Lichens as biomonitors of sulphur dioxide and nitrogen dioxide pollution in Colombo and suburbs

Abstract

Biological monitoring is an efficient and inexpensive method for monitoring air quality. Studies have confirmed that lichens are among the most reliable accumulators of airborne inorganic contaminants. As limited research has been done in tropics with regard to biological monitoring, a study was carried out to monitor lichen diversity and distribution in selected locations in Colombo and its suburbs.

Thirty one sites located on six radial transects diverging from Colombo Fort were selected for this study. Eight trees from three trees species, (Cocos nucifera, Mangifera indica, and Artocarpus heterophyllus) within each site (1km2) were chosen for the study. Coverage and frequency of corticolous all lichens found on selected trees were recorded by using 250 cm2 grids. After studying their morphology, anatomy, reproductive structures and chemistry, lichens collected were identified using keys. In addition, land use pattern, traffic density, pH of substrates, exposure levels of these sites to light were also recorded. Ambient SO2 and NO2 levels at each site were determined using passive samplers having filter pads coated with the absorbing reagents, ethylene glycol and acetone, for SO2 and NaI, NaOH and Ethylene glycol for NO2. Data were statistically analyzed by using the mean comparison, correlation and by principal component analysis to investigate relationship between diversity of lichens and environmental parameters.

Forty seven genera of lichens were identified, out of them ten genera are sensitive to air pollutants. The highest atmospheric SO2 and NO2 levels (48.35µg/m3 and 42.825µg/m3) as well as the lowest lichen diversity (0.8374) were recorded from the site located in Colombo Fort. When land use pattern changed from village to urban and with increase of traffic density of sites, diversity of lichens reduced significantly. The relationship between diversity of lichens and levels of SO2 and NO2 was negatively correlated but significant only with levels of NO2. Further, a significant negative correlation was found between the pH of substrates and levels of SO2 and NO2. Principal component analysis revealed that principal component 1(PC1), PC2 and PC3 explain 38%, 20% and 15% of the total variation in the data set respectively. Biplot developed by PC1 vs PC2 revealed that low lichen diversity class is clearly separated from other classes due to increased concentrations of SO2, NO2, land use pattern and the traffic density, all of which are included in PC1. Bark pH values of Cocos nucifera and Artocarpus heterophyllus that are represented by PC2 also had contributed to above grouping. Index of atmospheric purity (IAP) values increased along all transect with gradual decrease of SO2 and NO2 levels when moving away from the city.

Absence of several pollutant sensitive tropical lichens and decrease of lichen diversity in sites with high levels of SO2 and NO2, indicate that tropical lichens have the potential to be used as indicators in air pollution monitoring work.