Abstract:Large green tides of Ulva prolifera had become an annual event from 2007 to 2019 in the southern Yellow Sea and millions of tons of U. prolifera which were not salvaged settled to the bottom every year, releasing a large amount of biogenic elements such as C and N through degradation by microorganisms. This study discusses the effect of different temperature regimes on the release and the composition of dissolved organic matter (DOM) during the decomposition of U. prolifera. The results indicated that temperature significantly influenced the release of dissolved organic carbon (DOC) during 0~7 days (P<0.05): the DOC concentration at 20℃was significantly higher than that at 15℃ and 25℃. There was no significant difference in DOC concentration at different temperatures from day 7 to day 30 (P>0.05). For dissolved organic nitrogen (DON), temperature did not significantly affect the release of DON during the first 7 days (P>0.05). There were significant differences in DON concentration at different temperatures after 7 days (P<0.05): the concentration of DON was significantly lower at higher temperature (i.e. 25℃). The reason for this was that some organic nitrogen was converted to inorganic forms of nitrogen by microorganism during day 7 to day 30 and a higher microorganism abundance at 25℃ led to the transformation of more DON. The composition and fluorescence characteristics of fluorescent dissolved organic matter (FDOM) were evaluated by an excitation-emission matrix combined with parallel factor analysis (EEMs-PARAFAC). Three distinct dissolved organic matter fluorescent components (i.e. C1, C2, and C3) were identified using parallel factor analysis. C1, C2, and C3 were considered as tryptophan-like substances (also called protein-like substances), tryptophan-like substances and humic-like substances, respectively. The results showed that temperature had no significant impact on the composition of FDOM through comparing the relative content of three fluorescent components at different temperatures (P>0.05). While the fluorescence intensities of C2 and C3 were larger with the higher temperature (i.e. 25℃). Temperature affecting the fluorescence intensity markedly (P<0.05) may be caused by quicker microorganism decomposition rate under the higher temperature.
