (
Agarum
cribosum, L. saccharina
) which are thought to
enhance the turbulence in water
owing over the
lamina and hence allow greater uptake of raw
fl
Lessonia nigrissens
and
Durvillea
