Abstract. Iodine is an essential trace element for all mammals and its bioavailability in terrestrial systems depends on its accumulation in soils but also on its release into the aquatic system. In tropical systems retention and mobilization of iodine in soils and related concentrations in streamflow are poorly understood. We, therefore, investigated the relationship between solid phase iodine binding on hillslope soils and the iodine and dissolved organic carbon (DOC) mobilisation to a connected stream. Our study was conducted in a pristine pre-montane rainforest in Costa Rica with old (up to nine My) and highly weathered volcanic soils. A total of nine soil profiles from two tributary sub-catchments to the main stream were sampled. Solid phase sequential extraction was used to identify iodine binding forms in soils. The water leachable iodine fraction was additionally assessed by batch soil leaching experiments. Stream water was sampled randomly over a period of five weeks. Results showed extremely high iodine concentrations in soils, ranging from 53–130 mg kg−1 (median: 69 mg kg−1), which is 13-fold higher than in temperate soils. In contrast, median water-soluble iodine was only 0.01 % (range: 0–0.4 %) of total soil iodine. Solid phase sequential extractions revealed iodine sorption to iron oxides (median: 79 % of total iodine) as the main retention factor. High enrichment and low mobilisation of iodine in soils caused relatively low iodine concentrations (0.77–1.26 μg L−1) in stream waters during base and even moderate high flow conditions. The significant correlation of iodine and DOC in soil leachates suggested transport of organically bound iodine from upper- to subsoil horizons and strong sorption of DOC-iodine complexes to iron oxides. Our results showed that the old and highly weathered tropical soils in the study area were highly enriched in iodine caused by strong retention of DOC bound iodine to iron oxides. As a result, iodine release from soils was low which resulted in low stream water iodine concentrations and subsequently in a low bioavailability.

Abstract. Anoxic microniches in sinking particles in lakes have been identified as important water phase production zones of monomethylmercury (MeHg) (endogenic MeHg). However, the production and decay of MeHg during organic matter (OM) decomposition in the water column and its relation to the total Hg concentration in seston are poorly understood. We investigated Hg speciation and chemical changes in sinking seston in a small and shallow (12-m-deep) eutrophic lake during phytoplankton blooms from April to November 2019. The results show that MeHg proportions are high in seston at the water surface (up to 22 %) and at the oxic-suboxic redox boundary (up to 26 %). During suboxic OM decomposition, and with decreasing redox-potential, the concentration and proportion of MeHg in seston strongly decrease (< 0.5 %) as the water depth increases. Under these conditions, total Hg concentrations show a 3.8 to 26-fold increase. In the hypolimnion environment, changes in MeHg proportions were minimal in sinking seston, and samples collected by sediment traps had MeHg values similar to those measured at the sediment-water interface, though higher MeHg concentrations were found deeper in the sediment. Our results indicate that cycling of MeHg and total Hg (THg) in seston within small productive lakes is largely controlled by the decomposition processes of settling seston and that the endogenic MeHg pool appears to be largely disconnected from the sedimentary MeHg pool.

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