The Diurnal Changes of Seawater pH and Alkalinity on the Coral Reefs of the Strait

of Malacca and the South China Sea

Zulfigar Yasin1,2, Nithiyaa Nilamani1, Norhanis Razalli1,

Zulfikar3 and Aileen Tan Shau Hwai1

1Marine Science Lab, School of Biological Sciences, Universiti Sains Malaysia,

11800 Minden, Penang, Malaysia.2Institute of Oceanography and Environment (INOS), Universiti Terengganu Malaysia, 21030 Kuala Terengganu,

Terengganu, Malaysia.3Universitas Malikussaleh, Lhokseumawe, Provinsi Acheh, Indonesia.

To investigate the diurnal variation of the seawater pH

and alkalinity in shallow water coral reef of Pulau Payar

(Strait of Malacca), Pulau Bidong and Pulau Gaya

(South China Sea)

N

400 km

Pulau Gaya

STUDY SITE

Sumatra

Pulau Payar

TarutaoNational Park

SOUTH CHINA SEA

Pulau Bidong

3 day period of observation at a shallow water coral reef area

In-situ measurement & water sample collection

Every 3 hours, water sample were collected and measured for the

Total Alkalinity (AT)

Hydrolab DS 5X recorded reading every 15 minute

(pH, Temperature, Salinity, Dissolved Oxygen, DO)

Aragonite saturation were determined using the CO2SYS program (Lewis and Wallace, 1998) through in situ AT, pH,

salinity, temperature data.

Parameters Unit Measurement Methods

Temperature °C Hydrolab DS 5X

Salinity ppt Hydrolab DS 5X

pH Hydrolab DS 5X

Dissolved Oxygen (DO) mg/L Hydrolab DS 5X

Total Alkalinity µmol/kg Potentiometric Titration (Dickson et al., 2007)

Aragonite Saturation ΩAragonite CO2SYS program (Lewis and Wallace, 1998)

• Decreasing surface

seawater pH trend from

the Strait of Malacca to

the South China Sea

more basic

less basic

day day daynight night night

• pH trend fluctuates daily following the diurnal pattern which is regulated by

the biological activity such as photosynthesis and respiration

Payar (SOM)

Bidong (SCS)

Gaya (SCS)

day day daynight night night

Payar (SOM)

Bidong (SCS)

Gaya (SCS)

• Total alkalinity (AT) did not show any diurnal trend through out this study.

• However, the largest range of AT was recorded at Payar with the value of 28.92 μmol/kg.

• Changes in AT can be attributed to calcification or carbonate dissolution (Murillo et al., 2014)

Parameter Symbol Unit Pulau Payar Pulau Bidong Pulau Gaya

Temperature T °C 29.96 ± 0.13 30.05 ± 0.30 30.70 ± 0.52

Salinity S ppt 32.02 ± 0.14 32.60 ± 0.07 32.46 ± 0.08

Total Alkalinity AT μmol kg-1 2140.77 ± 6.29 2161.78 ± 3.08 2160.45 ± 3.41

pH pHT 8.14 ± 0.02 8.01 ± 0.02 7.92 ± 0.01

ρCO2 in seawater ρCO2 μatm 288.59 ± 13.56 420.11 ± 22.86 531.47 ± 17.00

Carbonic acid H2CO3 μmol kg-1 7.36 ± 0.36 10.66 ± 0.63 13.30 ± 0.51

Bicarbonate ion HCO3- μmol kg-1 1523.50 ± 18.55 1654.45 ± 18.86 1718.85 ± 13.03

Carbonate ion CO32- μmol kg-1 250.49 ± 6.65 206.22 ± 8.25 179.82 ± 5.48

Hydrogen ion H+ μmol kg-1 7.32 x 10-3 ± 0.00 9.83 x 10-3 ± 0.00 1.19 x 10-2 ± 0.00

Calcite saturation Ωcalc 6.25 ± 0.17 5.12 ± 0.21 4.48 ± 0.14

Aragonite saturation Ωarag 4.16 ± 0.11 3.41 ± 0.14 2.99 ± 0.10

Parameter Symbol Unit Pulau Payar Pulau Bidong Pulau Gaya

Temperature T °C 29.96 ± 0.13 30.05 ± 0.30 30.70 ± 0.52

Salinity S ppt 32.02 ± 0.14 32.60 ± 0.07 32.46 ± 0.08

Total Alkalinity AT μmol kg-1 2140.77 ± 6.29 2161.78 ± 3.08 2160.45 ± 3.41

pH pHT 8.14 ± 0.02 8.01 ± 0.02 7.92 ± 0.01

ρCO2 in seawater ρCO2 μatm 288.59 ± 13.56 420.11 ± 22.86 531.47 ± 17.00

Carbonic acid H2CO3 μmol kg-1 7.36 ± 0.36 10.66 ± 0.63 13.30 ± 0.51

Bicarbonate ion HCO3- μmol kg-1 1523.50 ± 18.55 1654.45 ± 18.86 1718.85 ± 13.03

Carbonate ion CO32- μmol kg-1 250.49 ± 6.65 206.22 ± 8.25 179.82 ± 5.48

Hydrogen ion H+ μmol kg-1 7.32 x 10-3 ± 0.00 9.83 x 10-3 ± 0.00 1.19 x 10-2 ± 0.00

