The effect of Spartina alterniflora Loisel on trace metals accumulation in inter-tidal sediments
We present the distribution of Cd and Zn in sediment cores taken under Spartina alterniflora Loisel. marsh and compare with adjacent mud flats without plant cover. Vertical distribution of Cd and Zn in the mud flat and S. alterniflora cores were different. From 0 to 5 cm of depth Zn and Cd concentrations did not change with depth in the two cores but were higher in the mud flat. From 5 to 15 cm, corresponding to maximum root biomass under S. alterniflora, Zn and Cd concentrations remained constant in the mud flat cores, but increased by a factor of nearly 4 under S. alterniflora. Below this depth concentrations dropped in both cores. Manganese distribution in S. alterniflora cores suggests that oxides are precipitated at the depth of maximum root biomass. More oxic environment under S. alterniflora (average Eh = +5 mV) with low sulfide concentration (0.58 mg l-1) relative to the mud flat cores (average Eh = –204 mV; sulfide 47.1 mg l-1), may favor Fe and Mn oxi-hydroxides precipitation, causing an accumulation of Cd and Zn at the root zone. In the mud flat cores, Zn and Cd highest concentrations were at the sediment water interface, where oxic waters favor their precipitation with Fe and Mn oxi-hydroxides. The results discourages the indiscriminate use of salt marsh sediments in environmental monitoring of trace metals, since the possibility of post-deposition mobilization of trace metals due to root metabolism, will make correlation with trace metal loading rates impossible.
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