Distribution and optical properties of CDOM in the Arabian Sea during the 1995 Southwest Monsoon

Abstract

Spatial and temporal variability in concentration and optical properties of colored dissolved organic matter (CDOM) were examined in the Arabian Sea during the 1995 Southwest (Summer) Monsoon on NRL/Seasoar Cruise, 21 June–12 July 1995. Classical upwelling patterns were observed near the coast, with strong fronts characterized by cold water, high chlorophyll a, and high nutrients. CDOM concentration showed a negative correlation with temperature in the surface waters throughout the study region, reflecting upwelling as the major source of CDOM to surface waters. Three water mass types were identified and found to have distinctive excitation–emission matrix (EEM) fingerprints. Persian Gulf Water had two fluorescence maxima due to UV and visible humic substance fluorescence. Maximum CDOM concentrations were found in the core of this water mass. Upwelling water had five or six fluorescence peaks, due to new and old CDOM. Elevated CDOM concentrations at the depth of the chlorophyll a maximum indicated production of new CDOM associated with the phytoplankton bloom. Oligotrophic water had extremely low CDOM concentrations, and only UV-absorbing and fluorescing compounds were found. This is consistent with CDOM which has been subjected to extensive exposure to solar irradiation at the sea surface. A new fluorophore with an Ex/Em maximum at 280/340 nm is described from highly productive waters. Two new EEM fingerprints are presented, one for oligotrophic blue waters showing photoresistant DOM, and one for upwelling water that may be of biological origin.