Molecular size distribution and properties of DOM in Lake Kasumigaura and sediment pore waterUsing the calibration curves described in the previous section, we estimated the molecular size distribution of DOM in Lake Kasumigaura and its sediment pore water (Fig. 8). In all samples, five peaks were observed for UVA, while three peaks for TOC. The AMWUVA of peak top was estimated to be >35,000, 4500, 3700, 2800, and 2000 Da (Fig. 8A), and for NDIR DOC detection, the AMWTOC of peak top was observed to be >35,000, 3800, and 2000 Da (Fig. 8B). Comparison of the UVA- and NDIRdetectedpeaks indicatesbothsimilaritiesand differences.Both detectors exhibited a small peak for AMW > 35,000 Da. This observation is consistentwith a recent finding that the AMW of most DOM is <3000 Da (Chin and Gschwend, 1991; Conte and Piccolo, 1999). The difference of DOC >35,000 Da between UVA and TOC was also found. While the area of UVA in the >35,000 Da peak accounted for only w1% of total area of UVA, the area of TOC in the >35,000 Da peak accounted for w15% in lake DOM. This indicates that DOM> 35,000 Da could come from microbial biopolymers which consist of low UVA DOM. Many phytoplankton are known to release exopolymers (Decho, 1990), and these compounds could be a main source of high-molecular-weight DOM in Lake Kasumigaura. This result also shows the importance of multiple detectors since only the
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