The use of Sentinel Organisms to evaluate the health of metal contaminated forest ecosystems in the Western Cape, South Africa

Abstract

Chronic exposure to high levels of metals in the environment can cause severe damage to ecosystems and human health. The City of Cape Town, an immense contributor to atmospheric pollution lies beneath the Table Mountain range, one of the Seven Wonders of the World and renowned for its biodiversity and ancient, indigenous forests, yet no plan exist to monitor metal concentrations. These ecosystems are subjected to considerable metal inputs all year round, which greatly increases during brown haze episodes in winter. Soil, leaf litter, moss, lichen and millipedes are key organisms in forests and reliable indicators for determining critical loads and preventing broader impacts to forests, which leads to the main purpose of this study: to determine the health of metal-contaminated forest ecosystems in the western cape, the smallest biome in South Africa, yet of utmost importance to our environment and ultimately human health. The objectives of this study were a) to determine concentrations of prominent metals in soil, leaf litter and sentinel organisms in a pilot study b) to compare the dry and wet season with regard to: (i) seasonal fluctuations of the concentrations of the metals (ii) the oxidative stress effect of metals, using two markers indicative of induced oxidative stress (oxidative lipid damage products and glutathione (total GSH levels) in the pill millipede, Spaerotherium compressum, the moss, Hypnum cupressiforme and the lichen, Parmotrema sp. and c) to expose millipedes to metal contaminated soil for a period of six weeks: (i) to determine whether they accumulated metals and (ii) to assess the induced oxidative damage to lipids and redox status of glutathione in the pill millipede, Spaerotherium compressum. Three sampling sites in three afromontane forests in the Western Cape, Platbos, Orange Kloof and Newlands forest formed the study area. Five subsamples of soil, leaf litter and each sentinel organism were collected and chemically analysed to determine the metal concentrations, using an Inductively Coupled Plasma Mass Spectrophotometer (ICP–MS). The oxidative stress effect was determined in freeze dried samples. Millipedes were exposed in terrariums to a cocktail of metals Al, Fe and Mn for six weeks at control, low and high concentrations and analysed thereafter. Significant (P<0.05) findings in this study were the metal contamination patterns observed at the sites and forests in closest proximity of the City of Cape Town and related pollutant sources, especially vehicle volumes, and traffic behaviour. Even more significant (P<0.05) was the enhanced metal concentrations found during winter in the forest in closest proximity of the city, impacted by the brown haze phenomena. Metals may further have triggered an overproduction of ROS (reactive oxygen species) judging from the activated antioxidant, tGSH levels in an effort to scavenge ROS, as well as the MDA (malondialdehyde) levels (measured as TBARS), which indicated damage to cells. An important finding was that MDA levels were mostly higher in winter during the brown haze episodes signifying more damage in the organisms during that season. A programme to monitor metals in these forests was also suggested in view of a concern for the survival of forests and human health, as a result of the growing population, vehicle traffic and urbanization.