Photobleaching response of different sources of chromophoric dissolved organic matter exposed to natural solar radiation using absorption and excitation-emission matrix spectra

PLoS One. 2013 Oct 25;8(10):e77515. doi: 10.1371/journal.pone.0077515. eCollection 2013.

Abstract

CDOM biogeochemical cycle is driven by several physical and biological processes such as river input, biogeneration and photobleaching that act as primary sinks and sources of CDOM. Watershed-derived allochthonous (WDA) and phytoplankton-derived autochthonous (PDA) CDOM were exposed to 9 days of natural solar radiation to assess the photobleaching response of different CDOM sources, using absorption and fluorescence (excitation-emission matrix) spectroscopy. Our results showed a marked decrease in total dissolved nitrogen (TDN) concentration under natural sunlight exposure for both WDA and PDA CDOM, indicating photoproduction of ammonium from TDN. In contrast, photobleaching caused a marked increase in total dissolved phosphorus (TDP) concentration for both WDA and PDA CDOM. Thus TDN:TDP ratios decreased significantly both for WDA and PDA CDOM, which partially explained the seasonal dynamic of TDN:TDP ratio in Lake Taihu. Photobleaching rate of CDOM absorption a(254), was 0.032 m/MJ for WDA CDOM and 0.051 m/MJ for PDA CDOM from days 0-9, indicating that phototransformations were initially more rapid for the newly produced CDOM from phytoplankton than for the river CDOM. Extrapolation of these values to the field indicated that 3.9%-5.1% CDOM at the water surface was photobleached and mineralized every day in summer in Lake Taihu. Photobleaching caused the increase of spectral slope, spectral slope ratio and molecular size, indicating the CDOM mean molecular weight decrease which was favorable to further microbial degradation of mineralization. Three fluorescent components were validated in parallel factor analysis models calculated separately for WDA and PDA CDOM. Our study suggests that the humic-like fluorescence materials could be rapidly and easily photobleached for WDA and PDA CDOM, but the protein-like fluorescence materials was not photobleached and even increased from the transformation of the humic-like fluorescence substance to the protein-like fluorescence substance. Photobleaching was an important driver of CDOM and nutrients biogeochemistry in lake water.

Publication types

  • Research Support, Non-U.S. Gov't

MeSH terms

  • Absorption
  • Ammonium Compounds / metabolism
  • Fluorescence
  • Fresh Water / microbiology
  • Nitrogen / metabolism
  • Phosphorus / metabolism
  • Photobleaching*
  • Phytoplankton / physiology*
  • Rivers / microbiology
  • Seasons
  • Solar Energy
  • Spectrometry, Fluorescence / methods
  • Sunlight

Substances

  • Ammonium Compounds
  • Phosphorus
  • Nitrogen

Grants and funding

This study was jointly funded by the National Natural Science Foundation of China (grant 41230744), the Provincial Natural Science Foundation of Jiangsu of China (BK2012050), the Key Program of Nanjing Institute of Geography and Limnology, Chinese Academy of Sciences (NIGLAS2012135003) and the Major Projects on 1 Control and Rectification of Water Body Pollution (2012ZX07101-010). The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript.