AMMONIUM ION AND ORGANIC PHOSPHORUS AS MAJOR IN-SITU CONTRIBUTORS TO DISSOLVED FLUORESCENCE OF THE NEAR NORTHWESTERN BAY OF BENGAL

Dissolved fluorescence (DFI) was investigated in relation to nutrients , dissolved carbohydrate (DCHO), chlorophyll-a and salinity at 20 stat ions of the continental shelf waters off the central east coast of Ind ia including the river Godavari. For the DFI, the major inputs identif ied were the humus-rich influx from land at the surface and the solubi lised fluorescent matter from resuspended bottom particles rich in set tled/adsorbed humic flocculates and biomolecules at depth. Photodegrad ation of DFI operative at the surface during sunny days is high around noon at low salinities, while surface accumulation of DFI takes place at night. At the surface, fluorescent metabolites of ammonium and dis solved organic phosphorus (DOP) are released along with DCHO during ph otosynthesis which in turn is related to the availability of nutrients (nitrate, silicate and orthophosphate). Ammonium is the most importan t factor affecting DFI in the water column, particularly at lower conc entration of DFI (approximately 46 mFl), while DOP is important at hig her levels of DFI (approximately 53 mFl). The efficiency of DOP improv es at the expense of nutrients in the subsurface to the bottom. This m ay be due to the deep DOP, formed from the resuspended bottom particle s, being more fluorescent than metabolic compounds at the surface, i.e . the deep DFI and nutrients are not of a common origin. Steric factor s, chelation and molecular rigidity may cause the DOP of deep water to be a more efficient source of DFI. Ammonium is probably not influence d by these factors as the analysed constituent refers to the form of f ree (ionic).