b

An innovative approach for monitoring abiotic factors influencing

mangrove forest biodiversity in an estuarine ecosystemM. Atwell1; M.N. Wuddivira1; J. Gobin2; D.A. Robinson3

1Department of Food Production, University of the West Indies,2Department of Life Sciences, University of the West Indies,

3Centre for Ecology and Hydrology, Environment Centre, Wales, UK

*

The UN’s Millennium Ecosystem Assessment (2005)

targeted coastal wetlands as environments that “are

experiencing some of the most rapid degradation and

loss worldwide”.

Mangrove wetlands are important because they provide

a buffer, protecting vulnerable coastal and marine

ecosystems from rising sea levels and storm tides (Ewel

et al., 1998).

In Trinidad West Indies, land-use changes and sea

defenses impact water quality and alter mangrove forest

distribution and biodiversity.

This research therefore investigates the abiotic water

quality factors influencing mangrove zonation along the

South Oropouche River, Godineau Swamp.

An innovative measurement campaign using conventional sensors and

high spatial resolution geophysical imaging was adopted to monitor the

river water quality and salinity patterns. Seasonally at high tide,

measurements were obtained from two river channels, a 6km larger main

channel and a 2km shorter side channel (Fig. 1).

•Electromagnetic Induction (boat-mounted geophysical imaging) was used

to spatially map the salinity of the water non-invasively (Plate 1). River

water was spatially tested in situ for pH, dissolve oxygen (DO),

conductivity, temperature and turbidity using a U-10 multi parameter water

quality checker (Plate 2).

•Aerial photos were analyzed using GIS for the years 1962 and 2003 to

determine mangrove boundary change (Fig. 2). Species abundance was

identified using transects at 5m intervals along both sides of the river

banks. Data analysis included geostatistics with SGEMS, graphs and GIS.

Introduction1

Methodology2

Results3

Aerial photos (Fig.2) showed an overall decrease of

more than 10% in mangrove extent from 1962- 2003.

The apparent electrical conductivity (ECa) levels were

much lower in the main channel than in the side channel

both in the wet and dry season (Fig. 1). In the dry

season, however, saline water intrusion was very evident

as salt water moved further inland (Fig. 1b).

The pH and DO levels showed a marked decrease in the

side channel than in the main channel of the river (Figs.

3-6). Rhizophora mangle L. (red) as opposed to

Avicennia germinans (black) and Laguncularia

racemosa (white) was found under adverse water quality

conditions of low DO, pH levels and high levels of

salinity (Figs. 3-6).

Summary &

Conclusion

Rhizophora mangle (red), Avicennia germinans (black)

and Laguncularia racemosa (white) species of mangroves

were identified along the river banks. The red mangroves

thrived in areas of adverse water quality conditions and

indeed are the hardiest of the three species.

Salinity and water quality parameters play a major role in

determining mangrove species diversity in a tropical

estuarine ecosystem.

The novel use of boat-mounted geophysical imaging has

therefore proven to be a useful tool in determining saline

water intrusion and in the characterization of spatial and

temporal patterns of the abiotic factors affecting mangrove

zonation pattern.

5

4

Our results showed a significant decrease in

mangrove species diversity as adverse water

quality conditions prevailed. This is partly due

to saline water intrusion particularly in the dry

season, which takes place with increased tidal

inundation extending as far as 6km upstream

(Fig. 1).

Plate 1. Boat-mounted geophysical imaging Plate 2. U-10 Water Quality Checker

Fig.3: Percentage species abundance as a function of pH

(Main channel)

0

20

40

60

80

100

120

16 730

1727

2443

3347

4115

4822

5718

Distance (m)

% A

bu

nd

an

ce o

f

do

min

an

t sp

eci

es

6.8

7.0

7.2

7.4

7.6

7.8

8.0

8.2

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2000.00

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3000.00

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4000.00

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pH

White Black Red pH

Fig. 5: Percentage species abundance as a function of

Dissolved Oxygen (DO) (Main channel)

0

20

40

60

80

100

120

16 4941157

1727

2350

2676

3347

3813

4333

4822

5494

6039

Distance (m)

% A

bu

nd

ance

of

do

min

ant

spec

ies

0

1

2

3

4

5

6

7

8

0.00000

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1000.00

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2000.00

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3000.00

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4000.00

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5000.00

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6000.00

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DO

(m

g/l)

White Black Red DO

Fig.2: Aerial extent of mangrove boundary for the years 1962

(pink) and 2003 (pattern)

References1. Ewel, K.C., Twilley, L.L. and Ong, J.E. 1998.

Different kinds of mangrove forests provide different

goods and services. Global Ecology and Biogeography

letters. 7:83-94.

2. “South Oropouche, Trinidad.” 2009. 61o29’56”N

and 10o13’13”E. Google Earth. August 29,

2009. Accessed August 29 2009.

3. “South Oropouche, Trinidad.” 2010. 61o29’56”N

and 10o13’13” E. Google Earth. February 19,

2010. Accessed February 19 2010.

4. Millennium Ecosystem Assessment "Synthesis

Report" (2005) 16.

Discussion

Fig. 4: Percentage species abundance as a function of

pH (Side channel)

0

20

40

60

80

100

120

16 253

494

730

983

1249

1535

1829

2079

2159

Distance (m)

% A

bu

nd

an

ce o

f

do

min

an

t sp

eci

es

7.2

7.4

7.6

7.8

8.0

8.2

0.00 500.00 1000.00 1500.00 2000.00 2500.00

pH

White Black Red pH

Fig. 6: Percentage species abundance as a function of Dissolved

Oxygen (DO) (Side channel)

0

20

40

60

80

100

120

16 253

494

730

983

1249

1535

1829

2079

2159

Distance (m)

% A

bu

nd

an

ce

of

do

min

an

t s

pe

cie

s

012345678

0.00 500.00 1000.00 1500.00 2000.00 2500.00

DO

(m

g/l)

White Black Red DO

(a) (b)

This adverse water quality may also be due to

low levels of pH and DO in the stagnant side

channel as a result of decomposition of organic

matter and run-off from acidified mangrove

soils (Figs. 4 and 6). In the side channel where

adverse water quality conditions were prevalent,

only the red mangrove predominates, indicating

a loss of mangrove species diversity.

Fig. 1: Spatial map of ECa along the main and side channels of the river in (a) wet season and (b) dry season. Blue represents low

salinity while red represents high salinity.