Maria Lund Paulsen

The Greenland Ice Sheet is melting rapidly, increasing freshwater runoff to the coastal ocean around Greenland. Through this pathway, allochthonous material, including nutrients, sediments, and organic carbon, is transported to coastal waters. The impacts of these inputs on coastal carbon cycling are poorly resolved, and accelerating climate change prompts closer examination of the character and fate of allochthonous material reaching Arctic coasts. In this study, we have taken a closer look at quantity, quality, and transformation of organic matter (OM) in surface waters of an oligotrophic high Arctic fjord influenced by glacial and proglacial runoff. We examined dissolved, suspended, and sinking OM by combining in situ observations along a river-to-sea gradient with experiments quantifying bioavailable carbon fractions, production of transparent exopolymer particles (TEPs), and OM flocculation. We found that dissolved organic carbon (DOC) concentrations in glacial and proglacial river waters were comparatively low (

Glacial retreat due to climate warming alters the pathway through which meltwater enters Arctic fjords. In the Tyrolerfjord–Young Sound system (NE Greenland), meltwater is delivered by two contrasting rivers: the Tyroler River, which flows directly from the glacier into the fjord, and the Zackenberg River, which passes through a proglacial lake. We investigated the impact of these different glacial sources on the pelagic system and fjord sediment biogeochemistry, with a focus on carbon and iron cycling. We quantified particulate organic carbon and particulate organic nitrogen, as well as δ ¹³ C and δ ¹⁵ N of the organic matter in the suspended and sinking fractions in the water column. In sediment, we quantified total organic carbon (TOC) and total nitrogen, δ ¹³ C and δ ¹⁵ N of the organic matter, porewater concentrations of Fe, Mn, and different fractions of solid‐phase Fe, O ₂ microprofiles and sulfate reduction rates. We find that the passage through a proglacial lake decreases the impact of the glacier on the fjord, as the lake acts as a trap for glacial material, decreasing sediment input to the fjord system. In the fjord sediments, a stronger redox‐cycling of iron was found further away from the rivers, which is mainly driven by the higher TOC content. Overall, our data suggest that, with glacial retreat, the impact of glaciers on the marine and the benthic systems in fjords will become weaker, and reduce long‐term carbon sequestration in Arctic fjord sediments.

Particulate (POM) and dissolved (DOM) organic matter in the ocean are important components of the Earth's biogeochemical cycle. The two are in a constant state of dynamic change as a result of physical and biochemical processes; however, they are mostly treated as two distinct entities, separated operationally by a filter. We studied the seasonal transition of DOM and POM pools and their drivers in a sub-Arctic fjord by means of monthly environmental sampling and by performing experiments at selected time points. For the experiments, surface water (5 m) was either pre-filtered through a GF/F filter (0.7 µm) or left unfiltered, followed by 36 h incubations. Before and after incubation, samples were collected for dissolved and particulate organic carbon concentrations (DOC, POC), extracellular polymeric substances (EPSs), microbial community (flow cytometry), and molecular composition of DOM (high-performance liquid chromatography coupled to high-resolution mass spectrometry – HPLC-HRMS). During the biologically productive period, when environmental POC concentrations were high (April, June, September), the filtered water showed an increase in POC concentrations. While POC concentrations increased in September, DOM lability decreased based on changes in the average hydrogen saturation and aromaticity of DOM molecules. In contrast, during the winter period (December and February), when environmental POC concentrations were low, lower concentrations of POC were measured at the end of the experiments compared to at the start. The change in POC concentrations was significantly different between the biologically productive period and the winter period (t test; p<0.05). Simultaneously, the DOM pool became more labile during the incubation period, as indicated by changes in the average hydrogen saturation, aromaticity, and oxygen saturation, with implications for carbon cycling. The change in POC was not directly associated with an antagonistic change in DOC concentrations, highlighting the complexity of organic matter transformations, making the dynamics between POC and DOC difficult to quantify. However, in both periods, bacterial activity and EPS concentrations increased throughout the incubations, showing that bacterial degradation and physical DOM aggregation drive the transformations of POM and DOM in concert but at varying degrees under different environmental conditions.

