Seasonal Changes in Dissolved Organic Matter Composition in a Patagonian Fjord Affected by Glacier Melt Inputs

Matthew G Marshall,Giovanni Daneri,Rodrigo Torres,Hong Chin Ng,Laura F Robinson,Alexander Beaton,Alejandra Urra,Jemma L Wadham,Helena V Pryer,Anne M Kellerman,Jon R Hawkings,Robert G M Spencer

doi:10.3389/fmars.2021.612386

Abstract

Biogeochemical processes in fjords are likely affected by changes in surrounding glacier cover but very little is known about how meltwater directly influences dissolved organic matter (DOM) in fjords. Moreover, the data available are restricted to a handful of northern hemisphere sites. Here we analyze seasonal and spatial variation in dissolved organic carbon (DOC) concentration and DOM composition (spectrofluorescence, ultrahigh resolution mass spectrometry) in Baker-Martinez Fjord, Chilean Patagonia (48°S), to infer the impacts of rapid regional deglaciation on fjord DOM. We show that surface layer DOC concentrations do not vary significantly between seasons, but DOM composition is sensitive to differences in riverine inputs. In summer, higher protein-like fluorescence reflects increased glacial meltwater inputs, whilst molecular level data show weaker influence from marine DOM due to more intense stratification. We postulate that the shifting seasonal balance of riverine and marine waters affects the supply of biolabile peptides and organic nitrogen cycling in the surface layer. Trends in DOM composition with increasing salinity are consistent with patterns in estuaries (i.e. preferential removal of aromatic compounds and increasing relative contribution of unsaturated and heteroatom-rich DOM from marine sources). Preliminary estimates also suggest that at least 10% of the annual organic carbon stock in this fjord is supplied by the four largest, glacially fed rivers and that these inputs are dominated by dissolved (84%) over particulate organic carbon. Riverine DOC may therefore be an important carbon subsidy to bacterial communities in the inner fjord. The overall findings highlight the biogeochemical sensitivity of a Patagonian fjord to changes in glacier melt input, which likely has relevance for other glaciated fjords in a warming climate.

Highlights

Fjords are critical zones of fresh and marine water interaction (e.g., Bianchi et al, 2020), which support hotspots of ecological productivity (e.g., Iriarte et al, 2007) and important carbon sinks (e.g., Smith et al, 2015) in the mid- to high-latitudes
We have presented the first detailed analysis of dissolved organic matter (DOM) composition in a Chilean Patagonian fjord, using molecular level techniques that have improved understanding of DOM cycling in other aquatic environments
Our results suggest that significant seasonal differences in DOM composition in the surface waters of Baker-Martinez Fjord (BMF) are controlled by variations in river discharge linked to glacier melt cycles

Summary

Introduction

Fjords are critical zones of fresh and marine water interaction (e.g., Bianchi et al, 2020), which support hotspots of ecological productivity (e.g., Iriarte et al, 2007) and important carbon sinks (e.g., Smith et al, 2015) in the mid- to high-latitudes. We know very little about the direct influence of glaciers on dissolved organic matter (DOM) in fjords, especially beyond the effects on overall biolability and the fluorescent fraction (Paulsen et al, 2017, 2018). Dissolved organic matter is an important phase in the aquatic biogeochemical cycles of carbon and major nutrients (e.g., Burd et al, 2016) and its overall reactivity is strongly linked to its composition (Amon and Benner, 1994; Sun et al, 1997; Hopkinson et al, 1998; Mostovaya et al, 2017). DOM cycling in fjords is sensitive to major changes in the upstream landscape (Asmala et al, 2016; Ward et al, 2017), which is primarily driven by rapid rates of glacier retreat in large regions of the higher latitudes (Zemp et al, 2015)

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