A Study of Mangrove Habitat
1. Introduction
1.1 Aims
- To understand the structure and functioning of a mangrove.
- To learn and practice basic ecological techniques.
- To use simple field equipment to measure environmental factors.
- To identify the common mangrove organisms.
- To identify and interpret adaptive features pertain to the mangrove
organisms.
1.2 General introduction about the mangrove habitat
Mangroves are intertidal coastal wetland ecosystems found in sheltered
tropical and subtropical shores. They receive inputs fro the sea through
regular tidal flushing and also from freshwater streams and rivers.
Mangrove plants (or simply called mangroves) are evergreen plants that
grow in sheltered areas away from significant wave action. They have
specialized morphological, physiological and structural characteristics that
allow them to adapt to such a peculiar physical environment. Different
species occupy different positions relative to the ocean based on their
tolerance towards
salinity.
Our field trip
The venue for the
study is the mangrove
along the coast near
Sai Keng.
Date: 19 / 4 / 2007
Time: 1300 – 1600
2. A sketch map of the shore (refer to Appendix 1)
3. Methods of study
3.1 Measurement of profile of the site
Instruments:
Meter ruler, clinometer
Procedures:
- Two wooden poles of the same height were placed on the soil surface
at two points 3 m apart.
- A clinometer was used to measure the angle of depression or angle of
elevation ( ).
- The vertical difference between the two points was calculated by
trigonometry.
3.2. Measurement of abiotic factors
Factor Instrument
Temperature Alcohol-in-glass thermometer
Light intensity Environmental comparator with light
3 m
A diagram showing the measurement of vertical difference
probe
Relative humidity Wet-and-dry bulb thermometer
Air movement Hand-held wind-meter
Salinity Refractometer
pH of water pH paper
Procedures:
- Temperature
At 0 m point of the transect line, the thermometer was placed in the air,
allowed to stabilize and the temperature was recorded.
The thermometer was held under the canopy of the plants, allowed to
stabilize and the temperature was recorded.
- Salinity of soil water
After placing the soil water sample on the refractometer, the liquid
level was noted, which indicated the concentration of sodium chloride
in the sample.
- Humidity
The relative humidity at various microhabitats was measured by
whirling the hygrometer until the temperature readings were constant.
(i.e. about 1 – 2 minutes)
The wet and dry bulb temperatures were recorded. The difference was
calculated and checked against the scale to get the relative humidity.
- Wind speed
The speed of wind was recorded by holding the hand-held wind-meter
against the wind. The direction of the wind was also noted.
- pH of soil water
A sample of soil water was collected in a vial.
The pH of the soil water was then checked with pH paper on the spot.
- Light intensity
Using the environmental comparator with light probe, the light
intensity under canopy was measured in lux.
3.3 Investigation of distribution of organisms
First of all, a belt transect was set up to investigate the distribution of
organisms. A belt transect was preferred to a line transect as the former was,
instead of a single line, a strip are along which animals and plants were
counted and studied. It was a more useful and accurate method for showing
the rapid change from one type of vegetation to another on the shore where
a line transect did not cover enough organisms to show this.
Procedures:
- A typical stretch of land running down to the sea was selected starting
above the high tide mark.
- A transect line perpendicular to the sea line was laid down in a region
where there was apparent transition of vegetation.
- The zero end of the transect line was tied to the stem of a shrub at the
ground level at the back of the shore.
- A half metre quadrat frame was placed against the transect line at 3 m
interval starting from zero mark.
3.3.1 Animal capturing and sampling
Instruments:
Aquarium net, forceps, plastic vials, quadrats, transect line, petri
dish
Procedures:
- A transect line perpendicular to the sea line was laid down in
a region of the mangrove community showing distinct
zonation patterns.
- The end of the transect line in contact with the plants at the
back of the shore was taken as the zero point. Quadrats were
placed every 3 m from zero point on the left side (viewed
from the shore) of the transect line.
- Animals on surface of mud and under stones were searched
within the quadrats. The animals were identified, counted
and their locations were noted.
