The effects of iron ore dust on mangroves in Western Australia: Lack of evidence for stomatal damage

Anecdotal evidence suggests that iron ore dust derived from industrial shiploading activities in north-western Australia may be more injurious to mangroves than is naturally-derived dust, because of its more angular structure and presumed ability to damage stomatal cells. Abaxial hairs on the most common mangrove, Avicennia marina (Forfk). Vierh., have been thought to exacerbate this effect through trapping and retaining dust. This study examined this hypothesis. Leaves were collected from dusty industrial areas and natural environments. Leaves in dusty environments were chosen on the basis of their thick coating of iron ore dust on both leaf surfaces. Approximately 3,000 stomata were examined in detail using light microscopy. Hair density, stomatal aperture, cell condition and presence of dust were also noted. Despite there often being a visible layer of dust on the abaxial and adaxial surfaces of the leaf, evidence for dust of any kind within stomatal spaces was noted, in total, only three times. The lack of visible dust in stomata was attributed to three factors; the density and morphology of the abaxial hairs, which prevent dust from entering the space between the hairs, and the improbability of dust circulating in the stagnant air within that space and moving against gravity past the guard cells. It was concluded that if iron ore dust affects mangroves, it must do so by some other mechanism, such as either increased temperature, shading or a restriction of transpiration by the thickness of the dust on the abaxial surface.