Particulate (POM) and dissolved (DOM) organic matter in the ocean are important components of the Earth’s biogeochemical cycle and in constant dynamic change through physical and biochemical processes. However, they are mostly treated as two distinct entities, separated operationally by a filter. We studied the transition between the DOM and POM pools and its drivers in different seasons in a sub-Arctic fjord by monthly environmental sampling and performing aggregation-dissolution experiments. For the experiments, surface water (5 m) was either pre-filtered through a GF/F filter (0.7 µm), or left unfiltered, followed by 36 h incubations. Before and after the incubation, samples were collected for dissolved and particulate organic carbon concentrations (DOC, POC), microbial community (flow cytometry) and in-depth analysis of the molecular composition of DOM (HPLC-HRMS). During the biologically productive period, when environmental POC concentrations were high (April, June, September), the filtered water showed a rapid increase of POC concentrations by up to 88 % within 36 h, indicating net-aggregation processes. During this process in September, DOM lability decreased based on changes in average hydrogen saturation and aromaticity of DOM molecules. In contrast, during the winter period (December, February), when environmental POC concentrations were low, the experiments indicated a dissolution of POC with a net-loss up to 58 %. Simultaneously, the DOM pool became more labile during the incubation period indicated by changes in average hydrogen saturation, aromaticity, and oxygen saturation. In both periods, bacterial activity increased throughout the incubation, showing that bacterial degradation likely plays a role in the transformation of POM and DOM. Our data highlights the importance of both physically driven DOM aggregation and biologically driven POM dissolution during different periods of the year, together determining the fate of the OM pool in high latitude marine ecosystems.

Particulate (POM) and dissolved (DOM) organic matter in the ocean are important components of the Earth’s biogeochemical cycle and in constant dynamic change through physical and biochemical processes. However, they are mostly treated as two distinct entities, separated operationally by a filter. We studied the transition between the DOM and POM pools and its drivers in different seasons in a sub-Arctic fjord by monthly environmental sampling and performing aggregation-dissolution experiments. For the experiments, surface water (5 m) was either pre-filtered through a GF/F filter (0.7 µm), or left unfiltered, followed by 36 h incubations. Before and after the incubation, samples were collected for dissolved and particulate organic carbon concentrations (DOC, POC), microbial community (flow cytometry) and in-depth analysis of the molecular composition of DOM (HPLC-HRMS). During the biologically productive period, when environmental POC concentrations were high (April, June, September), the filtered water showed a rapid increase of POC concentrations by up to 88 % within 36 h, indicating net-aggregation processes. During this process in September, DOM lability decreased based on changes in average hydrogen saturation and aromaticity of DOM molecules. In contrast, during the winter period (December, February), when environmental POC concentrations were low, the experiments indicated a dissolution of POC with a net-loss up to 58 %. Simultaneously, the DOM pool became more labile during the incubation period indicated by changes in average hydrogen saturation, aromaticity, and oxygen saturation. In both periods, bacterial activity increased throughout the incubation, showing that bacterial degradation likely plays a role in the transformation of POM and DOM. Our data highlights the importance of both physically driven DOM aggregation and biologically driven POM dissolution during different periods of the year, together determining the fate of the OM pool in high latitude marine ecosystems.