3.3.2 Vegetation analysis
Instruments:
Quadrats, transect line, metre rule
Procedures:
- A transect line perpendicular to the sea line was laid
down in a region of the mangrove community where
there is apparent transition of vegetation.
- The end of the transect line in contact with the plants at
the back of the short was taken as the zero point.
Quadrats were placed every 3 m from zero point on the
left side (viewed from the shore) of the transect line.
- The plants found in the quadrats were identified. Their
number was counted and their height and width were
recorded. Their percentage cover was determined by
looking at the quadrat vertically from the top.
- Profile diagrams of the vegetation were plotted and
superimposed on a leveling shore profile. The adaptive
features of the plants were identified and the relevant
features of the immediate surroundings of the plants
were also noted down.
4. Results
4.1 Height profile of the mangrove along the transect line
can be found in Appendix 2.
A table showing data for the height profile
Distance from
zero point / m
Horizontal
distance / m
Difference in
height / cm
Height above zero
point / m
0 3 7.8 1.705 + 0.078 = 1.783
3 3 36.8 1.337 + 0.368 = 1.705
6 3 21.0 1.127 + 0.210 = 1.337
9 3 0 1.127 + 0 = 1.127
12 3 21.0 0.917 + 0.210 = 1.127
15 3 21.0 0.707 + 0.210 = 0.917
18 3 26.2 0.445 + 0.262 = 0.707
21 3 21.0 0.235 + 0.210 = 0.445
24 3 15.7 0.078 + 0.157 = 0.235
27 3 7.8 0 + 0.078 = 0.078
30 0
4.2 Data regarding the abiotic factors
A table showing abiotic factors of different sampling points
Determined factorSampling point
0 m 3 m 6 m 9 m 12 m 15 m 18 m 21 m 24 m 27 m
Soil
temperature / ℃
Soil surface 23 24 22.5 22 19.5 21 25 22.0 21.0 21
Inside the soil 22.5 20 20.5 20 20 20 23.5 21.0 20.5 22
Temperature below canopy / ℃ 21.5 24 25 22 21.5 20 23.0 23.0 22
Light intensity below canopy / lux 36 30 26 23.5 52 48 44 64 76.0 80
Relative humidity / % 64 73 69 70 76 69 72 69 65 57
pH of soil 7 8 7 6 6 7 8 6 7 8
Wind speed / m.p.h. 5.5 6 3 7 5 8 3 10 3 3.5
Salinity of soil water / % 3.0 3.3 4.5 3 3.0 2.6 3.1 3.0 3.2 3.5
4.3 Data regarding the biotic factors
A table showing the plant species along the transect line
SpeciesPosition of stem
along transect line
Circumference of
stem / m
Maximum
height / m
Percentage
coverage / %
Immediate
surroundings
Aegiceras
corniculatum0 m 0.07 1.25 75 Pneumatophores
Aegiceras
corniculatum6 m 0.11 1.48 50 Pneumatophores
Aegiceras
corniculatum9 m 0.41 1.5 100
Pneumatophores,
droppers
Avicennia marina 3 m 0.16 1.47 50 Pneumatophores
Kandelia candel 15 m 0.25 1.4 75 Pneumatophores
A table showing the animal species along the transect line
AnimalPosition of quadrat
0 m 3 m 6 m 9 m 12 m 15 m 18 m 21 m 24 m 27 m
Earthworm 0 0 0 0 0 1 0 0 0 0
Crab 0 3 0 4 1 3 0 0 1 1
Mud snail 0 8 6 01 0 0 25 3 0 15
Other snails 127 0 0 10 17 37 0 85 26 90
Periwinkle 0 0 0 0 0 0 0 0 14 0
Bivalve 0 0 0 0 0 0 40 0 1 0
Rock oyster 0 0 0 0 0 0 1 3 10 13
(* Note: Measurement was made in terms of number of individuals)
(Refer to Appendix 3 for the histograms showing distribution of organisms)
5. Discussions
5.1 Zonation of plants along transect line
Along the transect line, there were altogether three types of plants found.
