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 haveconfirmed that Iichens are among the most rei iable accumulators of airborne inorganic contaminants.As limited research has been done in tropics with regard to biological monitoring, a study was carriedout 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 thisstudy. Eight trees from three trees species, (Cocos nucifera, Mangifera indica, and Artocarpusheterophyllus) within each site (1 km ') were chosen for the study. Coverage and frequency ofcorticolous all lichens found on selected trees were recorded by using 250 em- grids. After studyingtheir morphology, anatomy, reproductive structures and chemistry, lichens collected were identifiedusing keys. In addition, land use pattern, traffic density, pH of substrates, exposure levels of thesesites to light were also recorded. Ambient SO, and NO, levels at each site were determined usingpassive samplers having filter pads coated with-the absorbing reagents, ethylene glycol and acetone,for SO, and Nal, NaOH and Ethylene glycol for NO,. Data were statistically analyzed by using themean comparison, correlation and by principal component analysis to investigate relationship betweendiversity of Iichens and environmental parameters.

Forty seven genera of lichens were identified, out of them ten genera are sensitive to air pollutants.The highest atmospheric SO, and NO, levels (48.35/lg/m1 and 42.825~lg/mJ) as well as the lowestlichen diversity (0.8374) were recorded from the site located in Colombo Fort. When land use patternchanged from village to urban and with increase of traffic density of sites, diversity oflichens reducedsignificantly. The relationship between diversity oflichens and levels of SO, and NO, was negativelycorrelated but significant only with levels of NO, Further, a significant negative correfation was foundbetween the pH of substrates and levels of SO, and NO,. Principal component analysis revealed thatprincipal component I(PC I), PC2 and PC3 explain 38%, 20% and 15% of the total variation in thedata set respectively. Biplot developed by PC I vs PC2 revealed that low lichen diversity class isclearly separated from other classes due to increased concentrations of SO" NO, land use patternand the traffic density, all of which are included in PC I. Bark pH values "of Cocos nucifera andArtocarpus heterophyllus that are represented by PC2 also had contributed to above grouping.Index of atmospheric purity (lAP) values increased along all transect with gradual decrease of SO,and N021eveis when moving away from the city.

Absence of several pollutant sensitive tropical lichens and decrease of lichen diversity in sites withhigh levels of SO, and NO" indicate that tropical lichens have the potential to be used as indicators inair pollution monitoring work.