Amino acid and chlorin based degradation indicators in freshwater systems

Patrick E Stücheli,Thomas Larsen,Bernhard Wehrli,Carsten J Schubert

doi:10.1016/j.gca.2021.04.006

Abstract

Lakes cover a global area that is about 35 times smaller than the oceans, but carbon burial in lakes and oceans are on the same order of magnitude. Hence, understanding the processes for such high organic carbon burial in lacustrine systems is essential. We applied proxies typically used for marine environments including amino acid (AA) content and their nitrogen stable isotope composition to the water columns and sediments of three lakes that differ in their trophic states and depositions rates of sedimentary terrestrial organic matter. Additionally, we used carbon isotope fingerprinting of AAs to characterise their sources and fates. We show that this set of proxies tracks sources and degradation processes in eutrophic lakes with high sedimentary total organic carbon and nitrogen content. Those lakes also have a high total hydrolysable amino acid (THAA) content as well as higher pigment concentrations. While the Chlorin degradation Index (CI) showed increasing values with depth, the patterns were less consistent for the Degradation Index (DI). In general, all parameters failed to describe degradation in the most oligotrophic lake due to the very low organic carbon and nitrogen content in the sediment. We show that AAs are mostly of autochthonous origin and that AA contribute 5–45% to the organic material in plankton, POM, and sediment. Proxies based on AA for bacterial reworking (ΣV) or trophic level (TL) showed increasing values in the water column but relatively stable values in the sediments. Furthermore, we show that methanotrophic bacteria led to increased values for the bacterial reworking proxy (ΣV) and extraordinarily low δ13C AA values (−30 to −60‰).

Highlights

About 1.2% of the Earth surface and 3% of Earth’s continental land surface is covered by lakes (Downing et al, 2006), an area much smaller than the area occupied⇑ Corresponding author at: Eawag, Swiss Federal Institute of Aquatic Science and Technology, Department of Surface Waters – Research and Management, Seestrasse 79, 6047 Kastanienbaum, Switzerland.by oceans (71%)
We applied proxies typically used for marine environments including amino acid (AA) content and their nitrogen stable isotope composition to the water columns and sediments of three lakes that differ in their trophic states and depositions rates of sedimentary terrestrial organic matter
The amount of total hydrolysable amino acid (THAA) showed a similar pattern than Corg for both basins with decreasing content (Lake Zug N: from 227 mmol gÀ1 to 24 mmol gÀ1; Lake Zug S: 265 mmol gÀ1 to 27 mmol gÀ1) with increasing sediment depth (Fig. 2)

Summary

Introduction

Owing in large part to transportation of terrestrial organic material (OM) into lakes, the global burial of organic carbon in lacustrine systems (0.04–0.09 Pg C yrÀ1) is estimated to be up to three-quarters (Mendonca et al, 2017) of the burial occurring in the oceans (0.12 Pg C yrÀ1) (Sarmiento and Sundquist, 1992). It is estimated that inland waters annually receive about 1.9 Pg C yrÀ1 from the terrestrial landscape (Cole et al, 2007) showing how important terrestrial organic carbon is.

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Discussion

Conclusion