variability in photobleaching yields and their related impacts on optical conditions in subtropical lakes changes in the concentration and spectral absorption of chromophoric dissolved organic matter cdom may strongly condition the optical properties of tropical and subtropical water bodies we examined the spatial distribution of cdomrelated absorption spectral slope and vertical attenuation of solar radiation in two shallow lakes in the esteros del iber wetland system in situ measurements were made to examine spatial variations in photobleaching yields in natural lake conditions the results showed that fresh allochthonous cdom is more susceptible to phototransformations than either aged allochthonous organic matter or autochthonous sources if the distances from sources are considered as proxies for residence time based on measured changes in absorption spectral slope in relation to solar ultraviolet irradiance a model was developed which used cdom as a nonconservative tracer of water masses spatial changes in cdom absorption within the lake were then used to compare photo related transformations to those associated with conservative mixing