Temporal variations of trace metals and a metalloid in temperate estuarine mangrove sediments.

Mangrove sediments are strong modulators of organic matter (OM) content and pollutant dynamics, acting both as sinks and sources of these components. This study aimed to assess temporal dynamics of OM within temperate mangrove sediments and their ability to sequester pollutants. Specifically, levels of trace metals (Fe, Cu, Zn, Pb, Cd) and a metalloid (As) were examined within mangrove and mudflat sediments located in a high-energy environment in Mangawhai Harbour Estuary, northern New Zealand. Sediment cores were collected from a mangrove stand and adjacent mudflats at three sediment depths during different months over a year. Variations in OM and elements were compared to rainfall and temperature patterns observed during the sampling period. All element concentrations, except for those of As, were significantly higher in mangrove compared to mudflat sediments during the entire sampling period. This is consistent with the well-reported ability of mangroves to trap suspended particles and OM. In addition, we observed a decreasing trend in trace metal concentrations with increasing sediment depth within mangrove habitat, which correlated well with decreasing OM content. Our results also suggested that most elements had different, but significant, temporal variations throughout the year, especially in mangrove sediments. Overall, the concentrations of Cu, Zn, Pb, Cd, and As in mangrove sediments increased during summer, whereas maximum levels of Fe and OM were observed in winter. This temporal pattern was determined to be related to OM and redox cycling as a result of changes in effluent input rates and physical/chemical environments during different seasons.

Spatial variation of heavy metals in sediments within a temperate mangrove ecosystem in northern New Zealand.

Spatial dynamics of heavy metals (Cd, Cu, Fe, Pb, and Zn) and metalloid As were investigated along the sedimentary column, inside and outside mangrove stands, and within the upper, middle, and lower Mangawhai Estuary, New Zealand. Organic matter contents (1.89 to 17.15%) were 10 times higher inside versus outside mangroves, and decreased sharply with depth to 30 cm. Cu, Fe, and Zn were highest in the upper estuary and Pb was highest close to a road (lower estuary), reflecting road run-off. A mangrove island (lower estuary; high energy) had lowest heavy metal and highest As concentrations (resulting from high absorption of As by the carbonate rich sediments). Most of the analysed sediment parameters in our study were higher inside versus outside mangroves, suggesting the accumulation of organic matter and build up capacity to buffer pollutants. Our results show that mangrove stands tend to accumulate heavy metals and may act as natural filters of pollution of coastal waterways.