Fluorescence variability of marine and terrestrial colloids: Examining size fractions of chromophoric dissolved organic matter in the Damariscotta River estuary

Abstract

Marine chromophoric dissolved organic matter (CDOM) imparts highly variable optical signatures in surface waters over short spatial and temporal scales, but the cause of that variability is poorly understood. A major fraction of dissolved organic matter in seawater is colloidal in size and can cycle quite rapidly, potentially contributing to the observed variability in CDOM. The relationship between marine colloids and CDOM optical variability was examined using flow field-flow fractionation (FlFFF) to partition the colloidal organic phase into a continuum of molecular sizes for optical characterization by excitation emission matrix spectroscopy (EEMS). Colloidal organic matter in surface seawater of the Damariscotta River estuary showed 2 major peaks in apparent abundance, spanning at ∼ 1–5 kDa and ∼ 15–150 kDa in size, respectively. The relative magnitude of these peaks changed systematically with the phase of phytoplankton blooms during 2003 and 2004, implying a relationship between colloid size distribution and bloom dynamics. Of the two colloidal sizes, the 1–5 kDa fraction was far more variable in apparent abundance than the larger colloidal matter. EEMS results reveal a compositional partitioning of protein-like and humic-like fluorescence between size fractions. Protein-like materials occurred primarily in the smallest colloid size fraction while humic-type materials resided mainly in the larger colloidal phase. These findings suggest that the fluorescence signature of bulk dissolved organic matter results from a collage of chromophores having optical characteristics that differ according to size of the molecular constituents. The colloidal-sized fluorescence characteristics of marine derived CDOM were contrasted with bulk CDOM to provide fundamental information on the distribution and forms of CDOM in Maine coastal waters. The findings here indicate that colloidal processes will have significant effects on the character and variability in the optical signature of surface seawaters.