Estuarine systems

Estuarine SystemsEstuarine Systems
Dr M Belal HossainDr M Belal Hossain

Course overview:
1.Definition and uses of estuary
2.Estuarine circulation: water and salt
3.Estuarine communities
4.Adaptation of organisms
5.Threats and management of estuaries

Definition:
Place where rivers meet the sea
A semi-enclosed coastal body of water where sea
water is measurbly diluted with fresh water
derived from land drainage.

USE of Estuaries and coast:
fisheries (potting, trawling, etc.) 
telecommunications cables 
urban areas, infrastructure 
alternative energy generation (wind, tidal, wave) 
conventional energy generation (nuclear, coal, oil, gas) 
land claim 
coastal defence  military uses 
navigation/shipping  recreation/tourism 
safety (lifeboats/coastguards)  water abstraction
ports/harbours  aquaculture 
dredging/spoil disposal  agriculture 
aggregate extraction  waste discharge 
industry (petrochemical, food, etc.)  education, research 
oil and gas exploration/extraction 
barrages (amenity, safety) 
wildlife, conservation 

Salt and water circulation in estuaries
•Fresh water flow
•Tidal influence
•Topography (Bottom Condition)
•Wind action
•Current
•Upwelling
•Cyclone

1. Salt-wedge estuary :
Salt water moves into it in the shape of a wedge,
with fresh water flowing over it
Highly stratified
Small tidal range
Large inflow of river water
Fluvially dominated
Apparent halocline is present in that estuary
Fluvial sediment and
sediment size increases landward
Example : Mississippi,
Karnaphuli river estuary etc.

2. Partially mixed estuary :
Moderate tidal range
Halocline less well-defined.
Two-layer system forms :
fresh water on surface, flowing sea ward
and salt water on bottom, flowing landward.
Marine sediments and
sediment size decreases landward.
Example : Chesapeake, Thames etc.
Pashur River in Bangladesh

3. Well mixed estuary :
Tidal currents strong relative to river flow.
Usually large tidal range,
channel broad and shallow.
Water column completely mixed.
Salinity will decrease progressively upward.
In some cases lateral salinity may occur.
Sediment size decreases land ward.
Marine sediments on left bank and
fluvial sediments on right bank.
Fine sediments accumulate at head of estuary
and coarser sediment accumulates at mouth.
Example : Firth of Forth, Hudson River,
Delaware River,Bakkhali River Estuary in Cox’s Bazar.
Mathamuhuri River Estuary in Cox’s Bazar.

4. Inversed estuary :4. Inversed estuary :
Inflow of freshwater low.Inflow of freshwater low.
Tidal amplitude low.Tidal amplitude low.
Evaporation very high.Evaporation very high.
Hypersaline prevails in that estuary.Hypersaline prevails in that estuary.
A seaward oriented salt wedge developsA seaward oriented salt wedge develops
during the ebb.during the ebb.
Example : Broad Sound in Australia.Example : Broad Sound in Australia.
Naf River Estuary in Teknaf.Naf River Estuary in Teknaf.
nextnext

Estuarine communities
Saltmarsh, seagrass bed, mudflat/sand, pelagic,
Mangrove

Mixture of freshwater and marine organisms, only few real
brackish-water species; diversity lowest in brackish water (5-
10‰)

1. Saltmarsh Community (intertidal)
Salt marshes are a mosaic of snaking channels called tidal creeks
that fill with seawater during high tides and drain during low tides.
Fish species including flounder and mullet live most of their lives in
marsh creeks. Dominated by marshgrasses (flowering plants) as
high as 2 m, which trap nutrient-rich sediments most plant
tissues are not grazed but get into detrital food web slow decay and
deep sediment, saltmarshes growth upwards, eventually filling the
estuary and becoming land

2. Seagrass bed (inter- to subtidal)
Eelgrass (temperate) and Turtlegrass (tropical)
few seaweeds, which do not grow well on muddy sediment
many epiphytic diatoms on seagrass contribute to primary
production and serve as food for snails
habitat for sessile animals (hydroids)
seagrass biomass ends up in detritus
manatees and sea turtles graze Turtlegrass

3. Mud/Sandflats
•Primary producers: algae, mostly benthic diatoms and
dinoflagellates
•cyanobacteria mats on mudflats
•production 10% or less of seagrass beds and saltmarshes,
decreasing with grain size of sediment (mud more productive
than sand)
•macro- and meiobenthos, often detrivores, living of deposits
from seagrasses and marshes birds important grazers

4. Plankton Community
•High production by nutrients imported by the freshwater inflow
•Highest production and biomass at intermediate salinities. At
head of estuary, nutrient concentrations are high but turbidity by
sediments suspended in river water is high as well so that
phytoplankton remains light-limited; as sediments sink out of the
water column along the river plume and water turbidity decreases,
phytoplankton can make use of high nutrient concentrations at
intermediate salinities.
•benthic filter-feeders profit from plankton production
•high sedimentation of plankton from estuarine plumes can cause
oxygen consumption and anoxic sediments at the seaward edge
(even hypoxia in the water; for example Mississippi River plume)

