I. Introduction
A. An important coastal environment frequently
associated with barrier islands is the estuary
that separates the mainland from the
islands. An estuary is a semi-enclosed portion of ocean that is somewhat
isolated by land, and in which freshwater and salt water mix.
B. A glance at a globe
demonstrates how vital the influence of estuaries has been on the location
of large and small cities throughout the world (
C. For humans estuaries
provide:
1. Navigable harbors and waterways for the transfer of:
a. Food
b. Raw materials
c. Manufactured goods
2. This activity is economically indispensible to a
country's commercial and industrial practices.
3. Recreational sites
4. Commercial fishing
D. For
years the wetlands along the fringes of estuaries were considered useless
and served as convenient repositories for the disposal of industrial and
domestic waste.
E. Now, cleaned up and
reclaimed by landfill, this valuable land provides opportunities for urban
expansion into the sea. This, in itself, is a major threat to the
environment.
F. These increasingly
stressed estuarine and wetland environments are among the most
biologically productive earth environments.
1. The organic production of marshes, increased by
Nutrients supplied
by rivers, can be as high as 5-10 tons per year of organic matter per
acre, compared with 1 ton per year per acre for a carefully fertilized
and tended wheat field, or less than 0.5 tons per year per acre for the open
ocean or a desert. (unit= gms Carbon/m2/year)
II. How do estuaries differ from other oceanic
environments?
A. The shallowness of the water differentiates them from other non-coastal,
marine environments, but two other factors separate them even from other coastal
environments:
THE ENORMOUS DAILY AND SEASONABLE VARIABILITY IN SALINITY, AND
TEMPERATURE
B. DEPTH -
Estuaries are very shallow compared to open ocean.
1. Changes in the
nature of water with respect to distance and time are much greater in these
shallower coastal waters so river runoff and tidal currents have a very
significant effect on the nature of the water.
C. SALINITY
1. River runoff has
the direct effect of reducing salinity in the surface layer in areas
where mixing is not significant, and throughout the water column where mixing
occurs.
2. In temperate and
tropical climates the runoff of the rivers will be at the maximum during the
rainy season and will decrease the salinity significantly below the oceans average
of 35 o/oo.
3. Varies with tide
and seasons
D. TEMPERATURE
1. The shallow
water permits very great ranges in temperature to occur on a yearly and
even a daily basis. Sea ice forms in many estuaries in high latitudes.
2. Seasonal changes
in temperature can be most easily detected in the coastal regions of the
mid-latitudes, where surface temperatures are at a minimum in winter and
reach maximum values in late summer.
3. Strong
thermoclines may develop in areas where mixing does not occur.
a. Here very
high-temperature surface water may form a relatively thin layer. Mixing reduces the surface temperature by
distributing the sun's heat through a greater vertical column of water, thus
pushing the thermocline deeper and making it less pronounced.
4. Both tidal
currents and prevailing winds can have a significant effect on surface
temperatures.
III. Origin and Classification of estuaries
A. The word estuary comes from the Latin word aestuarium meaning tidal. It refers to a semi-enclosed coastal body of water in which sea water is significantly
diluted by fresh water from land runoff.
B. Origin of estuaries
1. Essentially all
estuaries exist today due to the fact
that in the last 18,000 years
sea level has been raised
approximately 120 meters (394
ft) owing to the melting of
much of the major continental
glaciers covering portions
of North America, Europe and
Epoch (1.6 - 0.01 mya).
2. Geologically estuaries are short-lived, temporary
features because their basins are quickly destroyed by
sediment influx from the rivers and surrounding land mass.
a. Different processes are responsible for this
1) Freshwater marshes slowly extend out into the
river-end of the estuary and saltwater marshes
and bars tend to close the seaward part.
2) Estuaries and fjords that are cut into solid
rock are less likely to be affected by these
processes.
3) Deltas at river mouths in the estuary
often
begin the process of infilling the estuary.
4) In addition, large quantities of sediment can
be carried into the estuary from the offshore
areas.
5) The cutback
of the coastline by marine erosion
slowly eats away
at, and will eventually destroy
an estuary.
b. If sea level were to lower, most estuaries would
rapidly disappear, to be replaced by rivers cutting
into their deposits.
If sea level were to remain con-
stant or rise just a few meters, the processes of des-
truction would not be stopped. Only a considerable
rise in sea level would maintain or rejuvenate present
estuaries.
C. Classification of estuaries by origin:
1. Coastal Plain
- low-lying coasts with drowned river
systems
a.
b.
c.
2. Fjord - glaciated valleys that are U-shaped
with steep
walls. They usually
have a glacial deposit forming a
sill near the ocean entrance.
a.
3. Bar-built - shallow estuaries separated from
the open
ocean by bars composed of sand deposited parallel to
the coast by wave action.
