Characterization of CDOM in an organic-rich river and surrounding coastal ocean in the South Atlantic Bight

Abstract

Listen

Translate article

Variability in chromophoric dissolved organic carbon (CDOM) was characterized in and around the Cape Fear River and Onslow Bay, North Carolina USA. The river end member of the study region are extremely rich in CDOM, thus the Cape Fear River serves as a point source of CDOM-rich water into the southeastern At- lantic bight. The river plume is easily traceable and gen- erally extends in a southwesterly direction along the coastline into Long Bay. Depending on physical pro- cesses and river flow, the plume can meander somewhat and may even turn northward for short periods of time. The oceanic end member of this study was the Gulf Stream. Samples from the Gulf Stream were obtained up to 97 km off shore. The experimental approach focused on the qualitative and quantitative description of CDOM from fresh-to-oceanic waters. CDOM was characterized by excitation emission matrix (EEM) fluorescence and UV/VIS spectroscopy. Variability of CDOM absorption in the relatively small area of the Cape Fear River estuary and surrounding coastal ocean was very high. The ob- served range of variability of CDOM absorption coeffi- cient, aCDOM(350), extended over nearly the entire range of CDOM absorption in the literature: 0.046 = aCDOM(350) = 29.9 m -1 . Changes in CDOM absorption spectrum slope coefficient S, were small in the Cape Fear River plume area, but relatively large in Onslow Bay. CDOM EEM spectra indicated that a radical change in composition of CDOM occurs along the river-to-oceanic salinity gradi- ent. CDOM in the coastal ocean was characterized by strong reduction of the three principal intensity peaks: A, C, and M, and a prominent contribution of the T peak to the fluorescence spectrum. The fluorescence intensity is linearly related with absorption. There is a strong inverse relationship between salinity and CDOM absorption, however our data show that this relationship may be de- pendent on river flow. The distribution of the slope coef- ficient and the percent contribution of respective peak in- tensities to the total EEM intensity showed that CDOM undergone conservative mixing until it approached oceanic salinity. Thus, CDOM is so concentrated in the river that mixing and other physical processes mask pho- tochemical or biological alteration of its composition.