Photobleaching Response of Different Sources of Chromophoric Dissolved Organic Matter Exposed to Natural Solar Radiation Using Absorption and Excitation–Emission Matrix Spectra

Yunlin Zhang,Mingzhu Wang,Boqiang Qin,Christopher L Osburn,Xiaohan Liu,Yongqiang Zhou,Wei-Chun Chin

doi:10.1371/journal.pone.0077515

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.

Highlights

Chromophoric dissolved organic matter (CDOM), a major reservoir of organic carbon in aquatic environments, plays a central role in many physical, chemical and biological processes in aquatic ecosystems, including inhibiting attenuation of harmful ultraviolet radiation [1], affecting carbon budgets, nutrient availability and ecosystem productivity [2,3,4], disturbing water color parameters and estimates of primary production from remote sensing reflectances [5,6]
The ratio of total dissolved nitrogen (TDN) to total dissolved phosphorus (TDP) decreased with the exposure time for both Watershed-derived allochthonous (WDA) and phytoplankton-derived autochthonous (PDA) CDOM, due to the contrast variation trend
Our results showed a marked decrease in TDN concentration under natural sunlight exposure for both WDA and PDA CDOM, indicating photoproduction of ammonium and the release of ammonia into the atmosphere from TDN

Summary

Introduction

Chromophoric dissolved organic matter (CDOM), a major reservoir of organic carbon in aquatic environments, plays a central role in many physical, chemical and biological processes in aquatic ecosystems, including inhibiting attenuation of harmful ultraviolet radiation [1], affecting carbon budgets, nutrient availability and ecosystem productivity [2,3,4], disturbing water color parameters and estimates of primary production from remote sensing reflectances [5,6]. Large and shallow eutrophic lake ecosystems, such as Lake Taihu, China, and its surrounding rivers, are complex and dynamic environments of intense CDOM cycling in which the quantity, quality and distribution of CDOM reflect a balance between inputs and decomposition. In these systems, the sources of CDOM include the terrestrial river inputs and seasonal in situ production from frequent algal bloom and dense macrophytes [7,8,9]. Photobleaching results in the partial remineralization of CDOM in lakes, thereby affecting nutrient dynamics in the euphotic zone, there is little knowledge about the photoinduced transformations that mediate these dynamics in lakes [20]

Objectives

Methods

Results

Conclusion