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Coral skeletons as a recorder of metal pollution: Environmental monitoring in the gulf of Thailand
Author(s)
Date Issued
January 1, 2016
Type
Book Chapter
Abstract
Coral skeletons provide useful information on aquatic environments in which corals grew, and they also offer to use as recorders of pollution history from urbanized hinterlands to coral reefs. In order to monitor pollutant discharges from urban areas to the Gulf of Thailand since the 1980s, we collected Porites corals from Khang Khao Island about 50 km southeast of Bangkok in 1985, 1998, 2001 and 2008. The coral collection periods since the 1980s coincided with a series of laws enacted by the Thai government to curb environmental pollution. To determine the skeletal growth of the samples, oxygen isotopes (18O/16O as δ18O) in coral aragonite was measured by stable isotope mass spectrometry. A cyclical change in d18O is observed to record an annual change in seawater salinity, and then the coral growth rate is estimated at ~18 mm/year on average. Using the skeletal δ18O method, the coral chronology was established in the Gulf of Thailand. Next we used a recently developed laser ablation inductively coupled plasma mass spectrometry (LA-ICPMS) method to assess the impact of metal pollution on coral skeletons taken from the Gulf of Thailand since the 1980s. The extent of anthropogenic contribution by riverine input to the gulf, including aerosol deposits, was assessed by comparing metal-calcium (Me/Ca) ratios of Khang Khao corals to those of Rukan-sho, a relatively unpolluted coral reef, Okinawa. In this comparison, high riverine inputs of Ba, V, Cd and Pb were observed from the Me/Ca values in the Thai coral samples. Since Ba concentration in seawater around Khang Khao Island largely depends on input from the rivers, especially the Chao Phraya River, the Ba/Ca ratios in Khang Khao corals reflect by the high concentrations of riverine input, showing cyclical variations like those of oxygen isotopes in the coral samples. The V/Ca ratios of Khang Khao corals showed a higher average value than that of the Rukan-sho coral, suggesting anthropogenic vanadium inputs due to fuel oil pollution in the Gulf since the late 1990s. Higher Cd/Ca ratios were observed in Khang Khao corals compared to that of Rukan-sho coral, indicating that the Cd concentration in the Gulf continuously suffered from anthropogenic input since 1983. The levels of Cd in the coral indicate a gradual decrease in the Gulf in the late 1990s, with a drastic drop in concentration from the 1980s. The historical variation in Pb/Ca ratios recorded in the coral skeletons suggests the Gulf of Thailand suffered from anthropogenic lead from 1985 to 2001. The Pb/Ca values recorded in Khang Khao Island corals suggest that the Gulf has been polluted by anthropogenic Pb from the early 1990s. After the use of leaded gasoline was banned in Thailand since 1995, the Pb/Ca in the Khang Khao Island corals showed a remarkable decrease, indicating that regulatory control has limited anthropogenic Pb inputs to the Gulf of Thailand. In conclusion, the coral archival record of the metals (V, Cd, Hg and Pb) strongly suggests the success of the environmental laws and regulations by the Thai Government introduced since the middle 1990s.
Citation
Coral Reefs Ecosystems Environmental Impact and Current Threats, 121-153, 2016