SEA SURFACE TEMPERATURE RISING TREND AND ITS … · Mentawai Islands are one of the outer regions of Indonesia, which is located in the western part of Sumatera and directly adjacent

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International Journal of Civil Engineering and Technology (IJCIET) Volume 8, Issue 10, October 2017, pp. 725–734, Article ID: IJCIET_08_10_076

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ISSN Print: 0976-6308 and ISSN Online: 0976-6316

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SEA SURFACE TEMPERATURE RISING TREND

AND ITS INFLUENCE ON THE CORAL

MORTALITY IN PAGAI STRAIT, MENTAWAI

ISLANDS, INDONESIA

U. J. Wisha

Research Institute of Coastal Resources and Vulnerability

Ministry of Marine Affairs and Fisheries, Padang - 25245, West Sumatera, Indonesia

H. Khoirunnisa

Geomatics Engineering Department

State Polytechnics of Batam, Batam - 29461, Riau Islands, Indonesia

ABSTRACT

Pagai Strait is threatened by warm water temperature. The Indian Ocean Dipole

(IOD) is the major factor which influences the water temperature anomaly in the West

Sumatera, which resulting in the high rate of coral mortality (bleaching). This study

aimed to identify trends and changes in temperature anomalies that occurred during

the last few years and their impact on coral bleaching in the Pagai Strait. The

regional sea surface temperature (SST) was analyzed spatially and graphically,

inverse distance weighted (IDW) is employed to process the spatial analysis of

temperature data. The supporting coral's data is conducted by the previous research

which employing the Line Intercept Transect (LIT) method, then calculate the percent

cover and mortality index. The temperature trend is increased ± 3oC for 5 years due to

the IOD event which occurring along the year, in 2016 the dipole mode condition was

dominated by a negative value. It causes the high temperature in the western waters of

Sumatera. The most pernicious impact is coral bleaching phenomenon, which is very

alarming due to the low of percent cover and high mortality. It indicates that the water

condition is not supporting the marine life.

Keywords: Sea surface temperature, IOD, coral bleaching and Pagai Strait

Cite this Article: U. J. Wisha and Khoirunnisa H. Sea Surface Temperature Rising

Trend and its Influence on the Coral mortality in Pagai Strait, Mentawai ISLANDS,

Indonesia. International Journal of Civil Engineering and Technology, 8(10), 2017,

pp. 725–734

http://www.iaeme.com/IJCIET/issues.asp?JType=IJCIET&VType=8&IType=10

U. J. Wisha and Khoirunnisa H.


1. INTRODUCTION

Mentawai Islands are one of the outer regions of Indonesia, which is located in the western

part of Sumatera and directly adjacent to the Indian Ocean. Pagai Strait is located between

North Pagai Island and South Pagai Island of Mentawai Islands. As the outermost water

region, the water circulation in Pagai Strait is influenced by the processes that happened in the

Indian Ocean such as the Indian Ocean Dipole (IOD). According to [21], the occurrence of

IOD has an influence on the rainfall intensity and the sea surface temperature in the western

part of Sumatera.

IOD is a phenomenon couple between the atmosphere and ocean, which are characterized

by differences anomaly bi-polar temperature sea surface in the tropical eastern Indian ocean

(the Indonesian waters around Sumatra and Java) and in the tropical western Indian ocean

(east coast of the African continent) [20]. The Indian Ocean Dipole index is defined as the

difference of sea surface temperature anomalies latitude 10oS – equatorial / longitude 90

o-

110oE or Equatorial Indian Ocean to the east and latitude 10°S-10°N / longitude 50

o-70

oE or

western part of the Equatorial Indian Ocean [14]. The index value > 0.35 were classified as

IOD (+) and < -0.35 classified as IOD (-). The IOD (+) means that the sea surface temperature

on the East Coast of Africa is higher than in the West Coast of Sumatra, the opposite is the

IOD (-) where the warm phase occurs on the west coast of Sumatera, so the convection

becomes increases [4].

Sea surface temperature anomalies (SSTa) during the positive phase of the IOD is

characterized by lower SSTa in the eastern waters of the Indian Ocean and the warmer waters

in the western. The SST anomalies occur near the south coast of Java and western Sumatra

and declining in the long term around ± 3°C [14]; [22]; [23].

Coral bleaching is a stress-related response that can be triggered by elevated sea surface

temperatures (SST). Recent increases in the frequency of coral bleaching have led to concerns

that increases in marine temperatures may threaten entire coral reef regions. A rise in regional

SST of 0.1oC results in 35% and 42% increase in the geographic extent and intensity of coral

bleaching [9]. Coral bleaching, the whitening of corals due to loss of symbiotic algae and/or

their pigments, is a response that can be triggered by a variety of stresses acting at local

scales. These stresses include solar radiation, salinity shock, sedimentation, disease, and

temperature increases.

