Environmental Engineering Reference
In-Depth Information
from top to bottom, e.g., Mississippi
River.
(b) Partially mixed or moderately strati-
ed estuary: When the freshwater and
the tidal water mix in equal proportion
due to turbulence caused by periodicity
of waves, such estuary is formed. Due to
this, the energy is dissipated in vertical
mixing thus creating a complex pattern
of layers and water masses, e.g., Ches-
apeake Bay.
(c) Completely mixed or vertically
homogenous estuary: When the tidal
action is more than the freshwater dis-
charge, the water tends to mix well from
top to bottom and the salinity is rela-
tively high. When there is wide variation
in salinity and temperature, then hori-
zontal estuaries are formed, e.g., bar-
built estuaries.
4. On the basis of systems energetics: Odum
(
1971
) have described estuaries on the basis
of systems energetics as stated here:
(a) Physically stressed systems of wide
latitudinal range: These include mainly
the high-energy breaking waves, strong
tidal currents, severe temperature and
salinity shocks, low dissolved oxygen
level during night or high rate of sedi-
mentation. Due to severe environmental
stress
high-velocity tidal channels, sedimentary
deltas and hypersaline lagoons are inclu-
ded in this category.
(b) Natural arctic ecosystems with ice
stress: Extreme cold conditions (in
Arctic and Antarctic coasts) exert con-
siderable stress on the intertidal zones of
the areas in which light is limiting due to
very short summer season. Productivity
is extremely low in these systems due to
poor diversity of phytoplankton. In most
of the times of the year, the phyto-
plankton species remain in encysted
condition to get rid of the chilled water.
(c) Natural temperate coastal ecosystems
with seasonal programming and
reproduction: Seasonal changes in pri-
mary productivity of the ocean and the
behaviour of animals are the common
characteristics in estuarine biology. The
more subdued tides, waves and currents
in the semi-enclosed basins provide
energy subsidies rather than stresses in
comparison to deeper sounds and off-
shore waters which often get charged
with nutrients and organic matter from
fertile shallow zones. Temperate estuar-
ies, though very fertile, are vulnerable to
damages caused by pollution, dredging,
lling and diking. Some important fea-
tures of the temperate estuaries include
tide pools, salt marshes, eel grass (
in this
system,
few species
Zos-
(
opportunistic species
) are able to thrive
in this
uctuating condition. It is because
of this fact such systems exhibit extre-
mely poor species diversity. The index of
dominance is high in these systems owing
to the ability of fewer numbers of species
to cope up with adverse environmental
conditions. However, in areas of inter-
tidal zone, sharp zonation of species and
seasonal replacement of communities is
very distinct because adaptation is more
ef
fl
tera
sp.) beds, seaweed bottoms, kelp
beds, oyster reefs and mud
ats, which
harbour dense population of clams and
sea worms.
(d) Natural tropical coastal ecosystems of
high diversity: These systems are
characterized by minimum environmen-
tal stress, and therefore, negligible
energy is spent by the species for adap-
tive modi
fl
cations. Due to congenial
environment, the species diversity is
very high in these systems. Examples of
such systems are mangroves, seagrass
meadows, salt marsh grass and seaweed
ciently accomplished by species
replacement along a gradient than by
adaptation within the species. Rocky sea
fronts,
intertidal
rocks, sand beaches,
