BG4.4

Aquatic biogeochemical cycles of carbon, nitrogen and phosphorus. From measurements to understanding hydrochemical patterns and processes

BG4.4

Aquatic biogeochemical cycles of carbon, nitrogen and phosphorus. From measurements to understanding hydrochemical patterns and processes

Co-organized by HS13

Convener: Magdalena Bieroza | Co-conveners: Andrea Butturini, Diane McKnight

vPICO presentations

| Tue, 27 Apr, 09:00–10:30 (CEST)

vPICO presentations: Tue, 27 Apr

Chairperson: Magdalena Bieroza

09:00–09:05

5-minute convener introduction

09:05–09:07

EGU21-14774

Column experiments show that cycling of both nitrogen and phosphorus is altered by dissolved organic carbon in river sediments

Marc Stutter, Daniel Graeber, and Gabriele Weigelhofer

Since agriculture and wider development have altered simultaneously runoff, pollution and natural structures in catchments (e.g. wetlands, floodplains, soil drainage, riparian trees) aquatic ecosystems deviate from background concentrations of N and P, but also organic C (OC). Hence mechanistic studies coupling OC, N and P are needed and whilst data coupling OC:N is becoming more available and interpreted this is not yet the case for aquatic OC:P. Column flow experiments (excluding light) allow preliminary controlled study of microbial biogeochemical processes in benthic sediments exposed to factorial nutrients (here +C, +NP, +CNP using simple dissolved substrates glucose, nitrate, and phosphate).

Based on the stoichiometric theory, we tested the hypothesis that bioavailable DOC will stimulate the heterotrophic uptake of soluble reactive P (SRP) and dissolved inorganic nitrogen in stream sediments. Glucose-C additions increased nutrient uptake, but also NP additions enhanced consumption of native and added OC. The effects of C addition were stronger on N than P uptake, presumably because labile C stimulated both assimilation and denitrification, while adsorption (unaffected by the presence or not of OC) formed a part of P uptake. Internal biogeochemical cycling lessened net nutrient uptake due to N and P recycling into dissolved organically-complexed forms (DOP and DON).

Simple column experiments point to mechanisms whereby availability of organic carbon can stimulate N and P sequestration in the bed of nutrient-polluted streams. This should promote further studies coupling OC with N and, especially P, towards better knowledge and ability to incorporate coupled macronutrient cycles into nutrient models and, potentially, ecosystem management.

How to cite: Stutter, M., Graeber, D., and Weigelhofer, G.: Column experiments show that cycling of both nitrogen and phosphorus is altered by dissolved organic carbon in river sediments, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-14774, https://doi.org/10.5194/egusphere-egu21-14774, 2021.

09:07–09:09

EGU21-1658

ECS

Microbial-driven Impact of the Topmost Bed Sediment Layer on Aquatic Phosphate Fluxes

Simeon Choo, Olaf Dellwig, Janine Wäge-Recchioni, and Heide Schulz-Vogt

There is a longstanding principle that the uppermost layer of aquatic sediment is the primary regulator of nutrient loads in the bottom water zone, pertaining to the fact that it is significantly biological in nature and thus the site of a myriad of biota-associated processes. Nevertheless, although this principle is seemingly obvious, there is unusually scant literature corroborating the impact of the uppermost sediment layer on water column nutrient fluxes, in particular soluble reactive phosphorus (SRP). It has also been theorized that in certain environments, large bacteria play a major role in phosphorus cycling in the sediment. This challenges the prevailing dogma that the control of bottom water phosphate (PO₄^3-) is mainly attributed to the SRP flux contribution from iron (Fe) oxide-bound P in sediment or remineralisation under anoxia and warming conditions respectively. In this study, elevated temperature as well as anoxic incubation treatments were set up to demonstrate that in response to an increased level of PO₄^3- being released under stressful conditions, the topmost bed sediment layer (TBSL) has an unmistakable impact on P sequestration and stabilisation of the bottom water PO₄^3- fluxes. Likewise, we also show that large filamentous microorganisms residing in the TBSL were seemingly active in polyphosphate (polyP) accumulation during these stress-inducing conditions. This therefore strongly points to a new and important biological sink for the SRP flux at the benthic layer of an aquatic environment.