Research about the environmental and physical aspects is still rare conducted in the Pagai

Strait due to its long distance from the city center and limited access reaching the location,

resulting in the sustainable monitoring of sea temperature and corals are minimal. Based on

the previous research by [6] percentage of coral cover included in the category of poor-

moderate with coral mortality index is moderate to high. These results show that the condition

of corals in the Pagai Strait very alarming. The purpose of this study was to identify trends

and changes in temperature anomalies that occurred during the last few years and their impact

on coral bleaching in the Pagai Strait.

2. MATERIALS AND METHODS

The materials consist of the annual time series temperature data around Pagai Strait that

downloaded from ECMWF (European Centre for Medium-Range Weather Forecasts) web

page (http://www.ecmwf.int/en/research/climate-reanalysis/browse-reanalysis-datasets) in the

form of reanalysis data in the region of 2oS – 3

oS and 100

oE – 100.25

oE and it has the

resolution as well as 0.125o. The other supporting data are obtained from a previous study

conducted by [6] such as life form composition, percent cover of corals, mortality index

calculation, and bleaching documentation.

Sea Surface Temperature Rising Trend and its Influence on the Coral mortality in Pagai Strait,

Mentawai ISLANDS, Indonesia


2.1. Sea Surface Temperature Analysis

Table 2 Set-up for generate IDW processing

Indicator Implemented in IDW Processing

Projection Coordinate

system

Geographic (Longitude/Latitude)

World Geographic System (WGS)

1984

Geoprocessing-

Environment Setting

Processing Extent:

Top = -2.50 Left = 100.00

Bottom = -3.00 Right = 100.25

ArcToolbox - Spatial

Analyst Tool

IDW Set up:

Output Cell Size = 0.001

Number of Points = 12

Search Radius = Variable

The temperature data are analyzed graphically and spatially by employing spatial analysis

tool Inverse Distance Weighted (IDW). IDW is used in this study was performed to analyze

the distribution temperature in Pagai Strait seawaters annually, it helps to lay out the ECMWF

temperature data to be well interpreted the process of distribution, set up for generating IDW

is shown in Table 1.

2.2. Coral Assessment Analysis

The secondary data that obtained from previous research is then modified and analyzed

statistically. According to [6] the coral data observed on the April 15th, 2016 by employing

line intercept Transect (LIT) method, LIT employed to define the benthic community based

on life form developed. Coral community is characterized by the life form category that gives

information about coral community morphology descriptively. Bed habitat component and the

length of the transitional cover which is found along the transect line (10x10 m) is then

grouped by their growth form, the coral cover criteria are shown in Table 2. The cover

calculation has employed the formula below:

% coral cover = �� (��)

�� (��) x 100 (1)

The rate of damage is related to the high rate of coral condition change (living coral and

coral demise), the coral mortality ratio is known by calculating the mortality index (MI) by

the formula below:

Mortality Index (MI) = % & �& ��

% ��' � �� & �& ��(��' �� (2)

The value of MI that closer to 0 described that there is no change in the coral life

community, therefore if the MI value is closer to 1 described that there is a high rate of

mortality. The MI category is shown in Table 2.

Table 2 Percent coral cover criteria

Category Percent Cover (%) Mortality index (MI)

Poor 0-24.9 0-0.249

Moderate 25-49.9 0.25-0.499

High 50-74.9 0.5-0.749

Very high 75-100 0.75-1

Source: Ministry of Environment and Forestry, Republic of Indonesia, regulation no. 4, 2011



2.3. Research Location

The observed point consists of 9 points which represent the condition of Pagai Strait, 6 points

around Siruso Island and 3 points around the Sijaojao atoll (Figure 1). In those stations, the

coral observation is conducted in the April 2016, employed LIT method with the depth ranged

from 5-10 meters in the coral reef area.

Figure 1 Research location map

3. RESULTS AND DISCUSSION

3.1. Correlation between IOD and SST in Pagai Strait

According to Figure 2, the IOD condition in the year of 2016 was dominantly by the negative

IOD. It has the value of -0.47 of IOD index. It shows that the SST condition in the eastern

Indian Ocean should become warmer than the normal condition [14]. It was proven in Figure

5, there was a positive anomaly in the SST condition in the Pagai Strait during 2016. The SST

reached the highest value in the year of 2016.

In addition, the anomaly condition of SST in Pagai strait can be observed by the annual

spatial distribution at every year (Figure 4). It shows that the highest condition of SST

occurred in the year of 2016. The SST condition in Pagai strait was rising during 5 years since

2011 until 2016.