5. Mangroves
•Mangroves replace saltmarshes in tropical regions (60-
75% of tropical and subtropical coastlines), upper tidal
zone
•High salinity tolerance: broad distribution from high in
the estuary to almost fully marine water, but wave-
protected zone
•Mangroves: 12 genera, 60 species of flowering,
terrestrial trees and shrubs restricted to mangrove
swamps; shallow and far-reaching roots; airial roots help
in oxygen supply because the sediments are anoxic;
Ecological importance:
• Host and feed breeding birds
• protect shoreline from erosion during tropical
storms
• important fisheries and supply (boat and fire wood)
in native people

Adaptation to Estuarine environment:
A very harsh environment with conditions less than optimal for
either plant or animal species.
Salinity variations – at any one point in the mid region of an
estuary, this can change from virtually completely fresh water to
seawater over a period of six hours.
Temperature variations – shallow waters can heat up to the
surrounding air temperature on a hot day, but deeper waters
remain cool. Cooler or warmer water can also flow in from the
sea during tidal action or from the river.
Oxygen level variations – levels of oxygen in the water also
vary considerably. Where eutrophication is taking place it can
drop right down, eg in salt marshes and in deeper water areas.
Right next to such an area in the mainstream of the estuary the
oxygen levels can be normal.
Tidal Currents: Changes direction in every six hours

Plants and animals that can tolerate only slight changes in
salinity are called stenohaline . These organisms usually
live in either freshwater or saltwater environments. Most
stenohaline organisms cannot tolerate the rapid changes in
salinity that occur during each tidal cycle in an estuary.
Plants and animals that can tolerate a wide range of
salinities are called euryhaline. These are the plants and
animals most often found in the brackish waters of
estuaries. There are far fewer euryhaline than stenohaline
organisms because it requires a lot of energy to adapt to
constantly changing salinities.

Some organisms have evolved special physical structures to
cope with changing salinity. The smooth cordgrass (Spartina
alterniflora) found in salt marshes, for example, has special
filters on its roots to remove salts from the water it absorbs.
This plant also expels excess salt through its leaves.
Pickleweed (Salicornia sp.) is an edible halophylic (salt-loving)
plant that tolerates the unique and constantly changing
environment of the salt-marsh estuary. Its Latin name Salicornia
means "salt horns," and describes the shape of the top sections of
the plant that are filled with salt.

During low tides when they are exposed to low-salinity water, oysters
close up their shells and stop feeding. Isolated in their shells, oysters
switch from aerobic respiration (breathing oxygen through their gills)
to anaerobic respiration, which does not require oxygen. Many hours
later, when the high tides return and the salinity and oxygen levels in
the water are considerably higher, the oysters open their shells and
return to feeding and breathing oxygen.

Most are adapted to survive in a limited range of conditions. For
example, many animals live only where the water is
salty. Others live only where the water is fresh. In the Hudson
estuary, oysters and seahorses live near New York City, where
the water is almost as salty as ocean water.
Zebra mussels and yellow perch live in fresh water upstream,
farther away from the sea.

Like American shad, adapt to livingin both salt and fresh water as
they migrate between the Atlantic Ocean and the Hudson.
Barnacles feed and breathe underwater.
At low tide, they shut their shells
to keep water inside.
The fiddler crab breathes air
and lives in a burrow in the mud.
The crab enters the burrow and
plugs the entrance with mud as the
tide rises. This traps air in the
burrow, allowing the crab to
breathe during high tide.

The body fluids of a fish has salt and mineral content lower
than sea water – this means that in the sea a fish will actually
lose water and tend to dehydrate. To compensate the fish
extracts the salt from the water it swallows and excretes it in
highly concentrated urine – in the process retaining water in
its body.
In fresh water the biogenic salt content is higher than the
surrounding water – this means that water will tend to enter the
fish’s body and make it swell up and burst! Here the fish must
extract as much salts from the water as possible and retain it in its
body, in the process excreting as much water as possible in the

Estuary margin development - population growth and coastal
settlement.
Increased demands for recreational uses - such as boating and
fishing.
Development in estuaries - such as marine farms and marinas.
Catchment development - such as forestry and agriculture.
Land clearance and reclamation.
Excavation and dredging for example for boat ramps and boat
channels.
Introduction of invasive species such as Spartina
Resource extraction – such as fishing.
Long term climate changes including sea-level rise.

What you can do
•Don’t dump rubbish in estuaries (including garden rubbish,
which may include invasive weeds).
•Don’t plant exotic species in our estuaries.
•Prevent pollutants getting into stormwater drains and flowing
into estuaries and coastal areas - wash your car, boat keels,
motors and trailers on the grass - not on paved surfaces. Fix oil
leaks. Don’t put petrol, paints, solvents, and garden chemicals
down stormwater drains.
•Boat owners should dispose of rubbish and sewage on-shore,
and maintain bilge pumps to reduce oil leaks.
Fence off the edge of estuaries to prevent stock access.
•Avoid trampling across saltmarshes, mangroves and seagrass
beds. They are sensitive to trampling and can take a long time
to recover.
•Plant or leave a buffer strip of plants along streams and at the
edge of estuaries to help stabilise the banks and trap nutrients.
These buffers act as a filter between land and waterways

ReferencesReferences
1. The ecology of estuaries –Mclusky and Mike Elliott1. The ecology of estuaries –Mclusky and Mike Elliott
2. Ecology- Odum2. Ecology- Odum
3. Introductory Oceanography- Thurman3. Introductory Oceanography- Thurman
4.Oceanography- Grant Gross4.Oceanography- Grant Gross

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