How to cite: Choo, S., Dellwig, O., Wäge-Recchioni, J., and Schulz-Vogt, H.: Microbial-driven Impact of the Topmost Bed Sediment Layer on Aquatic Phosphate Fluxes, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-1658, https://doi.org/10.5194/egusphere-egu21-1658, 2021.

09:09–09:11

EGU21-3031

ECS

The role of organic matter and bacterial physiology on river metabolism at low flow

Masihullah Hasanyar, Nicolas Flipo, Thomas Romary, and Shuaitao Wang

Development of accurate water quality modelling tools is necessary for integrated water quality management of river systems. The existing water quality models can simulate dissolved oxygen (DO) concentration quite well in rivers, however, there are discrepancies during summer low flow season which are assumed to be due to the heterotrophic bacterial decomposition of organic matter (OM) (Wang, 2019). Therefore, we used the C-RIVE biogeochemical model in order to evaluate the influence of controlling parameters on the DO simulations at low flow.

Four Sobol’ sensitivity analyses (SA) were carried out based on an evolving strategy of reduction in the number of parameters and hiding the inter-parameter interactions. The studied parameters are bacterial (such as growth rate of bacteria), OM-related (repartition and degradation of OM into constituent fractions) and physical (for instance reaeration of river due to navigation and wind) whose variation ranges are selected based on a detailed literature review.

Bacterial growth and mortality rates are by far the two most influential parameters followed by bacterial yield and the share of biodegradable dissolved organic matter (BDOM). More refined SA results indicate that depending on the net bacterial growth (=growth – mortality) being low or high, the bacterial yield and BDOM concentration are the most influential parameters, respectively. Reaeration constant due to navigation and the bacterial uptake of substrate are the other two influential parameters identified in this work.

The results of this study highlight the importance of accurate in-situ sampling and measurement of these influential parameters in order to reduce modelling uncertainties, as well as the necessity for a suitable sampling frequency in order to characterize potential bacterial community switch during transient events such as combined sewer overflows.

References:

Wang, S. (2019). Simulation Du Métabolisme de La Seine Par Assimilation de Données En Continu. These de doctorat, Paris Sciences et Lettres

How to cite: Hasanyar, M., Flipo, N., Romary, T., and Wang, S.: The role of organic matter and bacterial physiology on river metabolism at low flow, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-3031, https://doi.org/10.5194/egusphere-egu21-3031, 2021.

Discussion

09:11–09:13

EGU21-1443

ECS

Interaction between carbon cycling and phytoplankton community succession in hydropower reservoirs: Evidence from stable carbon isotope analysis

Xiao jing, Wang baoli, Qiu Xiao-long, Yang Mei-ling, and Liu Cong-qiang

09:13–09:15

EGU21-4836

ECS

Online counter gradient LC-FT-ICR-MS enables detection of highly polar natural organic matter fractions

Limei Han, Jan M. Kaesler, Chang Peng, Thorsten Reemtsma, and Oliver J. Lechtenfeld

