Trace Metal Dynamics in Mangrove Sediments Within Temperate Estuaries
Mangrove sediments have a high capacity to retain organic matter because of the low mineralization processes induced by the waterlogged conditions around them. The accumulation of organic matter and deposition of fine particles within mangrove forests are favorable for anaerobic metabolic processes that produce inorganic sulphur, which has a high capacity to sequester trace metals. This means that mangrove sediments have the ability to protect surrounding environments from trace metal contamination. However, changes in redox conditions in mangrove sediments as a result of natural or anthropogenic activities may result in trace metals being released to overlying waters. The released metals from mangrove sediments are a serious environmental threat worldwide because of their persistent nature, and they tend to accumulate in estuarine food chains. Such complex dynamics of metal cycling within temperate mangroves and their effects on surrounding ecosystems require further investigation. Thus, a comprehensive field-based study on the factors influencing the redox conditions of mangrove sediments is needed to understand trace metal dynamics in these habitats, especially in temperate areas, such as New Zealand. Hence, this study aims to enhance our knowledge on the process involved in metal cycling induced by natural and anthropogenic factors in temperate mangrove sediments, which could be a useful benchmark for estuarine management in temperate regions, including New Zealand. Such estuarine management approach should consider several physical and ecological factors because the distribution of trace metals in mangrove sediments may exclusively vary on a site-by-site basis. Hence, in this study, four different mangrove stands at Mangawhai Harbour and two stands at Manukau Harbour, both located in northern New Zealand, were selected as study locations, which represent rural and urban temperate mangrove stands, respectively. The study aims to understand trace metal distributions up to 30 cm sediment depth, representing the majority of mangrove live root zone which may influence metal distribution patterns. Overall, the results indicate that organic matter highly controls trace metal cycling in mangrove sediments and the variation of organic matter within the system is influenced by several natural and anthropogenic factors. Especially, local weather pattern, fluctuations in anthropogenic inputs, and hydrographic condition are major factors controlling organic matter and trace metal distributions in these mangrove sediments. The results also revealed significant variations in trace metal levels between mangrove sediments and adjacent mudflats, which indicate that mangrove sediments have the inherent sequestering capacity. However, the results also indicate that there are possible impacts of trace metal contamination to surrounding environments during pre- and post-mangrove removal assessment. Hence, the study utilizes a unique and rare opportunity especially during the mangrove removal period to provide field-based scientific information that physical disturbances in the mangrove ecosystem could change the habitat from a sink to a source of trace metals. In summary, the findings of the study show that mangrove sediments could act as both natural filters and secondary sources of trace metals depending on several factors influencing sediment redox conditions. Thus, mangrove ecosystems may be important agents to minimize trace metal contamination in estuarine and coastal ecosystems worldwide. Overall, the outcomes of the study provide essential scientific information on trace metal cycling in mangrove sediments, which is likely to assist with estuarine management and conservation strategies in New Zealand and worldwide. Also, the study highlights the necessity for periodic scientific assessments on trace metals in mangrove ecosystems, especially before making management decisions involving their removal in New Zealand and worldwide.