The SST fluctuation has been influenced by the IOD, it can be observed by the Figure 2

and 5. The IOD condition during August – December 2015 have the highest value, it is called

by the positive IOD. It influenced to Pagai Strait condition, especially in SST value. It is

shown in Figure 4 and 5, that the condition of SST in 2015 is shallower than the SST in 2016.

In addition to the IOD, the distribution of SST in the Pagai Strait is influenced by the

seasonal condition. The highest condition of the SST has occurred on April 2016 (Figure 3).

Figure 3 shows the monthly distribution of SST for the year of 2011 to 2016.

The lowest condition on SST at Pagai Strait had been occurring in September 2011 as a

transitional period. The SST condition has been correlated by the chlorophyll, while the SST

is going to the highest condition, then the chlorophyll concentration has the low condition

[19]; [11]. The chlorophyll condition is related to the condition of coral.

Besides on the IOD and SST condition, the wind velocity has also influenced the

condition of the coral at the Pagai Strait. The wind velocity is the one thing of the IOD’s

regulation. At the time of the negative IOD, especially in the year of 2016, it enhanced the

role of monsoonal variation [13]; [10].




The first transitional period exists during April to May, from the southeasterly wind-

driven pushes the water in the Indian Ocean after the northeasterly wind driven [19]; [2]; [7].

Figure 2 Dipole mode index (DMI) in the years of 2012-2016

3.2. Annual SST Trend During Last 5 Years

The monthly SST fluctuations have the same pattern for 5 years (Figure 3). In the wet season

(Nov, Dec, Jan) the STT is relatively lower than the dry season (May, Jun, Jul), according to

[16] the IOD switch over to warming in the western region of Sumatera happened during May

and July respectively, surface heat fluxes and horizontal advection caused warm SST

(anomaly) and the contribution of the latter decreased after august.

During the transitional season, the SST condition is following the season before, the

highest SST is measured in 2016 where the dipole mode status is negative, and the lower SST

is measured in 2015 where the dipole mode status is dominantly positive, it shows that the

water temperature is influenced by IOD events, IOD event supported by ENSO (El-Nino

Southern Oscillation) is the major factors which control the rainfall intensity and the surface

temperature anomaly in the West Sumatera [18]. Those coupled ocean-atmospheric

phenomena in which convection, winds, and thermocline play an important role in the

development of SST anomalies associated with the IOD [17].

The SST ranged from 30.8-33.8oC, the SST enhanced ± 3

oC for 5 years, it is clear that

those conditions trigger the bleaching regionally, especially in the west region of Sumatera.

According to [17]. During IOD events, biological productivity of the eastern Indian Ocean

increases and this in turn leads to the death of corals over a large area.

Figure 3 Mothly temperature fluctuation during last 5 years



Figure 4 Annual surface temperature distribution around Pagai Strait

The rising of surface temperature threatened the Pagai Strait (Figure 4), during 3 years

(2011-2013) the temperature enhances insignificantly almost 1oC, Figure 4 shows that the

temperature is higher around the Siruso Island than in the Sijaojao Atoll, and the surface

temperature degraded along the strait, the region which bordered directly by the Indian Ocean

is warmer, according to [5] the main cause of the surface warming is a decrease in the

upwelling-related oceanic cooling, it occurs in the thermocline dome region because of a

slowdown of the wind-driven Ekman pumping.

The negative and positive events of IOD are commensurable during those years (Figure

3), this condition may give an influence on the bleaching events due to the sensitiveness of

the corals with the enhancing temperature. According to [8] coral reef responses to rapid

climate change: physiological responses to interacting factors, such as temperature,

acidification, and nutrients, meanwhile the coast and constraints imposed by acclimation and

adaptation.

In 2014, the average temperature distribution is decreased slightly -0.15oC and followed

by the average in the next year (2015), the average of surface temperature becomes lower than

before (Figure 4 and 5). Those condition are affected by the positive dipole mode domination

along those years, but the condition changes significantly in 2016 which the average

temperature almost reaching 33oC. These conditions resulting in the changes of water quality,

inhibits the biogeochemical processes and the most pernicious impact, it affects the bleaching

event to be enhanced. In addition, according to [12] the rising temperatures, ocean

acidification, and ocean de-oxygenation will cause substantial changes in the physical,

chemical and biological environment, which will then affect the ocean’s biogeochemical

cycles and ecosystems.




Figure 5 Annual temperature trend around Pagai Strait during last 5 years

3.3. Corals Existing Condition and Bleaching Identification

The higher temperature average in the Pagai Strait was directly resulting the high rate of coral

mortality in the station St1, St2, and St3 which were located around Siruso Island. The

percentage of living corals and coral demise are equal in those stations (Table 3) and the

mortality index shows that this area is threatened enough by the increasing of temperature,

with the category of moderate to high mortality.