Natural organic matter (NOM) is a highly complex mixture of natural organic molecules. The hyphenation of liquid chromatography (LC) with ESI-FT-ICR MS for the molecular characterization of NOM have shown to be a promising approach. However, due to changing solvent composition during gradient elution in LC-FT-ICR-MS, ionization conditions also change throughout the chromatographic separation process. In this study, we applied a post-LC column counter gradient (CG) to ensure stable solvent conditions for transient ESI-MS signals. SRFA and a peat pore water samples were used as representative dissolved NOM samples for method development and validation. Our results show that in the isocratic range segment, the TIC intensities of CG was increased by a factor of 1.5 fold, as compared to the standard gradient (SG) method. In addition, the application of the CASI mode for low abundance fractions revealed over 3 times more molecular formulas (especially for CHNO, CHOS, CHNOS formula classes) than in full scan mode. The number of detected highly polar NOM compounds (with elemental ratios H/C < 1, O/C > 0.6) were more than 20 times larger for CG-LC mode as compared to direct infusion (DI) (5715 vs 266 MF).We conclude that the new CG-LC-FT-ICR MS method achieved a novel insight into the highly polar fractions of NOM, which are inaccessible in conventional DI measurements.

Key words: Natural organic matter (NOM), online LC-FT-ICR MS method, CASI mode, ESI

How to cite: Han, L., Kaesler, J. M., Peng, C., Reemtsma, T., and Lechtenfeld, O. J.: Online counter gradient LC-FT-ICR-MS enables detection of highly polar natural organic matter fractions, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-4836, https://doi.org/10.5194/egusphere-egu21-4836, 2021.

09:15–09:17

EGU21-4970

ECS

Spatial changes in instream DOM quality under base flow conditions linked with different DOM sources in small forested catchments

Maria Paula da Silva, Katharina Blaurock, Burkhard Beudert, Jan H. Fleckenstein, Luisa Hopp, Stefan Peiffer, Thorsten Reemtsma, and Oliver J. Lechtenfeld

Dissolved organic matter (DOM) plays an important role in aquatic systems controlling metal bioavailability and mobility, and nutrient cycling. The quantity and quality of instream DOM determine its role and behavior in aquatic systems. Headwater streams are particularly sensitive systems to study the changes in DOM dynamics due to their immediate interface with adjoining hillslopes and the high soil-to- water-area ratio. However, the controls, sources and mobilization processes of DOM export in natural forested catchments are still poorly understood. The objective of this study is to combine high temporal resolution spectroscopic DOM analysis with high-resolution mass spectrometry data to characterize the spatial changes in DOM composition along a low-order stream in a forested catchment (Bavarian Forest National Park - Germany) during base flow conditions. Furthermore, the study aims to link the patterns found instream with fingerprints of DOM end member sources. DOM quality was monitored over 1.5 year in three sites along the stream using absorption indices indicators of aromaticity (SUVA₂₅₄) and molecular weight (E₂:E₃) and data aggregating quality parameters and intensity based on formulas derived from discrete samples analyzed by FT-ICR-MS. Additionally, three end members corresponding to DOM in deep ground water, shallow ground water and water in the top layer of the soil were sampled in the catchment.

At base flow conditions, no significant changes in DOC concentration were observed spatially. Yet, absorption indices from DOM exhibited clear spatial patterns, with higher aromatic and lower molecular weight DOM at the lower floodplain compared to upstream. From the FT-ICR-MS data, however, high aggregate quality data showed a small gradient in DOM quality between the upper and lower parts of the catchment, with the relationship between DOC concentration and the relative intensity (RI) of molecular formulas being a better descriptor of the spatial changes. The patterns observed in formulas with an increase or decrease in RI at higher DOC concentrations is indicative of changes in DOM sources between upper and lower parts of the catchment. The characterization of end members could further elucidate the observed changes in RI of formulas. In the studied catchment, formulas with a decrease in RI at higher DOC concentration agree with the formulas most commonly found for DOM from the deep ground water, whereas the formulas being enriched at higher DOC concentration changed along the stream. At the upper part of the catchment, these formulas are the ones most abundantly present at the shallow ground water, whereas at the lower part these formulas are also representative of formulas found in the sample collected at the superficial layer of the soil. The monitoring of DOM amount and quality in the small forested catchments showed that DOM spatial variability is connected with the availability and mobilization of different sources, even during base flow conditions. The use of spectrophotometers allowed us to identify general trends in DOM quality and concentration, while FT-ICR-MS data was crucial to characterize DOM quality and link the findings instream with DOM sources.