Particulate (POM) and dissolved (DOM) organic matter in the ocean are important components of the Earth’s biogeochemical cycle and in constant dynamic change through physical and biochemical processes. However, they are mostly treated as two distinct entities, separated operationally by a filter. We studied the transition between the DOM and POM pools and its drivers in different seasons in a sub-Arctic fjord by monthly environmental sampling and performing aggregation-dissolution experiments. For the experiments, surface water (5 m) was either pre-filtered through a GF/F filter (0.7 µm), or left unfiltered, followed by 36 h incubations. Before and after the incubation, samples were collected for dissolved and particulate organic carbon concentrations (DOC, POC), microbial community (flow cytometry) and in-depth analysis of the molecular composition of DOM (HPLC-HRMS). During the biologically productive period, when environmental POC concentrations were high (April, June, September), the filtered water showed a rapid increase of POC concentrations by up to 88 % within 36 h, indicating net-aggregation processes. During this process in September, DOM lability decreased based on changes in average hydrogen saturation and aromaticity of DOM molecules. In contrast, during the winter period (December, February), when environmental POC concentrations were low, the experiments indicated a dissolution of POC with a net-loss up to 58 %. Simultaneously, the DOM pool became more labile during the incubation period indicated by changes in average hydrogen saturation, aromaticity, and oxygen saturation. In both periods, bacterial activity increased throughout the incubation, showing that bacterial degradation likely plays a role in the transformation of POM and DOM. Our data highlights the importance of both physically driven DOM aggregation and biologically driven POM dissolution during different periods of the year, together determining the fate of the OM pool in high latitude marine ecosystems.

Particulate (POM) and dissolved (DOM) organic matter in the ocean are important components of the Earth’s biogeochemical cycle and in constant dynamic change through physical and biochemical processes. However, they are mostly treated as two distinct entities, separated operationally by a filter. We studied the transition between the DOM and POM pools and its drivers in different seasons in a sub-Arctic fjord by monthly environmental sampling and performing aggregation-dissolution experiments. For the experiments, surface water (5 m) was either pre-filtered through a GF/F filter (0.7 µm), or left unfiltered, followed by 36 h incubations. Before and after the incubation, samples were collected for dissolved and particulate organic carbon concentrations (DOC, POC), microbial community (flow cytometry) and in-depth analysis of the molecular composition of DOM (HPLC-HRMS). During the biologically productive period, when environmental POC concentrations were high (April, June, September), the filtered water showed a rapid increase of POC concentrations by up to 88 % within 36 h, indicating net-aggregation processes. During this process in September, DOM lability decreased based on changes in average hydrogen saturation and aromaticity of DOM molecules. In contrast, during the winter period (December, February), when environmental POC concentrations were low, the experiments indicated a dissolution of POC with a net-loss up to 58 %. Simultaneously, the DOM pool became more labile during the incubation period indicated by changes in average hydrogen saturation, aromaticity, and oxygen saturation. In both periods, bacterial activity increased throughout the incubation, showing that bacterial degradation likely plays a role in the transformation of POM and DOM. Our data highlights the importance of both physically driven DOM aggregation and biologically driven POM dissolution during different periods of the year, together determining the fate of the OM pool in high latitude marine ecosystems.

Particulate (POM) and dissolved (DOM) organic matter in the ocean are important components of the Earth’s biogeochemical cycle and in constant dynamic change through physical and biochemical processes. However, they are mostly treated as two distinct entities, separated operationally by a filter. We studied the transition between the DOM and POM pools and its drivers in different seasons in a sub-Arctic fjord by monthly environmental sampling and performing aggregation-dissolution experiments. For the experiments, surface water (5 m) was either pre-filtered through a GF/F filter (0.7 µm), or left unfiltered, followed by 36 h incubations. Before and after the incubation, samples were collected for dissolved and particulate organic carbon concentrations (DOC, POC), microbial community (flow cytometry) and in-depth analysis of the molecular composition of DOM (HPLC-HRMS). During the biologically productive period, when environmental POC concentrations were high (April, June, September), the filtered water showed a rapid increase of POC concentrations by up to 88 % within 36 h, indicating net-aggregation processes. During this process in September, DOM lability decreased based on changes in average hydrogen saturation and aromaticity of DOM molecules. In contrast, during the winter period (December, February), when environmental POC concentrations were low, the experiments indicated a dissolution of POC with a net-loss up to 58 %. Simultaneously, the DOM pool became more labile during the incubation period indicated by changes in average hydrogen saturation, aromaticity, and oxygen saturation. In both periods, bacterial activity increased throughout the incubation, showing that bacterial degradation likely plays a role in the transformation of POM and DOM. Our data highlights the importance of both physically driven DOM aggregation and biologically driven POM dissolution during different periods of the year, together determining the fate of the OM pool in high latitude marine ecosystems.