They were Aegiceras corniculatum, Kandelia candel and Avicennia marina.
Aegiceras corniculatum were found from 0 – 9 m from the transect line.
Avicennia marina was found at 3 m from the transect line. Kandelia candel
was found at 15 m from the transect line.
Aegiceras corniculatum could be found towards the back of the shore while
Kandelia candel more outward, towards the seaside. Avicennia marina was
found occasionally.
However, the number of plants found along the transect line was not large
enough to give an accurate general trend of the plant species in the
mangrove. This could be accounted for by the limitation of the choice of our
transect belt.
5.2 Zonation of animals along transect
line
Along the transect line, there were five groups of
animals found. They are earthworms (annelids),
crabs (crustaceans), snails, periwinkles
(gastropods),
bivalves and
rock oysters
(bivalves).
Observing the general trend, there were
more snails, periwinkles and bivalves
Gastropod
Rock oystersattached on rock surface
towards the seaside. This could be because of the faster water current
towards the seaside, leading to a better circulation of nutrients, and enabling
more effective filter-feeding by some of the organisms.
In particular, there were more rock oysters
towards the seaside. This could be explained
by their adaptations that they could attach to
the surfaces of rocks, so that they would not be
washed away into the sea by the tides.
5.3 Adaptation of animals and
plants found in the mangrove
habitat
Adaptations of animals found in mangrove habitat
- To encounter wave action, soft-body animals such as bivalves,
periwinkles and rock oysters have hard calcareous shells to protect
them from the pounding action of waves. Periwinkles and mud snails
also have suctorial foot for attachment of rock surface by suction.
Crabs and shells of many bivalves are generally flat and round in shape
to reduce resistance to wave action. In addition, crabs have strong legs
for gripping. Earthworm, without hard shell, simply hides beneath
mud.
- To withstand desiccation at low tides, bivalves, periwinkles, mud snails
and rock oysters solve the problem by enclosing themselves in shells
and trapping a small amount of water within the shells. The shells are
impermeable and thus reduce water loss by evaporation. Periwinkles
and mud snails have
operculum that completely
covers the apertures of their
shells at low tides. Rock
oysters anchor together close
to each other on rock surface
to reduce water loss.
- Some mobile animals such as
earthworm and crabs simply
hide in mud cave, which is Mud caves
More crabs found towards the seaside
less affected by solar heat and evaporation, resulting a relative constant
temperature and humidity.
- For gaseous exchange, as most animals inhabiting in mangrove area
are marine in nature and their respiratory surfaces are susceptible to
desiccation in air, intertidal animals tend to enclose their respiratory
organs in a protective cavity to prevent them from desiccation.
- Molluscs such as bivalves and rock oysters have gills in the mantle
cavity can be kept moist and protected by the shells. Gaseous exchange
usually only occurs in the presence of water. The adaptation is that a
small amount of water is often trapped so that has exchange can still
occur during low tides.
- Gastropods such as periwinkles and mud snails, living at higher tidal
levels, have greater difficulties in keeping the gills moist. To encounter
this, gills are reduction and modification of mantle cavity as a lung for
aerial respiration.
- Mangroves at low tides may be washed by rainwater. This creates
stress to animals only adapted to salty marine environment. To
overcome salinity fluctuation, bivalves, rock oysters and gastropods
like periwinkles and mud snails close up their valves or operculum.
- For feeding, bivalves and rock oysters are filter feeders. They obtained
food by filtering minute food particles suspended in water. Periwinkles
and mud snails possess a radula for browsing the algae on rock
surfaces or plant structures.
- These intertidal animals have to expose their bodies when they feed, to
prevent desiccation, most of them feed during high tides and have their
bodies submerged. Crabs, however, feed at low tides and as
detritivores, they feed on any food washed in by tides.
Adaptations of plants found in mangrove habitat
- To establish the plant body in such a soft and unstable substratum,
many species of mangrove plants, such as Kandelia and Aegiceras,
develop branched,
looping aerial roots
arising from the trunk,
develop branched,
looping aerial roots
arising from the trunk
and lower branched.