How to cite: da Silva, M. P., Blaurock, K., Beudert, B., H. Fleckenstein, J., Hopp, L., Peiffer, S., Reemtsma, T., and J. Lechtenfeld, O.: Spatial changes in instream DOM quality under base flow conditions linked with different DOM sources in small forested catchments, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-4970, https://doi.org/10.5194/egusphere-egu21-4970, 2021.

09:17–09:19

EGU21-9553

ECS

The CO2 exchange of a small mountain lake as affected by the local thermo-topographically driven flow regime

Katharina Scholz, Tom Battin, Elisabet Ejarque, Albin Hammerle, Martin Kainz, Jakob Schelker, and Georg Wohlfahrt

Lakes receive large amounts of carbon (C) from the surrounding catchment and, together with the connecting streams, play an important and active role in the global C cycle. The received C can either be lost through the outflow and eventually transported to the ocean, or transformed and stored in sediments or outgassed to the atmosphere. Globally, lakes are estimated to emit 0.3 – 0.64 Pg C m-2 in form of CO₂ annually. Although subalpine and alpine lakes were observed to be supersaturated with CO₂, long-term measurements of lake-atmosphere CO₂ exchange are sparse. Several methods to quantify water-atmosphere gas exchange exist, like chambers, eddy covariance (EC), mass-balance or gradient based methods including boundary layer models (BLM), each having its own advantages and disadvantages. However, quantifying CO₂ exchange in aquatic ecosystems has often proved to be challenging. Here, both the BLM and the EC methods were used to estimate the air-water CO₂ exchange of Lake Lunz, a small lake situated in complex mountainous topography of the Austrian Alps. The results indicated that the lake was a small source of CO₂. Fluxes were affected by the thermo-topographic flow regime of the field site and its surroundings which drove the local wind pattern but also determined the local atmospheric CO₂ concentration. During most nights, a significant increase in atmospheric CO₂ was observed which decreased the differential CO2 concentration at the air-water interface and therefore led to decreased nocturnal CO₂ efflux. This diurnal pattern, however, was obscured in the EC measurements, because the method itself highly depends on the local wind regime. Because lakes are an integral part of mountain ranges which are characterized by catchments with complex topography, our findings are most likely of broader impact.

How to cite: Scholz, K., Battin, T., Ejarque, E., Hammerle, A., Kainz, M., Schelker, J., and Wohlfahrt, G.: The CO2 exchange of a small mountain lake as affected by the local thermo-topographically driven flow regime, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-9553, https://doi.org/10.5194/egusphere-egu21-9553, 2021.

09:19–09:21

EGU21-9863

ECS

Bioavailable DOC : reactive macronutrient ratios control heterotrophic nutrient assimilation - An experimental proof of the macronutrient-balance hypothesis

Daniel Graeber, Youngdoung Tenzin, Marc Stutter, Gabriele Weigelhofer, Tom Shatwell, Wolf von Tümpling, Jörg Tittel, and Dietrich Borchardt

We assess the “macronutrient-balance hypothesis,” which we define as: “Aquatic heterotrophic nutrient assimilation is controlled by the balance between the bioavailable DOC : reactive macronutrient stoichiometry and the microbial stoichiometric macronutrient demand.” Here we define the reactive macronutrients as the sum of dissolved inorganic nitrogen, soluble-reactive phosphorus (SRP), and dissolved bioavailable organic N (bDON) & P (bDOP). A global meta-analysis of monitoring data from various freshwaters suggests this hypothesis, yet clear experimental support is missing.

