Traces of sunlight in the organic matter biogeochemistry of two shallow subarctic lakes

Marttiina V Rantala,E Henriikka Kivilä,Tomi P Luoto,Liisa Nevalainen,John P Smol,Antti E K Ojala,Carsten Meyer-Jacob

doi:10.1007/s10533-021-00820-9

Abstract

Global environmental change alters the production, terrestrial export, and photodegradation of organic carbon in northern lakes. Sedimentary biogeochemical records can provide a unique means to understand the nature of these changes over long time scales, where observational data fall short. We deployed in situ experiments on two shallow subarctic lakes with contrasting light regimes; a clear tundra lake and a dark woodland lake, to first investigate the photochemical transformation of carbon and nitrogen elemental (C/N ratio) and isotope (δ13C, δ15N) composition in lake water particulate organic matter (POM) for downcore inferences. We then explored elemental, isotopic, and spectral (inferred lake water total organic carbon [TOC] and sediment chlorophyll a [CHLa]) fingerprints in the lake sediments to trace changes in aquatic production, terrestrial inputs and photodegradation before and after profound human impacts on the global carbon cycle prompted by industrialization. POM pool in both lakes displayed tentative evidence of UV photoreactivity, reflected as increasing δ13C and decreasing C/N values. Through time, the tundra lake sediments traced subtle shifts in primary production, while the woodland lake carried signals of changing terrestrial contributions, indicating shifts in terrestrial carbon export but possibly also photodegradation rates. Under global human impact, both lakes irrespective of their distinct carbon regimes displayed evidence of increased productivity but no conspicuous signs of increased terrestrial influence. Overall, sediment biogeochemistry can integrate a wealth of information on carbon regulation in northern lakes, while our results also point to the importance of considering the entire spectrum of photobiogeochemical fingerprints in sedimentary studies.

Highlights

Sunlight governs fundamental biotic and abiotic processes in shallow northern lake ecosystems including those tightly connected to the aquatic carbon balance
Isotopic, and spectral fingerprints in the lake sediments to trace changes in aquatic production, terrestrial inputs and photodegradation before and after profound human impacts on the global carbon cycle prompted by industrialization
Mean d13CPOM and d15NPOM values were slightly lower in the tundra lake (- 27.5% ± 0.7 SD / - 0.3% ± 0.0 SD) compared to the woodland lake (- 27.0% ± 1.2 SD), / 1.6% ± 1.7 SD) where, in turn, contribute to shaping the elemental (C/N) values were slightly lower (8.4 ± 1.2 SD) relative to the tundra lake (8.9 ± 1.5 SD)

Summary

Introduction

Sunlight governs fundamental biotic and abiotic processes in shallow northern lake ecosystems including those tightly connected to the aquatic carbon balance. Visible wavelengths of the solar spectrum fuel the conversion of inorganic carbon into organic forms through photosynthesis while light, in the ultraviolet (UV) range, disintegrates organic matter resulting either in partial photodegradation or complete photomineralization (Bertilsson and Tranvik 2000; Chen and Jaffe 2016; Cory and Kling 2018). Carbon sourced both from lake production and from the surrounding terrestrial environment contribute to the aquatic carbon cycle, while their reactivities and pathways within lakes often differ (Kellerman et al 2015). Among the most prominent symptoms of Arctic warming are widespread decreases in the length of the snow and ice cover period (Magnuson et al 2000; Callaghan et al 2010; Griffiths et al 2017) promoting the transfer of solar energy into aquatic ecosystems

Methods

Results

Conclusion