pH change induces shifts in the size and light absorption of dissolved organic matter Michael L. Pace • Isabel Reche • Jonathan J. Cole • Antonio Ferna ´ndez-Barbero • Ignacio P. Mazuecos • Yves T. Prairie Received: 26 August 2010 / Accepted: 25 January 2011 Ó Springer Science+Business Media B.V. 2011 Abstract Dissolved organic matter (DOM) influ- ences inland water ecosystems through its light absorbing qualities. We investigated how pH affects light absorption by DOM with pH manipulation experiments and with data from two lake surveys. We hypothesized that: (1) light absorption and photoble- aching of DOM would increase with increasing pH, and (2) as a result of photobleaching, molar absorp- tion (i.e. light absorbance at 440 nm/dissolved organic carbon concentration) would decrease among lakes with increasing pH. In experiments with filtered lake water both initial light absorption and photoble- aching rates increased at higher (i.e. more basic) pH along with a concomitant shift in the size of DOM toward larger colloidal materials measured by dynamic light scattering (DLS). Both scanning elec- tron microscopy (SEM) and atom force microscopy (AFM) revealed large colloidal to particulate-sized organic matter in alkaline relative to acidic treat- ments. In the lake surveys, molar absorption coeffi- cients were negatively related to pH across gradients similar to the experiments. Our results are consistent with a conceptual model in which at low pH DOM polymers and colloids are condensed limiting expo- sure of chromophores to light; at higher pH, polymers and colloids are expanded exposing chromophores to light resulting in greater initial light absorption and faster photobleaching. Hence, water transparency, which is significantly controlled by DOM, is sensitive to environmental changes that influence the pH and chemical composition of inland waters. Keywords Dissolved organic matter  Light absorption  pH  Lakes  Photobleaching  Colloids  CDOM Introduction Dissolved organic matter (DOM) is a heterogeneous pool of compounds that collectively influences many physical, chemical and biological properties of inland waters (Williamson et al. 1999; Wetzel 2001; Prairie 2008). The concentration of dissolved organic carbon M. L. Pace (&) Department of Environmental Sciences, University of Virginia, 291 McCormick Road, P.O. Box 400123, Charlottesville, VA 22904, USA e-mail: pacem@virginia.edu I. Reche  I. P. Mazuecos Departamento de Ecologia, Universidad de Granada, Granada, Spain J. J. Cole Cary Institute of Ecosystem Studies, Millbrook, NY, USA A. Ferna ´ndez-Barbero  I. P. Mazuecos Departamento de Fisica Aplicada, Universidad de Almeria, Almeria, Spain Y. T. Prairie De ´partement des Sciences Biologiques, Universite ´ du Que ´bec a ` Montre ´al, Montreal, QC, Canada 123 Biogeochemistry DOI 10.1007/s10533-011-9576-0