Diurnal variations of dissolved organic matter in the hydrothermal system of Green Island, Taiwan

摘要

Hydrothermal systems are increasingly recognized as important fields for the biogeochemical cycling of dissolved organic matter (DOM) in the ocean. While the quantity and quality of DOM have been characterized for various hydrothermal systems, little is known about the temporal variations of DOM and the underlying processes in these systems. Diurnal variations of DOM were investigated for the hot water area and the warm water area of the hydrothermal system in Green Island off southeast Taiwan at 3 h intervals, using measurement of dissolved organic carbon (DOC), absorption spectroscopy, and fluorescence excitation emission matrix parallel factor analysis (EEM-PARAFAC). The results of temperature, salinity, pH and DOC indicated that the warm water area had a third freshwater source, besides the seawater and the hot fluids from the hot water area, that was probably groundwater. The DOC concentration showed significant diurnal changes within 84-130 mu M at the warm water area. A negative correlation between the DOC concentration and temperature was observed at the warm water area, while the DOC concentration at the hot water area was largely independent of temperature and tide. Four fluorescent components were identified by PARAFAC, including tryptophan-like Cl and C4, and humic-like C2 and C3. The fluorescence intensities of Cl and C4 were higher at the hot and the warm water areas than those in the ambient seawater, suggesting that the former two areas were sources of fluorescent DOM for seawater. The fluorescence intensities of Cl and C2 at the hot water area were higher at night than those in the daytime, implying the effects of potential photobleaching. Overall, the low values of the humification index (HIX) and the high values of the fluorescence index (FI) and the autochthonous index (BIX) at the hot and the warm water areas revealed that the DOM might have a mainly autochthonous biological or bacterial origin.