Using macrophytes to improve the understanding and management of metal contaminated estuaries

Estuarine and coastal environments support a broad range of anthropogenic activities and many of these can have adverse interactions. Metal pollution is a particular concern because metals persist in the environment and can have a toxic effect on living organisms. Macrophyte species have been shown to be effective for monitoring and removing pollutants such as inorganic compounds and nitrogen, but their potential for removing heavy metals is still poorly understand. Understanding the ecophysiological responses of macrophyte species, in relation to metal pollution, is fundamental to developing effective monitoring and management strategies, and also to identifying potential bioremediation applications. This investigation focused on ascertaining which macrophyte species would be the best candidates for i) biomonitoring and ii) bioremediation potential of metal pollution in a heavily metal polluted estuarine/ coastal environment, the Derwent Estuary, in Hobart, Tasmania, Australia. The potential use of local macrophyte species as biomonitors of metal pollution was assessed firstly through a broad-scale survey looking at macrophyte distribution along a contamination gradient in the Derwent. Twelve species were evaluated including three species of seagrasses and nine macroalga. The macroalga Ulva australis appeared to be the best candidate for a local bioindicator, because of its widespread distribution and capacity to accumulate high metal content (Arsenic (As), Copper (Cu), Lead (Pb) and Zinc (Zn)) relative to the ambient concentrations in the Derwent Estuary. The next stage of this study reviewed the distribution of U. australis throughout the estuary over 3 years (2013-2015) to determine how reliable it might be as a biomonitor over time. The relative concentrations of As, Cu, Pb and Zn were evaluated, with only Pb and Zn showing a clear spatial trends of accumulation in the tissue. This strong spatial gradient in metal content in U. australis appears to be most strongly related to the metal concentrations in surface water. Zinc was by far the most significant metal contaminant, and showed clear seasonal differences in levels being greater in spring and summer compared with other seasons. As a result, the next level of investigation, looked at metal uptake mechanisms and the potential to use U. australis for remediation focused on Zn. In order to use macrophytes most effectively for biomonitoring or bioremediation it is important to understand how plants might respond under different levels of contamination; it is especially important to understand at what concentrations metal contamination might actually limit physiological function and/ or uptake response. However, in order to study the effect of heavy metals on the ultrastructure of U. australis, a new fixation protocol had to be developed to ensure the quality of specimen preservation for analysis using transmission electron microscopy (TEM). Five protocols were compared, and the most successful (2.5% glutaraldehyde, 0.05 M sodium cacodylate buffer and 2% paraformaldehyde) was applied in subsequent analyses. The physiological and cytochemical response of U. australis was assessed against three Zn concentrations (25 μg·L\(^{-1}\), 50 μg·L\(^{-1}\) and 100 μg·L\(^{-1}\) for seven days. There were marked utrastructural changes in U. australis associated with increasing Zn concentrations such as the incorporation of electron-dense bodies, cytoplasm retraction and compression of cellulose fibrils. However, the photosynthetic performance (Fv/Fm and ETRmax) and photosynthetic pigments (Chl a, Chl b, and carotenoids) were not affected by Zn concentrations. This suggests that despite the substantial cellular changes, the plant physiology was not adversely affected, indicating that U. australis would appear to be adaptable to high levels of Zn and as a result has great potential to be used as a bioindicator of metal pollution. The assessment of U. australis as a potential biomonitoring tool and for removing metals was further evaluated via in situ transplantation. Ultrastructural changes, such as the incorporation of electron-dense bodies in the cell wall and vacuoles, indicated metal accumulation at highly polluted sites. However, as in the laboratory experiment (Chapter 5), after 12 days of field deployment neither photosynthetic performance (Fv/Fm and ETRmax) nor photosynthetic pigments (Chl a and Chl b) were affected. Longer deployments (45 days), confirmed the metal bioaccumulation capacity of U. australis with further evidence of increases in metal content in thalli, which again suggests that this species could be a useful bioremediation tool. In conclusion, this study suggests that U. australis could be a valuable tool for both monitoring and management of heavy metal contamination in Australian estuaries and coast. This study also provides a new insight into the ecophysiological response of U. australis to metal contamination (both in vitro and in situ), particularly zinc, and describes a new and improved technique for fixation that will lead to better definition of cellular ultrastructure in this species. It is hoped that this information will be used to support and enhance existing management and monitoring strategies in metal contaminated environments