We assessed this hypothesis in a proof-of-concept experiment for waters from four different small agricultural streams. We used seven different bioavailable DOC (bDOC) : reactive N and bDOC : reactive P ratios, induced by seven different levels of alder leaf leachate addition. With these treatments and a stream-water specific bacterial inoculum, we conducted a separate 3-day experiment, with three independent replicates per combination of stream water, treatment and sampling occasion. Here, we extracted dissolved organic matter (DOM) fluorophores by measuring excitation-emission matrices with subsequent parallel factor decomposition (EEM-PARAFAC). We assessed the true bioavailability of DOC, DON, and the DOM fluorophores as solute concentration difference between the beginning and end of each experiment. Separately, we predicted bDOC and bDON concentrations based on the bioavailable fluorophores, which we compared to their true bioavailability measured before. Due to very low DOP concentrations, the DOP determination uncertainty was high, and we had to neglect DOP as part of the reactive P.

For bDOC and bDON, the bioavailability measurements agreed with the same fractions calculated indirectly from bioavailable DOM fluorophores (bDOC r² = 0.96, p < 0.001; bDON r² = 0.77, p < 0.001), hence we could predict bDOC and bDON concentrations based on the molecular composition of DOM. Moreover, we found that bDOC : reactive nutrient ratios at specific ranges (molar bDOC : reactive N = 2 − 17; molar bDOC : reactive P = 50 − 300) control microbial heterotrophic nutrient uptake.

In summary, the results of our simple laboratory experiment provide first proof that the bDOC : reactive macronutrient ratio strongly controls heterotrophic reactive macronutrient uptake. Combined with the previous large-scale monitoring evidence, our study implies that the “macronutrient-balance hypothesis” holds in many aquatic ecosystems. However, this hypothesis needs to be corroborated by further experiments with different DOC sources and assessments of changes in bDOC : reactive macronutrient ratios on freshwater carbon and nutrient cycles.

How to cite: Graeber, D., Tenzin, Y., Stutter, M., Weigelhofer, G., Shatwell, T., von Tümpling, W., Tittel, J., and Borchardt, D.: Bioavailable DOC : reactive macronutrient ratios control heterotrophic nutrient assimilation - An experimental proof of the macronutrient-balance hypothesis, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-9863, https://doi.org/10.5194/egusphere-egu21-9863, 2021.

09:21–09:23

EGU21-12646

ECS

A Case Study: The deployment of a novel in situ fluorimeter for monitoring biological contamination within the urban surface waters of Kolkata, India.

Bethany Fox, Robin Thorn, Tapan Dutta, and Darren Reynolds

With increasing pressures on water resources due to population, industrialization, agriculture, urbanization and climatic changes, improved temporal and spatial understanding of water quality is required. The development of new monitoring parameters, along with new monitoring technologies, are needed to provide real-time insight into the biogeochemical processes that underpin aquatic ecosystem health. Aquatic fluorescent organic matter (AFOM) has recently been explored for its potential to measure underpinning microbial activity within aquatic systems, which are essential in maintaining ecosystem health and function, with specific focus on the utilisation of tryptophan-like fluorescence (TLF or Peak T). In situ real-time portable fluorimeters have been extensively used for the identification and measurement of anthropogenic pollutants, such as polycyclic aromatic hydrocarbons (PAH) and optical brighteners. More recently, this portable fluorescence technology has been adapted for the monitoring and sensing of biological contamination, using microbially derived fluorescence signals (TLF).

The principal aim of this research was to deploy, for the first time in the field, the VLux TPro sensor (Chelsea Technologies Ltd., UK) and to assess the ability of this novel fluorescence-based sensor to detect the presence of biological contamination and elevation of microbial activity. This sensor has been developed to correct in situ real-time sensing data for optical interferences (caused by high turbidity and absorbance), as well as to provide quantitative fluorescence data by reporting in standardised quinine sulphate units (QSU). The urban surface waters within the city of Kolkata provide an interesting challenge for water quality sensors, allowing exploration of sensor performance in a range of water bodies ranging from turbid river waters to open sewer canals.

