Behaviour of the optical properties of coloured dissolved organic matter under conservative mixing C.A. Stedmon*, S. Markager Department of Marine Ecology, National Environmental Research Institute, P.O. Box 358, Frederiksborgvej 399, DK-4000 Roskilde, Denmark Received 15 September 2002; accepted 7 January 2003 Abstract The optical properties of coloured dissolved organic matter (CDOM) can be used, in some environments, to trace water masses and provide information about the dynamics of the dissolved organic fraction in natural waters. This work presents the results from a modelling exercise, laboratory experiment and field data, which describe the variations in the optical properties of CDOM during mixing. The exponential slope coefficient (S ) is frequently used to characterise different CDOM pools; however, its behaviour during conservative mixing of two different CDOM types is often misunderstood. Identification of a theoretical conservative mixing line allows the rapid identification of non-conservative processes (e.g. in situ production, flocculation and degradation) acting on the pool during mixing. The results suggest that some of the patterns reported in the literature could purely be a result of conservative mixing rather than a product of non-conservative processes. Ó 2003 Elsevier Science B.V. All rights reserved. Keywords: coloured dissolved organic matter; light absorption; S-coefficient; mixing; model 1. Introduction In coastal and inland waters, a large fraction of the dissolved organic matter (DOM) pool is coloured (Harvey & Boran, 1981, Chap. 9). The coloured fraction is often termed coloured dissolved organic matter (CDOM), and at high concentrations, its presence gives water a yellow/brown colour. CDOM light absorption is highest in the ultraviolet (UV) region and declines to near-zero levels in the red region of the spectrum. Its spectral behaviour can often be modelled successfully by a ðkÞ ¼ a ðk 0 Þ expðSðk 0 ÿ kÞÞ ð1Þ where a k is the absorption coefficient at a wavelength k and k 0 is a reference wavelength (Bricaud, Morel, & Prieur, 1981; Jerlov, 1968; Lundgren, 1976), which in this study was chosen to be 375 nm. The S is the spectral slope coefficient and defines as to how the absorption of CDOM decreases with increasing wavelength. With a knowledge of both a k and S, one is able to quantify and characterise the CDOM pool in natural waters and, thereby, also trace changes resulting from the production and removal of CDOM and the mixing of different pools (e.g. CDOM of terrestrial and marine origin) (Blough, Zafiriou, & Bonilla, 1993; Vodacek, Blough, Degran- depre, Peltzer, & Nelson, 1997). However, it is important to have an understanding of the dynamics of these coefficients during simple conservative mixing. Conser- vative mixing is the situation where a constituent behaves linearly with mixing of water masses, i.e. there is neither removal nor addition, only dilution. Salinity is typically a conservative constituent of water and can be used to trace the mixing of water masses. In coastal waters, where there is mixing of saline marine water with fresh water, the behaviour of a constituent (e.g. conservative or not) can, thereby, be assessed by examining the rela- tionship a constituent has with salinity. Fig. 1 shows conservative and non-conservative be- haviours. If CDOM behaves conservatively during mixing, its distribution is controlled by the physical mixing of two end-members causing its absorption at a given wavelength (as an expression for quantity, e.g. * Corresponding author. E-mail address: cst@dmu.dk (C.A. Stedmon). Estuarine, Coastal and Shelf Science 57 (2003) 1–7 0272-7714/03/$ - see front matter Ó 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0272-7714(03)00003-9 ARTICLE IN PRESS