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

Abstract

A natural planktonic assemblage from the St. Lawrence Estuary was isolated in eight 1,500‐liter outdoor meso‐cosms and subjected to combinations of fast or slow mixing regimes with natural solar radiation or natural solar radiation 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 (Chl a), particulate organic carbon (POC), and bacterial abundance in the mesocosms was followed over a 10‐d period. There was a net increase of Chl a, POC, and DOC in all systems over time; however, the slower mixing treatments had less accumulation than the systems with faster mixing. All systems displayed weak correlations of DOC with POC and Chl a. A significant effect of enhanced UVB radiation on concentrations of these bulk properties was not observed in any of the mesocosms. A strong correlation of CDOM absorbance loss (photobleaching) with absorbed radiation dose was observed in all treatments, with the fast mixing systems having larger absorbance losses and faster loss rates. Photobleaching was wavelength dependent, resulting in an increase in the spectral slope of CDOM absorption over time. Thus, although CDOM photobleaching may result in deeper penetration of light at all wavelengths, the ratios of UVB to ultraviolet‐A (UVA) and photosynthetically active radiation (PAR) are reduced. The effect of enhanced UVB radiation was unexpected, with no proportional increases in CDOM photo‐bleaching in the #x002BUVB treatments. Comparisons of the different treatments indicate that interactions of biological activity, mixing, and the in situ light field can influence CDOM absorbance properties and/or photoreactivity and that there is a possible role for UVB in the production of CDOM.