Potential biological indicators of heavy metal stress in mangrove ecosystems

Abstract

The effects of heavy metals Cu, Pb, and Zn on a number of biological structural components of Avicennia dominated mangrove ecosystems is reviews and their potential as biological indicators of metal stress discussed. Three main taxa groups were chosen for examination, representing key structural components of Avicennia systems at different trophic levels. As the primary producer, Avicennia marina (Forsk.) Vierh, the grey mangrove, was studied most intensely. Heloecious cordiformis (Milne Edwards, 1852), the semaphore crab, was chosen as a higher level consumer. A suite of benthic organisms were also studied, representing a range of trophic associations at the community level. Potential biological indicators of metal stress were investigated in each group, at various organisational levels. A combination of laboratory and field based experiments have shown that most biological indicators investigated have merit in terms of reflecting exposure and effect of heavy metal stress in Avicennia systems. Avciennia marina was found to be highly tolerant to Cu, Pb and Zn. Both field and laboratory studies revealed the three metals were accumulated in root tissue in proportion to sediment metal loadings, and Cu and Zn showed some limited mobility to leaf tissue. Zn was the only metal in leaf tissue. Zn was the correlated with sediment loadings. All metals in root tissue and Zn in leaves were appropriate for accumulative indication. In both field and laboratory studies, increases in peroxidase activity and decreases in photopigments (most notably the chlorophyll a/b ratio) were found with increasing Cu and Zn. Significant relationships with peroxide ase activity only, were found for Pb. Both peroxidise activity and photopigment may be employed as sensitive early warning biomarkers of metal stress in A. Marina. For H. Cordiformis, Cu and Zn were regulated in the hepatopancreas. Lead was accumulated in the hepatopancreas in proportion to sediment Pb levels, and Pb was the main metal loadings tended to support lower numbers of crabs, greater proportions of females with purple chelae (rather than orange) and less females in the population, and thus may be employed as population level indicators of metal stress. Monitoring of macrobenthic community assemblage diversity and abundance along with natural variables and low-level anthropogenic contaminnts including Cu, Pb, Zn, revealed no significant pollutant related patterns. Natural physiochemical sediment differences were found to be the main determinants in species assemblage patterns among sites. An integrated approach of both field and laboratory based testing, with a range of species measured at various organisational hierarchies, is most instructive in the assessment of metal impacts on mangrove ecosystems.