Organic Carbon and humic substances in sediments of Grado and Marano Lagoon

Mercury contamination in sediments is a major concern, especially, in bivalves farming area, due to the potential risk of bioaccumulation. A substantial fraction of the so-called refractory organic matter in sediments is constituted by humic substances (HS). HS are ecognized to play an important role in complexing heavy metals, mercury included, thus favoring their mobilization or accumulation in sediments. In the framework of the MIRACLE project (Mercury Interdisciplinary Research for Appropriate Clam farming in Lagoon Environment) sediment cores have been sampled in 15 stations located in Grado and Marano lagoon and in 4 stations in Aussa River, one of the main freshwater inputs in this environment. The aim of our study was the evaluation of the organic carbon content in sediments, the estimation of the contribution of humic substances to total organic carbon and the elemental characterization of HS. Riverine sediments showed high organic carbon values (mean 2.46±1.47 %), increasing upstream from the river mouth and C/N atomic ratios higher (8.7±2.14) than lagoon sediments. Organic carbon content was similar in surficial and sub-surficial lagoon sediments (mean values 1.28±0.53 and 1.28±0.62 respectively), but higher C/N ratio have been observed in the deeper levels of the cores (7.1±1.5 with respect to 8.0±1.6 in the surficial layer). Humic substances extracted from sediments showed a high carbon content (mean 50.5±1.5 %). Humic acids concentrations in sediments fall in the range 1.8-31 mg g-1. The contribution of humic carbon to total organic carbon was relevant (up to 39.5 %), pointing out the important role of HS in the accumulation and transformation of organic material in the lagoon environment. Preliminary data showed no differences of the humic contribution to total organic carbon among upper and deeper levels of the cores.