As the region with a favorable location, Siruso Island got an advantage due to the water

mass distribution with high rate of nutrient transport around it which is very supporting the

increase of net primary production, but this position has a bad impact due to anomalies of

temperature which circulated from the Indian Ocean and effect to the imbalance environment.

Table 3 Life form composition and mortality index in Pagai Strait

Station

Living

corals

Dead

corals

Another

biota Algae

Abiotic

component Mortality

Index (MI) %

St1 33.3 33.3 3.2 0 30.2 0.50

St2 30.5 22.4 1.2 0 45.9 0.42

St3 27.6 27.5 1.2 0 43.7 0.71

St4 23.4 29.9 0.0 0 46.7 0.56

St5 21.4 11.9 5.0 0 61.7 0.35

St6 32.0 24.7 2.8 0 40.5 0.43

St7 25.4 63.0 6.1 0 5.5 0.71

St8 21.0 71.7 1.0 0 6.3 0.77

St9 14.1 62.4 0.0 0 23.5 0.81

Source: Mutmainah and Clara, 2016 [6]



Table 4 Coral bleaching documentation sample in each observation station

Station Documentation Station Documentation Station Documentation

St1

Acropora Cytherea

St4

Acropora

Solitaryensis

St7

Fungia Scutaria

St2

Acropora Aculeus

St5

Acropora Nasuta

St8

Stylopora Pistillata

St3

Acropora

Abrotanoides

St6

Acropora

Valenciennesi

St9

Montipora Digitata

Source: Mutmainah and Clara, 2016 [6]

In the stations of St4, St5, and St6 the mortality index is categorized to moderate-high, but

the level of mortality is lower than Siruso Island stations. The location of St4, St5, and St6 is

called Tunang Bulag by the local, which is located between North Pagai Island and Siruso

Island. The position of Tunang Bulag is very proportional due to the high mass transport

circulation so that this area is more fertile and safe from temperature anomaly threats.

The last region is Sijaojao atoll which represented by the station St7, St8, and St9. The

coral cover is very poor and dominated by the dead corals, the mortality index is also high -

very high, this condition is very alarming due to the high mortality level of this region.

Sijaojao atoll is located near the local settlement called Sikakap Sub-district, which is the

main region with a high rate of activities. According to [1], multiple activities affect the

marine environment in concert, the cumulative impacts need to be managed to reduce the

long-term effect.

Sijaojao atoll is the most pernicious area and needs to be conducted a rehabilitation of

corals and marine biota, that endanger condition is caused by not only the temperature

anomalies but also the ocean acidification and the local contamination of disposal waste

which might be influencing. In addition, according to [12] ocean warming will not only affect

organisms and biogeochemical cycles directly but will also increase upper ocean

stratification. The changes in the ocean’s carbonate chemistry induced by the uptake of

anthropogenic carbon dioxide (CO2) (i.e. ocean acidification) will probably affect many

organisms and processes.

The sample of bleaching and demised corals is shown in Table 4. In every single station,

there are many kinds of coral which bleached, this condition inhibits the marine ecosystem

and disrupt the water fertility. It indicates that the water condition is not supported the marine

life. According to [15] climate change also exacerbates local stresses from declining water




quality and ever-exploitation of key species, driving reefs increasingly toward the tipping

point for functional collapse.

4. CONCLUSION

The sea surface temperature (SST) trends during 2011-2016 were increased and reach ±3oC, it

caused by the negative IOD events domination which triggers the high-temperature anomaly

in the west coast of Sumatera. The enhancing temperature triggers the bleaching phenomenon

which is supported by the low coral cover and high mortality. This condition is pernicious and

necessary to be rehabilitated in the marine ecosystem, especially in the Sijaojao atoll.

This study only discusses the temperature anomaly impact to the coral bleaching

phenomenon, actually, the main factors that influence is not only the rising temperatures but

also the ocean acidification and the marine pollution. For the next research is suggested those

parameters to be analyzed. This study also gives the important information to the people

around the world to reduce the greenhouse effect and global warming to control the climate

change which is undoubtedly avoided.

ACKNOWLEDGEMENTS

Acknowledgments and gratitude are given to ECMWF due to its deserving the temperature

data, and to Herdiana Mutmainah and Rani Santa Clara for giving their result data of LIT

corals analysis, and to everyone who helped to finish this article.

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Documents

SEA SURFACE TEMPERATURE RISING TREND AND ITS … · Mentawai Islands are one of the outer regions of Indonesia, which is located in the western part of Sumatera and directly adjacent