Abstract
We studied the effect of initial concentration on the decomposition processes of organic matter during the decomposition of cultivated phytoplankton (Skeletonema costatum) in seawater containing bacteria, HNF (Heterotrophic nanoflagellates), and zooplankton for knowing the characteristics on the formation of semirefractory organic matters in dissolved and particulate forms. In this study, the initial concentration of organic matter was changed from 4.1 to 73mg-C/L. The results showed that decomposition consisted of two different order reactions at POC (Particulate organic carbon). They were decomposition of labile and decomposition of semi-refractory. The turnover time for labile was 9-11days and the one for semi-refractory POC was 29-88 days at 20°C. This was independent of the initial concentrations of organic matter. About 10 % of the initial concentration of TOC remained as semi-refractory POM. This suggests that about 10% of organic matter of cultivated phytoplankton (S. costatum) is consisted as semi-refractory POC. The time-dependence of the POC/PON (PON: Particulate organic nitrogen) ratio was also independent on these initial concentrations. The variation pattern of DOC (Dissolved organic carbon) was independent of the initial concentrations of organic matter. DOC concentration was nearly constant after the first few days although the amount of POC decreased steadily during the decomposition process. This suggests that the organic matter derived from the degraded plankton did not accumulate in dissolved form but decomposed into inorganic carbon very quickly and a part of DOC remained as semi-refractory DOC.
