Fluorescence and size characterization of dissolved organic matter in riverine and sea waters in the northern Gulf of Mexico

Zhengzhen Zhou

Abstract

Riverine export of dissolved organic matter (DOM) is an important component in marine carbon budget but the composition and phase partitioning are poorly quantified. Monthly water samples were collected from the lower Mississippi and Pearl rivers between January 2009 to August 2011 for DOM characterization using the fluorescence excitation emission matrix (FluoEEM) technique, coupled with parallel factor analysis (PARAFAC), and flow field-flow fractionation technique. DOM in the Pearl River showed higher dissolved organic carbon (DOC) concentration, temporal fluctuation, and aromaticity, reflecting instantaneous inputs of DOM from local soil and plant litter. In contrast, DOM in the Mississippi River exhibited lower abundance, seasonal variability and aromaticity, but higher proportion of protein-type fluorophores, corresponding to integrated signals and enhanced photochemical degradation largely resulting from of prolonged water residence time, as well as in situ phytoplankton production. Distinct DOM characteristics in these two river contribute to explain the effect of source, hydrology and human impacts on composition and colloidal size distribution of riverine DOM. In addition to terrestrial inputs of organic matter, oil is another source of organic matter in the Gulf of Mexico. The unprecedented Deepwater Horizon oil spill in April 2010 introduced large amount of crude oil into the Gulf of Mexico. The FluoEEM technique and PARAFAC modeling were also used to examine the fate, transport and transformation of oil in the water column during this oil spill. Oil greatly altered the optical properties of DOM in the entire water column during the oil spill. Persistent influence of oil on deep waters was observed even 15 months after the spill. Three major oil components were characterized by FluoEEM and PARAFAC modeling. The oil component ratios varied consistently and quantitatively with degradation states of oil and can be used as an index to track oil in the water column. The chemical evolution of oil and its degradation pathways observed in the field were further tested and confirmed with results from controlled laboratory experiments using Macondo crude oil. Degradation half lives of PAH, n -Alkanes and oil components were identified and can help understand the degradation pathways of different oil components.