Dissolved　organic　matter　(DOM)　is　a　critical　component　in　aquatic　ecosystems,　yet　its　seasonal　variability　and　reactivity　remain　not　well　constrained.　These　were　investigated　at　the　land-ocean　interface　of　a　subtropical　river　(Minjiang　River,　SE　China),　using　absorption　and　fluorescence　spectroscopy.　The　annual　export　flux　of　dissolved　organic　carbon　(DOC)　from　the　Minjiang　River　(5.48　x　10(10)　g　year(-1))　was　highest　among　the　rivers　adjacent　to　the　Taiwan　Strait,　with　72%　occurring　in　spring　and　summer.　The　freshwater　absorption　coefficient　a(280),　DOC-specific　UV　absorbance　SUVA(254)　and　humification　index　HIX　were　higher,　while　the　spectral　slope　S275-295　and　biological　index　BIX　were　lower　in　summer　than　in　winter.　This　suggests　intensified　export　of　terrestrial　aromatic　and　high　molecular　weight　constituents　in　the　rainy　summer　season.　Six　fluorescent　components　were　identified　from　428　samples,　including　humic-like　C1-C3,　tryptophan-like　C4　and　C6,　and　tyrosine-like　C5.　The　freshwater　levels　of　four　components　(C1,　C2,　C4,　and　C6)　were　lower　while　that　of　C5　was　higher　in　the　wet　season　than　in　the　dry　season,　suggesting　contrasting　seasonal　variations　of　different　constituents.　Laboratory　experiments　were　performed　to　assess　the　effects　of　photochemical　and　microbial　degradation　on　DOM.　Photo-degradation　removed　chromophoric　and　fluorescent　DOM　(CDOM　and　FDOM)　effectively,　which　was　stronger　(i)　for　high　molecular　weight/humic　constituents　and　(ii)　during　summer　under　higher　solar　radiation.　Microbial　degradation　under　laboratory　controlled　conditions　generally　showed　little　effect　on　DOC,　and　had　smaller　impact　on　CDOM　and　FDOM　in　winter　than　in　summer.　Overall,　this　study　showed　notable　seasonal　changes　in　the　chemical　composition　and　reactivity　of　DOM　at　the　land-ocean　interface,　and　demonstrated　the　significant　effects　of　photo-degradation.