The sensor data collected demonstrates the ability of the VLux to identify waters with high bacterial loads using Peak T fluorescence. Moderate and weak positive correlations are seen for Peak T and E. coli or total coliform counts, R2 = 0.55 and 0.38 respectively. However, a strong significant correlation is identified between Peak T and the total bacterial cell counts (R2 = 0.75). This demonstrates that Peak T should not be used as a species-specific enumerator in complex surface water matrices. It does, however, demonstrate the ability of the VLux to successfully measure optically corrected and quantitative Peak T fluorescence in QSU. Therefore, data regarding the activity and fast-acting dynamics of freshwater bacterial communities, in response to pollution events, can now be reliably sensed and collected. This was demonstrated by the elevated Peak T fluorescence intensity observed when biologically contaminated water entered the main river channel, enabling identification of contamination hotspots. Sensing data has been further validated by laboratory analysis of spot samples confirming the significant correlations between Peak T and bacteria and nutrient concentrations. Further field-based research is required to determine the feasibility of long-term catchment scale sensor deployment as part of a sensing network, for the monitoring of biological activity and pollution events in freshwaters.

Acknowledgements: This research was supported by the NERC-DST Indo-UK Water Quality Programme NE/R003106/1 and DST/TM/INDO-UK/2K17/30. We would also like to acknowledge Chelsea Technologies Ltd. for ongoing VLux TPro sensor support.

How to cite: Fox, B., Thorn, R., Dutta, T., and Reynolds, D.: A Case Study: The deployment of a novel in situ fluorimeter for monitoring biological contamination within the urban surface waters of Kolkata, India., EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-12646, https://doi.org/10.5194/egusphere-egu21-12646, 2021.

09:23–09:25

EGU21-12676

ECS

Wavelet Analysis Identifies Carbon Processes in a Subarctic Stream During Snowmelt Spring Flood

Danny Croghan, Pertti Ala-Aho, Annalea Lohila, Jeffrey Welker, Jussi Vuorenmaa, Mika Aurela, Kaisa-Riikka Mustonen, Bjørn Kløve, and Hannu Marttila

Snowmelt spring floods dominate the annual carbon flux in Arctic streams. However, climate change is altering their timing and magnitude due to changes in snow conditions, further altering the processes controlling the carbon cycle at the catchment scale. Current knowledge is limited by a lack of high-resolution data from Arctic areas. In this study we combine high-resolution biogeochemical-hydro-climatological variables with spectral wavelet analysis for new insights into carbon processes.

This study was conducted during the snowmelt spring flood period in a sub-arctic headwater catchment in Pallas-Ylläs national park, Finland (68°02′N, 24°16′W). We collected in-stream dissolved organic carbon (DOC), carbon dioxide (C0₂), and terrestrial C0₂flux alongside a suite of hydro-climatological variables measured at 30-minute intervals. Continuous wavelet transformations and wavelet coherence were produced to assess the relationship between hydro-climatological variables and carbon variables at different periodicities.

Wavelet transforms indicated that the onset of snowmelt caused the development of significant diel periodicity for in-stream DOC, CO₂ and terrestrial CO₂flux, while substantial periods of significant periodicity were observed at multiple day periodicities. Wavelet coherence analysis identified that DOC was consistently lead by flow and conductivity across daily and multiple daily scales suggesting that transport of carbon from the surface and shallow sub-surface pathways to the stream were the predominant processes controlling in-stream DOC. Interestingly for in-stream CO₂, groundwater level showed periodic rather than consistent spectral coherence suggesting it is not a consistent control on CO₂ in the spring flood. The strongest coherence for in-stream CO₂ was with in-stream O₂, which may suggest the importance of in-stream metabolism as a control on in-stream CO₂ dynamics. Terrestrial CO₂fluxwas controlled by notably different processes than in-stream Carbon and linked strongest to climatological variables. Photosynthetically active radiation (PAR) showed the strongest relationship with CO₂ terrestrial flux dynamics.