Prop roots
These aerial roots trap mud during tidal movement, and help to
increase the amount of soil.
- The prop roots of Kandelia supply air to the underlying roots.
Avicennia develop cable roots which spread horizontally and laterally
just below the soil surface to
anchor the plant firmly in soil.
- For gaseous exchange of the mangrove plants under the waterlogged
and almost anaerobic conditions. Avicennia produce erected aerial
roots called pneumatophores which extend upwards into the air at
intervals from the cable roots. Pneumatophores facilitate gas exchange
between the submerged roots and the atmosphere. The prop roots of
Aegicera and Kandelia form arched called knee joints which grow
above the soil surface for gas exchange. These structures bear many
large lenticels on their surface to increase the efficiency of gaseous
exchange.
- To encounter
high salinity, as
Aegicera,
Avicennia and
Kandelia are
halophytes, they
can overcome
the difficulty of
water absorption
which arises from the high salt concentration
in the surrounding water. They can accumulate low molecular
carbohydrates to keep the water potential of root cells even lower than
that of the surrounding water. Aegicera and Avicennia secret the excess
salts with the presence of salt glands in leaves. Aegicera and Kandelia
can prevent salts from entering the root xylem by an active pump
mechanism.
- To overcome dehydration, the leaves of Aegicera, Avicennia and
Kandelia have thick cuticle, epidermal hairs and sunken stomata to
reduce transpiration rate. Some of them store water in special multi-
layered water storage tissues.
- For adaptations for reproduction, to enhance success in seedling
development in tidal movements and unstable substratum, Aegicera
and Kandelia produce seeds that germinate inside the fruits. The fruits
Pneumatophores
are called droppers. When the
droppers detach from the mother
plants, the roots are already in the
early stage of development and can
establish themselves rapidly in the
substratum. The droppers have
freshly structures called hypocotyls,
which help them to float and
disperse. They are carried by water
until they reach a position where the
water is shallow enough for the roots
to come in contact with the
substratum. The droppers are
elongated and are heavier at the region near the lower tip. These allow
them to stick in the substratum in an upright position.
5.4 Developmental status of the mangrove ecosystem and
effect of human activities on the mangrove system
The mangrove in Sai Keng that we visited was not really far from human
habitation. We found some litter. In fact, the pollution was not so serious as
to harm the animals in the mangrove. However, if the trash is not dealt with
properly, some poisonous substance may leak out under microbial
decomposition to deteriorate the soil affecting the normal growth of
mangrove plants.
Development of a dropper into a young plant
As seen in the previous parts, environmental factors are exerting great
influence on the ecosystem of Mangrove. For example, a change in oxygen
content, as caused by a sudden increase in micro-organisms in the water as a
result of a large increase in organic waste released by villagers, can cause
suffocation of marine organisms in the area.
Moreover, domestic wastes (e.g. nitrogenous waste) released act as nutrients
which promote the growth of root system of mangrove plants and lead to
algae bloom. These abnormal vary in organisms population will disturb the
equilibrium of the ecosystem in mangrove.
According to the continuous construction works in Sai Keng village
recently, destruction to the mangroves in Sai Keng has brought an
irreversible damage. Construction wastes are dumped beside the mangrove
and the scenery of Sai Keng mangrove has been destroyed by the villa.
The roots of the mangrove plants and rock surfaces provide habitats for
oysters and help to impede water flow; thereby enhancing the deposition of
sediment in areas where it is already occurring. It is usually the case that the
fine, anoxic sediments under mangroves act as sinks for a variety of heavy
(trace) metals, which are scavenged from the overlying seawater by
Human settlement near Sai Keng mangrove
colloidal particles in the sediments. The nearby residents remove some
oysters for eating purposes. The disturbance of these underlying sediments
may create problems of trace metal contamination of seawater and loss of
sediment in the mangrove area.
Removal of rock oysters
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