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The bioavailability of macroalgal dissolved organic matter (DOM) was examined by decomposition experiments using released DOM from Ecklonia cava Kjellman (Phaeophyceae) living in Oura Bay, Shimoda, Izu Peninsula, Japan. The samples used for the decomposition experiments were obtained by enclosing the plants in bags. Based on the reduction rates of the concentrations of dissolved organic carbon (DOC), the turnover times of the macroalgal DOC were calculated to be between 24 and 172 d, with monthly-seasonal timescales. These values were mostly higher than those of phytoplanktonic DOC in previous studies (<1 mo). The relatively longer turnover time probably reflects the bio-refractory property of the macroalgal DOM. In most of the experiments (except for June), fucans and humic-like material were the major constituents of the released DOM. The fucans appeared to be partly decomposed during the experiments, but the compositional changes in the neutral carbohydrates in these seasons were less definite than those in June. The fluorescent intensity of the humic-like material did not decrease with time, suggesting a refractory character. Macroalgae are likely important DOM producers in Oura Bay, because the daily DOM production of E. cava accounts for 1.5 to 34% of DOM stock in Oura Bay per day. The concentration and the distribution of DOC inside and outside the bay strongly suggests that the released DOM was extensively exported out of the bay. These facts indicate that the macroalgal DOM contributes to marine DOM pools in a wider area including the adjacent coastal region.

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