Calcite saturation Ωcalc 6.25 ± 0.17 5.12 ± 0.21 4.48 ± 0.14

Aragonite saturation Ωarag 4.16 ± 0.11 3.41 ± 0.14 2.99 ± 0.10

Lower salinity and total

alkalinity but higher pH

Lower carbonate ion

concentration

Lower pH value and higher

total alkalinity than Payar

More carbonate ion

available in the seawater

GBPG –GayaB – BidongP – Payar

Pulau Bidong and Pulau Gaya (South China Sea)

Higher AT were recorded at these sites, 2161.78 μmol kg-1 and 2160.45 μmol kg-1

respectively.

The pH at both sites in the South China Sea are lower than Paya (St of Malacca) as

denoted by the higher H+ concentration in the seawater.

Aragonite saturation in the seawater at both sites were recorded at adequate (3.41)

and marginal (2.99) level respectively.

Pulau Payar (Strait of Malacca)

Lower AT (2140.77 μmol kg-1) means the capacity of the seawater buffer system to

neutralize acidic pollution is reduced.

At present, the seawater carbonate system is maintained because of the pH value

(pH 8.14).

Only a small amount of acidic pollution is needed to disrupt the carbonate system

equilibrium.

Optimal aragonite saturation state in Payar contributes to the high coral calcification

rate in the area.

Parameter Symbol Unit Pulau Payar Pulau Bidong Pulau Gaya

Temperature T °C 29.96 ± 0.13 30.05 ± 0.30 30.70 ± 0.52

Total Alkalinity AT μmol kg-1 2140.77 ± 6.29 2161.78 ± 3.08 2160.45 ± 3.41

pH pHT 8.14 ± 0.02 8.01 ± 0.02 7.92 ± 0.01

ρCO2 in seawater ρCO2 μatm 288.59 ± 13.56 420.11 ± 22.86 531.47 ± 17.00

Bicarbonate ion HCO3- μmol kg-1 1523.50 ± 18.55 1654.45 ± 18.86 1718.85 ± 13.03

Carbonate ion CO32- μmol kg-1 250.49 ± 6.65 206.22 ± 8.25 179.82 ± 5.48

Calcite saturation Ωcalc 6.25 ± 0.17 5.12 ± 0.21 4.48 ± 0.14

Aragonite saturation Ωarag

4.16 ± 0.11

(Optimal)

3.41 ± 0.14

(Adequate)

2.99 ± 0.10

(Marginal)

Strait of MalaccaSW of South

China Sea

SE of South

China Sea

Calcification Rate of Porites sp.

(g cm-2 yr-1)

2.1 (Payar)

Tanzil et al., 2013

2.1 (Redang)

Tanzil et al., 2013

0.7 (Spratly

Islands)

Ruixia et al., 2012

Source: Reefs at Risk Revisited (2011)

Studies on calcification rates and reef integrity in the region taking

into consideration local stressors

Develop and agree on a common methodology for the monitoring

of ocean acidification on coral reefs

Increase the number and spread of long term monitoring sites and

separation between OA and other anthropogenic effects on reefs

Develop baseline carbonate chemistry information and data sharing

for the Southeast Asia region and selected Class III sites.

Consider the depth of certain reefs and the effect OA on these and

associated biota

Integrate the findings of this group with the other physical and

chemical approaches of ocean acificication studies and beyond.

Relate to socio-economics of reef use and especially to fisheries.

Studies on calcification rates and reef integrity in the region

Increase the number and spread of long term monitoring sites

and separation between OA and other anthropogenic effects

on reefs

Develop and agree on a common methodology for the

monitoring of ocean acidification on coral reefs

Develop baseline carbonate chemistry information and data

sharing for the Southeast Asia region and selected Class III

sites.

Consider the depth of certain reefs and the effect OA on

these and associated biota

Integrate the findings of this group with the other physical and

chemical approaches of ocean acificication studies and

beyond. Relate to socio-economics of reef use and especially

to fisheries.

Pulau Payar

La

titu

de

Longitude

D

C

B

A

Genera St.A

St. B

St. C

St. D

Caulastrea

Montastrea

Acropora

Pectinia

Symphyllia

Diploastrea

Favia

Favites

Goniastrea

Porites

Total 4 8 7 7

zulfigar@umt.edu.myThank you for your kind attention

Template of study is adopted for reef monitoring studies by the Malaysia’s Marine Park in collaboration with Universiti Malaysia Terengganu and Universiti Sains Malaysia.

Current location of monitoring stations are in the:• Straits of Malacca (Sabang and Langkawi)• Eastern portion of the South China Sea (Bidong, Terengganu)• Western portion of the South China Sea (Pulau Gaya, Kota Kinabalu

We wish to thank the following institutions for their generous suport:

Ministry of Science and Technology of Malaysia (MOSTI)Ministry of Natural Resources and Environment (NRE)The Marine Park Department of MalaysiaUniversiti Sains Malaysia (USM)Universiti Malaysia Terengganu (UMT)