Seasonal changes of fluorescent dissolved organic matter in coastal waters off Visakhapatnam, Bay of Bengal, India

摘要

Visakhapatnam is at the centre of the east Indian coast. Although far away from any major estuary. its coastal region receives inputs from rivers like Ganga in the north and Godavari in the south. It also receives some inputs from the minor river Gosthani occurring 15 km towards north. During southwest monsoon season whenthe rivers experience flood, the fresh water input can reach high valuesWith an objective to make a fluorescence-based identification of these different water (external) sources as well as the in situ contributions to FDOM, we made a seasonal study during the year October 2010 to October 2011.From the water column, water samples (n = 116) were collected on three occasions (Premonsoon, Monsoon and Postmonsoon seasons) each representing one season viz.from different depths ca., 0, 10, 20, 30, 50, 75 and 100 m at six stations located at 10 m, 20 m, 30 m, 50 m, 75 m and 100 m isobaths in the coastal region off Visakhapatnam (Fig. 1).. Fluorescence Excitation Emission Matrix spectra were run for the water samples filtered through 0.22 μ membrane filters on a Horiba Jobin Yvon Specrofluorimeter. The fluorophore components extracted from these spectra by parallel factor (PARAFAC) analysis (Stedmon et al., 2008) were examined in relation to chlorophyll a determined fluorimetrically and salinity measured by CTD. Five fluorophoric components (Fig. 2) two protein (B: Tyrosine and T: Tryptophan) like and three humic (A: UV humic, M: Marine humic and C: Visible humic) like resulted (Table. 1). The tyrosine protein like component showed seasonally higher intensity in the premonsoon season (March to May) compared to other seasons in surface waters. The remaining Components showed more or less the same intensities year-round. The enrichment of tyrosine protein like component during premonsoon may be due to a stable and enhanced bacterial decay of phytoplankton and of photosynthetically produced organic matter. The humic like components (C1, C3 and C4) showed conservative behaviour at salinity < 31.06 in the coastal surface waters during monsoon and postmonsoon season. But during the premonsoon season showed less values compared to the conservative line attributed to photo bleaching. The humic like fluorophores, during the premonsoon season, had significant positive correlation with Chlorophyll a inferring that the humic like fluorophores are likely formed from chlorophyll a containing phytoplankton during the premonsoon season. Humification and fluorescence indices (Zsolnay et al., 1999; McKnight et al., 2001) showed significant seasonal trend. The humic like fluorophores showed significant vertical trend where as the protein like fluorophores did not, at the offshore stations (100 m isobaths). The maxima of the humic fluorophores occurred at subsurface in the premonsoon (Fig. 3a) and at surface in the monsoon season (Fig. 3b) attributed to losses (as explained above) and surface freshening by the monsoon driven riverine influx respectively.