Heavy metal contamination in Hong Kong mangroves : multiscale impacts on key ecosystem components

Abstract

Although the rate of mangrove destruction is slowing, the urbanisation and industrialisation of coastlines is causing unprecedented pollution in mangroves. Hong Kong’s mangroves are no exception, receiving inorganic and organic input from domestic, agricultural, and industrial activities. Pollution by heavy metals is a serious concern due to their toxicity, persistence, and bioaccumulation. Heavy metal contamination is commonly assessed in mangrove sediments, but current research fails to consider their bioaccumulation and affects in local flora and fauna. Mangrove plants display metal tolerance and perform phytoremediation in mangrove systems, however mangrove fauna are vulnerable to metal accumulation via direct uptake (e.g., gill surface) or dietary intake. Mangrove crabs are vital to the health and resilience of mangroves by performing key functional roles and metal toxicity experienced by these organisms may result in consequences for the wider ecosystem. Once accumulated, heavy metals may transfer throughout food webs and transcend ecosystems with unknown ramifications. By analysing the concentration of Al, As, Cd, Cr, Cu, Fe, Mn, Ni, Pb and Zn by Inductively Coupled Plasma Mass Spectrometry (ICP-MS) in mangrove sediments, flora, and fauna, I determined the spatial variation of heavy metal contamination and bioaccumulation in five mangrove stands across Hong Kong. Noting considerable levels of metal contamination at Mai Po Marshes Nature Reserve (MPMNR), I explored the physiological influence of heavy metals on keystone species by comparing the thermal tolerance of Parasesarma bidens exposed to varying levels of metal pollution in situ by analysing oxygen consumption and cardiac activity. The suppression of oxygen consumption and a reduced thermal optimum observed in P. bidens from MPMNR indicated narrowing and shifting thermal windows in crabs exposed to metal pollution, reducing their resilience to future climate change. In contrast, mangrove plants appeared to be tolerant of metal contamination with no spatial variation in bioaccumulation observed throughout sites. To investigate the tolerance of mangrove plants to metal pollution, I analysed the concentration of metals within different plant tissues of Kandelia obovata at MPMNR with results illustrating exclusion at the root level and reduced translocation to aerial plant tissues. Finally, I used heavy metal and stable isotope analysis to investigate the trophic transfer of metals within the brachyuran crab community at MPMNR and found the biomagnification of As and Cd in consumers. These results highlight the potential of heavy metals to bioaccumulate and biomagnify in food webs, which is particularly concerning for the management and conservation of protected areas such as MPMNR. My research indicates that heavy metals have the potential to influence mangroves at multiple scales of biological organisation: influencing the physiology of individuals, reducing the resilience of populations, and transferring through communities, all of which may influence the wider ecosystem. My findings highlight the importance of considering the impacts of metal pollution on mangrove systems which may be missed in current assessments of mangrove health, which focus on the loss and gain of forest area and consider quantity instead of quality.