Lagoons separating barrier
islands form the mainland are bar-built estuaries.
a.
b. Pamlico-Albemarle Sound is a combination of
this and a coastal plain estuary.
c.
4. Tectonic - produced by faulting or folding
which causes
a restricted down-dropped area into which rivers flow.
a.
b. Juan de Fuca Strait
c. These are typically deeper
than the estuaries
on a flooded coastal
plain.
5.
(bolide) impact that occurred during the Eocene (~ 35 mya).
The crater (1-2 km deep) and breccia generated by this
impact were
discovered from cores drilled down into rock
layers in the
region. Debris blasted from the crater up
into the atmosphere
has been found in the coastal
Ocean as far away
as
would have
included: seismic sea waves, acid rain, blast
waves, forest
fires, toxic gas clouds, flooding, etc.
IV. Circulation and mixing of water in estuaries
A. Mixing of fresh and saltwater in estuaries
results in long-term average flow seaward in the surface layers and a net
landward flow along the bottom. This is
called an estuarine circulation. We are concerned about
circulation in estuaries because it moves organisms and pollutants around.
Salinity differences resulting from different circulation patterns also affect
the environment for the organisms. Now we'll look at how it develops and how
estuaries can be classified based on the nature of the salinity distributions
within the estuary.
1. There is a continuum (a continuous extent,
series or whole with no sharp clear-cut
boundaries) of
circulation patterns in estuaries.
B. In an ideal estuary (no friction
between water masses, no tides, and no winds), low-salinity river water flows
in at the head of the bay and spreads out over the seawater beyond because it
is less dense.
Mass
Density =--------
Volume
1. This generates a more-or-less horizontal pycnocline.
PYCNOCLINE - A LAYER OF WATER IN WHICH A HIGH RATE
OF CHANGE IN DENSITY IN THE VERTICAL DIMENSION IS
PRESENT
C. The subsurface
saltwater in such an estuary forms a wedge with its thin end pointed upstream, so in its simplest
form this is called a salt-wedge estuary.
1. The simplified picture is modified by several factors:
a. Friction
between seaward-moving fresh water and the underlying seawater causes some
underlying water to be dragged up from below and mixed with the surface
layer. This process is called entrainment. This newly mixed water
cannot reenter the lower layer because its reduced salinity makes it less dense
than seawater. Therefore, ocean water
flows into the estuary along the bottom to replace that drawn seaward by
surface flows. Landward flow along the bottom is much greater in volume than
the flow of river water into the estuary.
2. Occurs where river flow is large and tidal range is low.
a. Mouths of the
1) In the
D. As fresh water discharge into an estuary
decreases, tidal effects become more important. The estuary may act
like a salt-wedge estuary only during floods. During low-flow periods, the same
estuary is usually strongly influenced by tides and tidal currents. Then it departs substantially from the
pattern of a simple salt-wedge stratification and becomes a moderately stratified or partially-mixed
estuary.
1. Tidal currents
cause turbulence (random
movements) throughout the water column, which, in turn, leads to mixing of salt
and fresh water masses. Consequently, more salt water is transferred from the
subsurface to the surface and some fresh water from the surface also mixes
downward.
2. Generates a very
different salinity pattern than the one observed in a salt-wedge estuary.
3. Salinity decreases in a landward direction
both in the surface and the subsurface layers.
4. Found in deeper
estuaries like Puget Sound and
E. Tides dominate currents in most estuaries.
Where tidal effects are relatively strong, waters in the estuary are less
stratified. In an
estuary with small river flow but large tides and
tidal currents, the waters may be
mixed almost completely
from top to bottom. These are called well-mixed estuaries.
1. Many small estuaries along the coast of
2. River discharge forms cloudlike parcels of low-salinity
water, which, when turbid with suspended sediment, are
easily recognizable from the air.
3.
tidal currents are so strong, Instead, because Pamlico
Sound is so shallow (nowhere much deeper than 30 feet)
winds mix the water column all the way to the bottom.
F. Because of their
great depths and irregular bottom topography, fjords, have complicated circulation systems. Many fjords have a sill or submerged ridge of glacial sediment
deposited at their mouths which cuts off most of the deeper water from
communication with the adjacent ocean.
1. Fresh water flowing into a fjord forms a low-salinity
surface layer that moves seaward, in a typical estuarine
circulation.
2. The deeper waters of a fjord may be almost completely
isolated from the surface circulation and may become ser-
iously depleted in oxygen due to lack of downward mixing of
oxygenated surface waters.
V. Environmental Issues in Estuaries
A.
Estuaries are extremely vulnerable to the activities of man
because
of their shallowness and nearness to land. They are
significantly
influenced by:
1. Nutrient-rich agricultural run-off,
causing O2 depletion
2. Sewage and industrial pollution
3.
