Interactions of ultraviolet-B radiation, mixing, and biological activity on photobleaching of natural chromophoric dissolved organic matter: A mesocosm study

A natural planktonic assemblage from the St. Lawrence Estuary was isolated in eight 1,500-liter outdoor mesocosms and subjected to combinations of fas t or slow mixing regimes with natural solar radiation or natural solar radi ation artificially enhanced with ultraviolet-B (UVB, 280-320 nm) radiation. The interdependent evolution of dissolved organic carbon (DOC), absorption by chromophoric dissolved organic matter (CDOM), chlorophyll a (CN a), par ticulate organic carbon (POC), and bacterial abundance-in the mesocosms was followed over a 10-d period. There was a net increase of Chi a, POC, and D OC in all systems over time; however, the slower mixing treatments had less accumulation than the systems with faster mixing. All systems displayed we ak correlations of DOC with POC and Chi a. A significant effect of enhanced WE radiation on concentrations of these bulk properties was not observed i n any of the mesocosms. A strong correlation of CDOM absorbance loss (photo bleaching) with absorbed radiation dose was observed in all treatments, wit h the fast mixing systems having larger absorbance losses and faster loss r ates. Photobleaching was wavelength dependent, resulting in an increase in the spectral slope of CDOM absorption over time. Thus, although CDOM photob leaching may result in deeper penetration of light at all wavelengths, the ratios of UVB to ultraviolet-A (UVA) and photosynthetically active radiatio n (PAR) are reduced. The effect of enhanced UVB radiation was unexpected, w ith no proportional increases in CDOM photo-bleaching in the +UVB treatment s. Comparisons of the different treatments indicate that interactions of bi ological activity, mixing, and the in situ light field can influence CDOM a bsorbance properties and/or photoreactivity and that there is a possible ro le for UVB in the production of CDOM.