Ammonium ion and organic phosphorus as major in-situ contributors to dissolved fluorescence of the near northwestern Bay of Bengal

Abstract

Dissolved fluorescence (DFl) was investigated in relation to nutrients, dissolved carbohydrate (DCHO), chlorophyll-a and salinity at 20 stations of the continental shelf waters off the central east coast of India including the river Godavari. For the DFl, the major inputs identified were the humus-rich influx from land at the surface and the solubilised fluorescent matter from resuspended bottom particles rich in settled/adsorbed humic flocculates and biomolecules at depth. Photodegradation of DFl operative at the surface during sunny days is high around noon at low salinities, while surface accumulation of DFl takes place at night. At the surface, fluorescent metabolites of ammonium and dissolved organic phosphorus (DOP) are released along with DCHO during photosynthesis which in turn is related to the availability of nutrients (nitrate, silicate and orthophosphate). Ammonium is the most important factor affecting DFl in the water column, particularly at lower concentration of DFl (∼46 mFl), while DOP is important at higher levels of DFl (∼53 mFl). The efficiency of DOP improves at the expense of nutrients in the subsurface to the bottom. This may be due to the deep DOP, formed from the resuspended bottom particles, being more fluorescent than metabolic compounds at the surface, i.e. the deep DFl and nutrients are not of a common origin. Steric factors, chelation and molecular rigidity may cause the DOP of deep water to be a more efficient source of DFl. Ammonium is probably not influenced by these factors as the analysed constituent refers to the form of free (ionic).