Photo-reactivity of dissolved organic carbon in the freshwater continuum

Balathandayuthabani Panneer Selvam,David Bastviken,Ana R A Soares,Jan Karlsson,Martin Berggren,Jean-François Lapierre

doi:10.1007/s00027-019-0653-0

Balathandayuthabani Panneer Selvam, David Bastviken + Show 4 more

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https://doi.org/10.1007/s00027-019-0653-0

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Abstract

The patterns in dissolved organic carbon (DOC) photo-mineralization along the freshwater continuum from land to sea are poorly known. Specifically, it has not been resolved how the photo-degradation of DOC into CO2 (PD) depends on the combination of intrinsic properties of DOC and extrinsic variables that affect the photo-reactions. We measured PD per unit of absorbed ultraviolet light energy (PD-Ew) in headwater streams, lakes, intermediate rivers and river mouths in Sweden. Surprisingly, no trend of decreasing PD-Ew was found with decreases in colored DOC. However, there was a relationship between PD-Ew and pH, best described by a quadratic (U-shaped) curve, indicating environmental control of photo-reactivity. Interestingly, the highest values for both of these variables were recorded for river mouths. Moreover, PD-Ew increased with proxy variables for the amount of autochthonous DOC in the water. Thus, changes in pH and autochthonous DOC input along the continuum may sustain high DOC photo-mineralization throughout continental aquatic networks.

Highlights

Inland waters play a substantial role in generating greenhouse gas emissions to the atmosphere, in part owing to organic carbon mineralization processes (Panneer Selvam et al 2014; Cole et al 2007; Tranvik et al 2009)
In the different types of freshwaters that we sampled along the aquatic continuum, the CDOM decreased strongly from lakes to intermediate rivers and to river mouths (Fig. S1), but there was no significant change from headwater streams to lakes
Terrestrial lateral export of dissolved organic carbon (DOC) is increasing with global warming (IPCC 2013; Frey and Smith 2005, Frey and McClelland 2009), and mineralization processes in inland waters play a key role for determining the degree by which this carbon is lost to the atmosphere rather than exported to coastal regions

Summary

Introduction

Inland waters play a substantial role in generating greenhouse gas emissions to the atmosphere, in part owing to organic carbon mineralization processes (Panneer Selvam et al 2014; Cole et al 2007; Tranvik et al 2009). In freshwaters, dissolved organic carbon (DOC) is subject to both bioand photo-mineralization, the latter has received less attention by the research community. The contribution from photomineralization to the total DOC processing during summer was less than 3% in the water column of a subtropical lagoon (Ziegler and Benner 2000), but 12% in epilimnia of boreal lakes (Granéli et al 1996) and as much as 70–95% in arctic lakes and rivers (Cory et al 2014). Vachon et al (2016) found that the proportion of lake pelagic CO2 produced due to photo-mineralization was higher in spring compared to in summer. There are indications that dark environments such as soils, ice-covered lakes, or deep hypolimnetic

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