ROMBLON STATE UNIVERSITY LEARNING MODULE
SAN AGUSTIN CAMPUS
College of Education BSED FISHERY
MODULE 5
ECOLOGY OF FISHES
PHYSICOCHEMICAL PROPERTIES OF
BRACKISH WATER
INTRODUCTION:
This module you will learn about the physicochemical characteristics of briny or brackish water
which vary easily due to both marine and brackish water influences. After the content discussion,
you will be given some assignments or assessment regarding the topic discussed. Do the task
honestly coupled with high interest so that you can benefit the most of it.
LEARNING OUTCOMES:
Understand and apply the concepts gained about the study of physicochemical properties of
brackish water in real life situations.
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ROMBLON STATE UNIVERSITY LEARNING MODULE
SAN AGUSTIN CAMPUS
College of Education BSED FISHERY
LEARNING OBJECTIVES:
1. Define what brackish water is
2. Discuss common features of estuaries
3. Highlight biological features of estuaries.
4. .
CONTENT EXPLORATION
What is Brackish Water?
Freshwater, brackish and sea water can be differentiated based on salinity. Brackish water or
briny water is water that has more salinity than fresh water, but not as much as seawater. It may
result from mixing of seawater with fresh water, as in estuaries, or may occur in brackish fossil
aquifers. Brackish water contains between 0.5 and 30 grams of salt per litre or 0.5 to 30 parts per
thousand (ppt or ‰). Thus, brackish water covers a range of salinity regimes. The salinity of
brackish surface waters can vary considerably over space and time. Brackish water condition
occurs when fresh water mixes with sea water. The most extensive brackish water habitats are
estuaries, where a river meets the sea. Brackish water is also found in salt marshes.
An estuary is a partially enclosed body of water where a river or stream meets the sea. Estuaries
form the transition zones between the ocean and river environments. They contain a mixture of
salt and fresh water with high levels of nutrients. They are among the most productive
ecosystems in the world. Estuaries filter out sediments and pollutants from land –based sources.
Estuaries are subject to marine influences (waves, tides, inflow of saline water) and river
influences (fresh water flows and sedimentation).
The daily tides greatly influence the estuary environments and are dependent on factors like
coastal shape, local winds, water depth, and water restrictions. The daily tides affect the
abundance and distribution of plants and animals.
Common Features of Estuaries
Rivers provide continuous inputs of sediments into estuaries. Turbidity is a measure of the
amount of suspended particles including fine sediments in water. Circulation within estuaries
redistributes the suspended sediments and maximum turbidity occurs where fresh water from
rivers meets with seawater.
There are chemical interactions between sediment particles and dissolved ions in sea water
resulting in attraction between particles causing flocculation (particles stick together).The heavy
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ROMBLON STATE UNIVERSITY LEARNING MODULE
SAN AGUSTIN CAMPUS
College of Education BSED FISHERY
particles settle out of the water column and are deposited. Most estuaries efficiently retain
dissolved and particulate materials and so estuaries are very susceptible to pollution.
Salinity constantly changes in estuaries. The regular salinity change in estuaries is an important
feature. The degree of salinity is influenced by tides, melting snow, heavy precipitation and dry
periods or droughts.
Salinity varies within an estuary. It can be weak upstream of estuaries (about 0.5ppt) and very
high downstream (up to 30 ppt).The zone where freshwater change to salt water is called
apycnocline. The degree of salinity can also be influenced by factors such as the level and
intensity of the tides, the melting of snow in the spring, heavy precipitation, and dry periods
during the summer season. An out-going tide can reduce salinity slightly.
The fresh water from rivers is lighter than seawater and flows on top of the seawater. If the
estuary is deep enough, it may form a salt wedge where the fresh water flows over seawater for
some distance into the sea while salt water flows under the fresh water for some distance into the
river with low salinity up and high salinity below). In salt wedge estuaries, there is low
turbulence and little mixing of water; there is very little salt in surface water and high salinity at
the bottom.
Temperature
Temperatures of estuaries tend to be higher than sea surface temperature because estuaries are
shallow and semi enclosed. Temperature is one of the main elements influencing reproduction of
invertebrates and fish. Cold temperatures can negatively affect fish in their planktonic stages,
slowing down the hatching and growth of the young.
Tides
Rise and fall of tides cause turbulence and upwelling of water and nutrients from the bottom to
the surface. With the rise and fall of tides, nutrients are also brought into the estuary. Estuaries
are usually funnel-shaped and incoming tides at the mouth tend to increase in amplitude as the
channel narrows. The frictional contact from the shore and bottom act against the tide and tend to
lower the height of the tide. The rise and fall of tides can be felt far up-river, especially in bigger
estuaries.
Currents
In the estuary, river and tidal currents play a significant role in mixing thelower and upper layers
of water. The estuarine circulation is important in determining the productivity of the estuaries.
Sediment
An estuary changes constantly and accumulates sediments from rivers, streams, brackish
marshes located inland, salt marshes and sand dunes located near the mouth of the estuary.
Sediments can be made of animal and plant matter, or inorganic material like mud or sand.
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ROMBLON STATE UNIVERSITY LEARNING MODULE
SAN AGUSTIN CAMPUS
College of Education BSED FISHERY
3.3 Biological Features of Estuaries
Phytoplanktons are minute plants such as dinoflagellates. Phytoplankton, along with bacteria and
fungi, are the basis of life in the estuary and are carried by the currents. In order to survive, they
must remain in a place where the salinity fits their needs. They use the sun and the nutrients from
rivers and salt marshes to feed themselves.
Zooplanktons are the tiny animal part of plankton. In estuaries, zooplankton benefit from an
abundant food supply: phytoplankton, microscopic algae, bacteria, and detritus coming from
dead plants and animals.
Only certain types of plants can flourish in estuaries, and each of these plants can grow in only
certain parts of the estuary. A major factor influencing the growth and distribution of plants in an
estuary is its salinity or the amount of salt in the water. Some plants tolerate high concentrations
of salt, and have special salt pore on their leave to lose excess salt. Other plants cannot tolerate
salt and grow only where seawater cannot reach. Some plants can tolerate moderate
concentrations of salt and can survive in brackish (or slightly salty) areas of the estuary.
Another factor affecting plants in an estuary is the amount of flooding. The longer and deeper an
area is flooded with water, the less oxygen is available in the soil. Plant roots need oxygen to
grow and survive and should be adapted to an oxygen shortage. Some plants transport oxygen
from special storage cells in their leaves and stems to roots.
Check your Understanding
Direction: Write your answer on a separate sheet of paper.
1. What is brackish water?
2. Explain the common features of estuaries.
3. Analyse the biological features of estuaries.
REFERENCES:
1. McConnell, Campbell and Stanley Brue. Economics: Principles, Problems,
and Policies 16th Edition. McGraw-Hill, 2005.
2. Samuelson, Paul and William Nordhaus. Economics 17th Edition. McGraw-
Hill, 2005.
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ROMBLON STATE UNIVERSITY LEARNING MODULE
SAN AGUSTIN CAMPUS
College of Education BSED FISHERY
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