A Study of Mangrove Habitat

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