Fluorescence peak integration ratio IC:IT as a new potential indicator tracing the compositional changes in chromophoric dissolved organic matter

The present study demonstrates that the ratio of fluorescence integration of peak C to peakT (I-c:I-T) can be used as an indicator tracing the compositional dynamics of chromophoric dissolved organic matter (CDOM). CDOM absorption and fluorescence spectroscopy and stable isotope delta C-13 were determined on a seasonal basis in seventeen Chinese inland waters as well as in a series of mixing and photodegradation experiments in the lab. A strong positive linear correlation was recorded between Ic:IT and the ratio of terrestrial humic-like Cl to tryptophan-like C4 (C1 :C4) derived by parallel factor analysis. The r(2) for the linear fitting between I-c:I-T and C1:C4 (r(2) = 0.80) was notably higher than between C1:C4 and other indices tested, including the ratio of CDOM absorption at 250 nm to 365 nm, i.e. alpha(250):alpha(365) (r(2) = 0.09), spectral slope (S275-295) (r(2) = 0.26), spectral slope ratio (S-R) (r(2) = 0.31), the humification index (HIX) (r(2) = 0.47), the recent autochthonous biological contribution index (BIX) (r(2) = 0.27), and a fluorescence index (FI370) (r(2) = 0.07). I-c:I-T exhibited larger variability than the remaining six indices and a closer correlation with stable isotope delta C-13 than that observed for alpha(250):alpha(365), S275-295, S-R, FI370, and BIX during field campaigns. Confirming our field observations, significant correlations were recorded between I-C:I-T and the remaining six indices, and I-c:I-T also demonstrated notably larger variability than the six other indices during our wastewater addition experiment. Compared with HIX, eutrophic water addition and photobleaching substantially decreased I-c:I-T but had no pronounced effect on alpha(250):alpha(365), S275-297, S-R, FI370, and BIX, further suggesting that Ic:IT is the most efficient indicator of the CDOM compositional dynamics. (C) 2016 Elsevier B.V. All rights reserved.