Origins and transformations of dissolved organic matter in large Arctic rivers

Karl Kaiser,Maria Canedo-Oropeza,Rainer M. W. Amon,Rachel McMahon

doi:10.1038/s41598-017-12729-1

Abstract

Arctic river watersheds are important components of the global climate system and show an amplified response to climate change. Here, we characterize origins and transformations of dissolved organic matter (DOM) in five major Arctic rivers (Kolyma, Lena, Yenisei, Ob, Mackenzie) over 3 years with seasonal sampling periods using measurements of carbohydrates, amino acids, bacterial biomarkers (D-amino acids), and plant protein biomarkers (hydroxyproline). A strong seasonal cycle of bioavailable DOM export was observed that correlated with discharge, vegetation, river morphology and water residence time. The chemical composition of bioavailable DOM was different among rivers reflecting unique characteristics of Arctic river watersheds. Trends in specific bacterial biomarkers were synchronous to changes in bacterial community compositions demonstrating that bacterial communities responded to the seasonal shifts in organic matter quality and chemical composition. Extensive heterotrophic processing of plant and soil-derived DOM resulted in major inputs of bacterial detritus, and bacterial organic matter accounted for 21–42% of DOC in all watersheds. Dissolved organic nitrogen sources were dominated by bacterially-derived nitrogen and important contributions of soluble plant protein during the Spring freshet. Overall, our results demonstrated the importance of watershed characteristics and bacterial metabolism in regulating DOM composition, reactivity and carbon fluxes in Arctic river watersheds.

Highlights

Arctic watersheds store approximately 50% of global soil organic carbon of which much is held in shallow continuous and discontinuous permafrost soils[1]
Concentrations of dissolved organic carbon (DOC), total hydrolysable amino acids (THAA), and total hydrolysable neutral sugars (THNS) exhibited pronounced changes connected to the seasonal rate of water flow in all Arctic rivers (Fig. 1, Table S1, Table S2)
In the Ob river, the highest concentrations of total hydrolysable amino acids (THAA) were observed at the end of the freshet whereas in other rivers highest concentrations occurred with the onset of the freshet

Summary

Introduction

Arctic watersheds store approximately 50% of global soil organic carbon of which much is held in shallow continuous and discontinuous permafrost soils[1]. Lignin and hydroxy-benzene biomarkers indicate that water-soluble components of fresh litter from angiosperm and gymnosperm plants are likely important sources of DOM in these rivers during the Spring freshet[10]. At low flow conditions in late summer, fall and winter, Arctic river DOM appears to carry a substantial fraction of DOM derived from mosses and peat bogs with an older radiocarbon signature indicative of deeper soil horizon drainage[9,12,13,14]. DOM transported in the rivers after the freshet, when flow is dominated by groundwater, bears the signature of older, extensively decomposed organic matter from soils, peat- and wetlands and exhibits drastically lower bioreactivity[10,24]

Methods

Results

Conclusion