Mangrove soils are considered as sinks for trace metals, protecting coastal waters from pollutions. However, the cycling of trace metals in mangroves is complex due to various biogeochemical processes across the intertidal zone, notably the dissolution of bearing phases resulting in high trace metal concentrations in porewaters. Previous studies demonstrated a decrease of trace metal stocks in mangrove soils seaward, possibly due to the export of dissolved metals through tidal pumping. Can Gio mangrove is the largest one in Vietnam, developing downstream Ho Chi Minh City (Viet Nam’s biggest industrial city). The objectives of the present study were to characterize the dynamics of trace metals in a tidal creek of the Can Gio mangrove that does not receive any upstream inputs and to identify the role of porewater seepage on their dynamics. To reach our goals, surface water and suspended particulate matters were collected every 2 h during two different tidal cycles (spring and neap tides) and at the two different seasons, dry and wet. Mangroves porewaters were also collected. In addition to particulate and dissolved trace metals, physico-chemical parameters and a groundwater tracer (Radon – 222Rn) were measured. The results showed that trace metal concentrations at flood tides, both in the dissolved and the particulate phases, were in the same range that those measured in the Can Gio Estuary. Then during ebb tides, we evidenced high inputs of dissolved Fe, Mn, Co, and Ni from mangrove soils. However, the dynamics of these inputs differed depending on the element considered. Mn was exported from the tidal creek in its dissolved form. However concerning Fe, and to a lesser extent Co and Ni, we suggest that, when delivered to the creek from the soils under dissolved forms, these trace metals precipitated because of different physicochemical characteristics between mangrove soils and tidal creek, notably higher dissolved oxygen concentrations and higher pH. Consequently, these elements were exported to the estuary in particulate forms. We suggest that budget studies of trace metals in mangroves should be developed like the ones concerning carbon to efficiently determine their role as a barrier for pollutants between land and sea.

中文翻译：

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热带红树林潮汐溪中的微量金属动力学：孔隙水渗流的影响（Can Gio，越南）

红树林土壤被认为是痕量金属的汇，保护沿海水域免受污染。然而，由于潮间带的各种生物地球化学过程，红树林中痕量金属的循环是复杂的，特别是轴承相的溶解导致孔隙水中痕量金属浓度高。先前的研究表明，向海的红树林土壤中的微量金属存量有所减少，这可能是由于潮汐泵送溶解金属的输出。Can Gio 红树林是越南最大的红树林，发展下游胡志明市（越南最大的工业城市）。本研究的目的是表征没有接受任何上游输入的 Can Gio 红树林潮汐小溪中痕量金属的动态，并确定孔隙水渗漏对其动态的作用。为了实现我们的目标，在两个不同的潮汐周期（春季和小潮）和两个不同的季节，干湿季节，每 2 小时收集一次地表水和悬浮颗粒物。还收集了红树林孔隙水。除了颗粒物和溶解的微量金属外，还测量了物理化学参数和地下水示踪剂 (Radon – 222Rn)。结果表明，潮汐中的微量金属浓度，无论是溶解相还是颗粒相，都与 Can Gio 河口测量的浓度范围相同。然后在退潮期间，我们证明了红树林土壤中溶解的铁、锰、钴和镍的高输入。然而，这些输入的动态因所考虑的元素而异。Mn 以其溶解形式从潮汐小溪中输出。然而关于 Fe，在较小程度上 Co 和 Ni，我们建议，当以溶解形式从土壤输送到小溪时，这些微量金属沉淀，因为红树林土壤和潮汐小溪之间的物理化学特征不同，特别是较高的溶解氧浓度和较高的 pH 值。因此，这些元素以颗粒形式出口到河口。我们建议对红树林中的微量金属进行预算研究，就像研究碳一样，以有效地确定它们作为陆地和海洋之间污染物屏障的作用。因此，这些元素以颗粒形式出口到河口。我们建议对红树林中的微量金属进行预算研究，就像研究碳一样，以有效地确定它们作为陆地和海洋之间污染物屏障的作用。因此，这些元素以颗粒形式出口到河口。我们建议对红树林中的微量金属进行预算研究，就像研究碳一样，以有效地确定它们作为陆地和海洋之间污染物屏障的作用。