Temperature changes induced by industries that use
large
amounts of water for cooling or during their
processing
(i.e., nuclear power)
4.
Changes in salinity induced by man (i.e., PCS Phosphate
pumps
65 million gallons/day of freshwater into the
Pamlico
to dewater their mine).
5.
Changes in turbidity (i.e., sediment load) of water
caused
by clearing forests for agriculture and towns.
6.
Real-estate development after draining and filling
of wetlands
7.
Waste disposal
8.
Commercial fishing
9.
Dredging
10. Shipping
11.
Habitat destruction
B.
Evidence of Man’s Impact
1. 70% of the
of the coast
2. Only about 6% of the
in under public ownership; the rest is
privately owned.
3. In the mid-1098’s about $4.2 billion in
retail sales
were generated in the coastal zone and
there were over
10 million boats, 80% of which were in the
30 coastal
states.
4. 7 of the largest cities in the
5. About 33% of
6. Shoreline property continues to increase
in value
7. About 40 % of US manufacturing is in
coastal counties
8. Many large energy facilities and oil
refineries are
on or very near the coast
9. Coastal developments have already destroyed
or built
over 20-50% of coastal habitats.
C.
Biological activity in estuaries
1. Estuaries are an incredibly rich
environment in terms
of the diversity of organisms. This
diversity is due to
the high level of nutrients (nitrate and
phosphate, etc.)
dissolved in the water. Nutrients collect
in estuaries
because the denser salt water flows in along the bottom
bringing with it organic debris from the
shallow ocean.
The river water also brings in nutrients
from the land
masses, which are the major nutrient source
to the ocean.
2. Because most estuaries are shallow the
light penetrates
all the way to the bottom contributing to
abundant plant
growth throughout the water column, but
especially on
the bottom where sinking generates the
greatest supply
of nutrients. Because plants are at the base of the food
chain here, as they are nearly
everywhere on Earth, this
leads to a rich source of food for all
kinds of biota.
The
abundant plant life also provides shelter for animals.
3.
These precious habitats, which are the most productive
of all
oceanic regions serve as the spawning
and breeding
grounds to support much of the marine
life in deeper parts
of the estuary. This in turn supports
much of the marine
life in the rich fishing grounds of the
continental shelves
a.Human
impacts can destroy these breeding grounds by
throwing
off the carefully-tuned cycle of the organisms
that
live in the very stressful environment of the estuaries.
1) Natural stress = drastic salinity and
temperature
changes
2)
Human activities significantly increase
the
stress due to the impacts described earlier
b. The
from 50-75%
of all commercially-caught marine fish are
“born
and raised” in estuaries.
4.
Altering one component of these complex food webs can affect
the
entire ecosystem.
a. Many of the pollutants adhere to
particles, sink to
the bottom and are, therefore, removed
from the water
column. However, they may be released back into the
system by the action of bottom feeders
and dwellers,
strong currents, and trawling that
stir up bottom
sediments.
D.
1. Most estuarine pollutants are flushed
out to sea
2.
fresh water in an estuary that is
calculated by comparing
the total fresh water content of the
estuary to the rate at
which fresh water enters. If it were
possible to remove all
the freshwater from an estuary and then
record how long it
took the rivers to replace this water, the
flushing time
would be determined. Many flushing times
are required to
remove old pollutants.
a. During the rainy season flushing
time is on the
order of weeks to months for the
Pamlico.
VI. Coastal Seas
A. Some coasts border on seas rather than
directly on the open ocean.
These
coastal seas differ in their degree of communication with the
open
ocean. The Gulf of Mexico and the
substantially
with the
dominate
their current systems. Places such as the
however,
have such limited communication with the open ocean that
the
ocean has no direct influence at all. Intermediate between
these
two situations is that of the
B. I'm
going to discuss the two general types of circulation patterns
that
result in these seas with restricted communication to the open
ocean,
using the Mediterranean and the
1. Evaporation
dominates precipitation =
a. Results in considerable vertical
circulation
because the surface waters become more
dense as
evaporation proceeds and therefore sink
down
into the underlying waters promoting
vertical
mixing. This sinking surface water carries
oxygen
with it and promotes animal life at depth.
The
sinking water is replaced by surface water
that
flows into the sea through the
The denser bottom waters flow out of the
Sea at a
Depth of 200-600 meters through the Strait.
2.
Precipitation dominates evaporation ==
a. Here the surface water is not made more
dense
by evaporation so it remains on the surface
and
does not sink into deeper parts of the
basin
carrying oxygen with it. The result is that
the
bottom is not flushed and becomes stagnant.
The
bottom of such seas can not support much
life,
which
may be limited to anaerobic bacteria.