Mangrove removal: Effects on trace metal concentrations in temperate estuarine sediments

Abstract Trace metal accumulation in mangrove sediments is of increasing global concern, as contaminants can easily remobilise to surrounding environments through a range of sediment geochemistry processes. Such processes are driven by various physical or biological disturbances, which tend to remobilise metals profligately. The present study aimed to investigate the effects of mangrove removal activities on trace metal concentrations in temperate mangrove sediments and adjacent mudflats. To understand patterns of trace metal mobilization due to the removal of mangroves, we measured levels of Fe, Cu, Zn, Pb, and Cd along with other sediment parameters (organic matter, pH, bulk density) during pre- and post-mangrove removal activities. Sediment cores from three different depths down to 30 cm were collected from the mangrove removal location and adjacent mudflats at Mangawhai Harbour Estuary, New Zealand, during pre/post-removal sampling events over a four-year period. Results indicate that levels of OM and trace metals were higher in mangrove sediments than adjacent mudflats, which is consistent with the inherent capacity of mangrove sediments to sequester trace metals. However, after the removal of mangroves, the levels OM and trace metals decreased in removal sites. During this period, we observed a decrease in OM and lower sediment pH levels, indicating an increase in sediment acidity which is associated with a lower sediment sequestering capacity. We also observed increases in trace metal concentrations in adjacent mudflats during the post-removal period, thus highlighting the far-reaching effects of mangrove removal activities. These findings provide important information to assist with future management strategies of temperate mangrove ecosystems.