Carbon Bioavailability in a High Arctic Fjord Influenced by Glacial Meltwater, NE Greenland

Maria L Paulsen,Oliver Müller,Mikael K Sejr,Thomas Juul-Pedersen,Eva F Møller,Antonio Delgado Huertas,Mathias Middelboe,Aud Larsen,Colin A Stedmon,Stiig Markager,Sophia E B Nielsen

doi:10.3389/fmars.2017.00176

Abstract

The land-to-ocean flux of organic carbon is increasing in glacierized regions in response to increasing temperatures in the Arctic (Hood et al., 2015). In order to understand the response in ecosystem metabolism to the organic carbon input it is essential to determine the bioavailability of the different carbon sources in the system. We quantified the bacterial turnover of organic carbon in a high Arctic fjord system (Young Sound, NE Greenland) during the ice-free period (July-October 2014) and assessed the quality and quantity of the 3 major organic carbon sources; 1) local phytoplankton production 2) runoff from land-terminating glaciers and a lowland river and 3) inflow from the ocean shelf. We found that despite relatively low concentrations of DOC in the rivers, the bioavailability of the river–DOC was significantly higher than in the fjord, and characterized by high cell-specific bacterial production and low C:N ratios. In contrast, the allochthonous DOC source entering via inflow of coastal shelf waters had elevated DOC concentrations with low bioavailability, high C:N and low specific bacterial production. The phytoplankton production in the fjord could not sustain the bacterial carbon demand, but was still the major source of organic carbon for bacterial growth. We assessed the bacterial community composition and found that communities were specific for the different water types i.e. the bacterial community of the coastal coastal inflow water could be traced mainly in the subsurface water, while the glacial river community strongly dominated the surface water in the fjord.

Highlights

Carbon consumption and mineralization by pelagic heterotrophic bacteria play a key role in marine ecosystems
When single taxa were correlated to specific environmental parameters we found that especially genera from the class Gammaproteobacteria showed correlations with specific dissolved organic matter (DOM) characteristics e.g., a strong positive correlation was found between the genus Glaciecola and bacterial production (r2 = 0.5283; p < 0.0001) with a maximum relative abundance of 25% when bacterial carbon production (BP) was highest
We hypothesize that the coastal shelf waters entering the fjord are characterized by high levels of terrestrial organic matter that originates predominantly from Siberian rivers

Summary

Introduction

Carbon consumption and mineralization by pelagic heterotrophic bacteria play a key role in marine ecosystems. Bacterial carbon turnover was traditionally suggested to be limited by low temperature in high latitude systems, and . Measurements of the bacterial respiration (BR) are required in order to estimate the bacterial carbon demand (BCD) and growth efficiency (BGE). Such measurements in Arctic systems are few and the BGE reported are highly variable, but all in the low end of those reported in other aquatic systems (del Giorgio and Cole, 1998). Other studies have demonstrated an inverse relationship between BGE and temperature (Rivkin and Legendre, 2001; Apple et al, 2006) leading to speculations that the low efficiency found in the Arctic may instead be a result of poor quality carbon sources (Middelboe et al, 2012). While the concentration of organic matter alone does not reflect the carbon quality (Kirchman et al, 2005), the elemental ratios of the dissolved organic matter (DOM) i.e., the C:N ratio has provided insight on the DOM bioavailability (del Giorgio and Cole, 1998; Pradeep Ram et al, 2003; Kragh and Søndergaard, 2004)

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Results

Conclusion