The concentration of TC in the surface sediment ranged between 33 and 55 gC Kg−1 dw (Fig. 3).Most of it was as OC, which varied from 30 to 45 gC Kg−1 dw, accounting usually for more than 70% of the TC. The highest values of both TC and OC concentrations were found at the reservoir tail and at S11; significant differences between the sediments during the low and the full water periods were not observed (Table 4). Total nitrogen (TN)was very low, ranging between 0.6 and 1.2 gN Kg−1- dw (Fig. 3). Contrarily to TC and OC, the TN highest concentrations appeared at themiddle reservoir body and near the dam;moreover, significant
differences (p b 0.001) were observed for TN concentrations between the lowand the fullwater periods, obtaining the highest values at the fullwater conditions (Fig. 3; Table 4). The lowTNconcentrations determined high C/N ratios, always exceeding the value of 30 (expressed in atoms); the significant differences (p b 0.006) observed for the C/N ratio between the different water conditions were clearly linked to the TN fluctuations.