Fluorescence peak integration ratio I_C:I_T as a new potential indicator tracing the compositional changes in...

The present study demonstrates that the ratio of fluorescence integration of peak C to peak T (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 δ¹³C 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 I_C:I_T and the ratio of terrestrial humic-like C1 to tryptophan-like C4 (C1:C4) derived by parallel factor analysis. The r² for the linear fitting between I_C:I_T and C1:C4 (r²=0.80) was notably higher than between C1:C4 and other indices tested, including the ratio of CDOM absorption at 250nm to 365nm, i.e. a(250):a(365) (r²=0.09), spectral slope (S_275-295) (r²=0.26), spectral slope ratio (S_R) (r²=0.31), the humification index (HIX) (r²=0.47), the recent autochthonous biological contribution index (BIX) (r²=0.27), and a fluorescence index (FI₃₇₀) (r²=0.07). I_C:I_T exhibited larger variability than the remaining six indices and a closer correlation with stable isotope δ¹³C than that observed for a(250):a(365), S_275-295, 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 a(250):a(365), S_275-297, S_R, FI₃₇₀, and BIX, further suggesting that I_C:I_T is the most efficient indicator of the CDOM compositional dynamics.