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　μM　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　C1　and　C4,　and　humic-like　C2　and　C3.　The　fluorescence　intensities　of　C1　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　C1　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.　©　2017　Elsevier　B.V.