Selenium, Chromium and Cobalt Diffusion into Mangrove Sediments: Radiotracer Experiment Evidence of Coupled Effects of Bioturbation and Rhizosphere

Microcosm experiments on the behaviour of Se, Cr and Co were carried out with mangrove sediments from Sepetiba Bay, Brazil. Three 8-cm length sediment cores were covered with tidal water spiked with 75 Se, 51 Cr and 60 Co to evaluate its behaviour within the sediments. Two cores retained almost all activities (99–100 %) within the uppermost centimetre layer, while the third core presented a deeper penetration of all radiotracers, displaying a second activity peak at the 3–4-cm depth interval, which evidenced benthic fauna bioturbation influence. This influence extended the diffusion into depths in which mangrove rhizosphere can retain the elements, suggesting increased retention efficiency. This mechanism of retention enhancement was proposed in addition to previous conceptual models describing trace elements behaviour in mangrove sediments. Increased bioturbation and rhizosphere development will probably increase this effect, while mangrove plant cover losses may promote a less efficient retention of elements recently diffused from tidal water.