University of Jyväskylä

Dissertation: 23.11.2017 Sunlit surface waters: exploring the photochemical reactivity of dissolved organic carbon (Gu)

Start date: Nov 23, 2017 12:00 PM

End date: Nov 23, 2017 03:00 PM

Location: Ylistönrinne, YAA303

YufeiGunettikuvaajaMerviK..jpg
Yufei Gu picture: Mervi K.
Ms Yufei Gu defends her doctoral dissertation in Environmental Science "Sunlit surface waters: exploring the photochemical reactivity of dissolved organic carbon". Opponent Professor Davide Vione (University of Turin, Italy) and custos FT Anssi Vähätalo (University of Jyväskylä). The doctoral dissertation is held in English.

In streams and lakes, water is often stained brown. Brown colour comes originally from soils when dissolved organic carbon and iron are transported from land to water. Dissolved organic carbon can make a long journey from soils through rivers and lakes into the ocean. Along this journey, sun light bleaches water colour and simultaneously breaks down a part of organic carbon into carbon dioxide through photochemical reactions.

World is changing. Climate change has increased the transport of organic matter and iron to surface waters. At the same time, reduction in acidic deposition have increased pH e.g., in many Finnish lakes. I wanted to examine how these environmental changes affect photochemical breakdown of organic carbon into inorganic one along the transport of water from lakes to the ocean.

My laboratory experiments with organic carbon from one lake show that iron enhances photochemical breakdown and the effect is strongest in acidic waters. Therefore, the current increase in the transport of iron from land to water is expected to increase photochemical breakdown of organic carbon in lakes.

In order to test the expectation from a laboratory study, we collected water samples from 59 Finnish and Swedish lakes with variable concentration of iron. I was glad to notice that the expectations held in these lakes. Photochemical breakdown increased with the higher iron content of water and acidity. Iron content and thus photochemical breakdown was largest in lakes, where peaty soils transported a lot of organic carbon and iron to water. Our findings indicate that the current environmental changes enhance photochemical breakdown of organic matter in particular in those lakes that receive high loads of organic matter and iron from land.

More information:

Yufei Gu, yufei.y.gu@jyu.fi, phu. 040 480 2691

Communications Officer Anitta Kananen, anitta.kananen@jyu.fi, 040 805 4142

The dissertation is published in the series Jyväskylä Studies in Biological and Environmental Science, number 337, 179 p., Jyväskylä 2017, ISSN: 1456-9701, ISBN: 978-951-39-7252-3(PDF). Permanent link to this publication: http://urn.fi/URN:ISBN:978-951-39-7252-3

Yufei Gu graduated from Tongji University with a Bachelor of Science degree in 2012. She studied as a doctoral student in University of Jyväskylä during 2013-2017.

Abstract

In surface waters, solar radiation can photochemically mineralise the dissolved organic carbon (DOC, a measure of dissolved organic matter, DOM) to dissolved inorganic carbon (DIC). This DIC photoproduction constitutes an essential yet vague flux in the aquatic carbon cycling. The present thesis is based on the empirical assessment of the DOC photochemical reactivity, which was determined as the spectral apparent quantum yields (AQY) for DIC photoproduction. First, AQYs were determined in DOM solutions to quantify the impact of pH and DOM-associated iron. Then boreal lake waters were used for assessing the alteration of DOC photoreactivity due to water quality and catchment property. By simulating DIC production, further, AQYs were used to approximate the photomineralisation of terrigenous DOC (tDOC) in coastal waters. Finally, the experimental protocols determining AQY were compared by four laboratories. The results demonstrated the variation of AQYs triggered by the laboratory-specific procedures was less than that across the examined inland waters. Up to 86 % of the DIC photoproduction in DOM solutions can be justified by iron-stimulated photoreactions with acidic pH, while the effect was negligible at pH > 7. This interaction between iron and acidity was similarly influential on DOC photoreactivity in boreal lake waters. Across lakes, the DOC photoreactivity was varied relevant to the water quality and catchment land use patterns. A high DOC photoreactivity can be expected when the contents of DOC and chromophoric DOM are high, more so in small lakes enriched by peaty soils. Although DOC photoreactivity in lakes was higher, the estimates revealed that solar radiation mineralised far more tDOC in marine waters, which may be attributed to the extensive spreading of tDOC during mixing over the coastal ocean.


More information

Yufei Gu
yufei.y.gu@jyu.fi
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