Our study highlights the unique processes controlling different parts of the carbon cycle in a headwater arctic catchment during the snowmelt spring flood. We highlight in-stream DOC as particularly vulnerable to changes in spring flood magnitude and timing given the importance of snowmelt dominated transport processes to DOC flux. To identify future changes in the Arctic carbon cycle, wavelet analysis shows potential as tool to analyse changes in processes in high-resolution datasets.

How to cite: Croghan, D., Ala-Aho, P., Lohila, A., Welker, J., Vuorenmaa, J., Aurela, M., Mustonen, K.-R., Kløve, B., and Marttila, H.: Wavelet Analysis Identifies Carbon Processes in a Subarctic Stream During Snowmelt Spring Flood, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-12676, https://doi.org/10.5194/egusphere-egu21-12676, 2021.

09:25–09:27

EGU21-12720

ECS

A pan-Arctic algorithm for DOC concentrations from CDOM spectra

Rafael Gonçalves-Araujo, Mats Granskog, Christopher Osburn, and Colin Stedmon

The surface layer of the Arctic Ocean carries a higher dissolved organic carbon (DOC) content than other ocean basins. Climate change impacts the Arctic aquatic DOC-pool by e.g., introducing DOC trapped in permafrost soils as they thaw and by increasing the terrestrial runoff and primary production. Sampling for DOC in the Arctic is rather challenging given its remoteness and difficult access to the region and that it is not possible yet to determine DOC concentrations from instruments deployed in the field. Compared to DOC, colored dissolved organic matter (CDOM) absorption spectroscopy is an easy-to-measure, relatively quick and cost-effective approach which is often closely related to DOC concentrations in water samples. In regions in close proximity to rivers, linear relationships between CDOM absorption at 350nm (a350) and DOC often can be found and, thus, have improved prediction of DOC using two end‐members. However, in regions with two or more end‐members of comparable DOC concentrations (shelf seas and oceanic waters) these relationships are difficult to derive, as there might be pools of similar concentration/intensity but different ratio of absorption to DOC (carbon specific absorption coefficient, a*). Here we present an algorithm to estimate DOC concentrations based on quantitative (a350) and qualitative (spectral absorption slope between 275 and 295nm, S275-295) properties of CDOM. The algorithm considers that there is a linear correlation between DOC and a350 but that the slope of the relationship (inverse of a*) varies depending on the exponent of the ultraviolet (UV) spectral slope (S275‐295), that is, the character or source of DOM. We compiled a Pan-Arctic dataset (n=3607) from a wide range of aquatic systems spanning lakes, rivers, estuaries, coastal and shelf seas and open ocean with salinity ranging from 0 to 35.3. DOC ranged between 19 and 2304µM, whereas a350 varied from 0.01–81.33m^-1 and S275-295 ranged 12–39µm^-1. The algorithm provided significant and robust (r²=0.93; p<0.0001) DOC estimates (pDOC), ranging 1–2598µM (RMSE=64µM). This indicates that, besides its simplicity, this method is capable of capturing the extremely high variability of DOC within the broad gradient of Arctic aquatic systems considered in this study. Apart from that, pDOC estimates could reproduce both DOC profiles and the DOC vs. salinity relationship across the Arctic Ocean (i.e., distinct sites with highly distinct hydrographic conditions). This potentially makes the method suitable for high-resolution and long-term in situ monitoring of DOC concentrations in Arctic aquatic systems from e.g., absorbance measurements from in situ nitrate sensors.

How to cite: Gonçalves-Araujo, R., Granskog, M., Osburn, C., and Stedmon, C.: A pan-Arctic algorithm for DOC concentrations from CDOM spectra, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-12720, https://doi.org/10.5194/egusphere-egu21-12720, 2021.

09:27–09:29

EGU21-13347

ECS

Dissolved oxygen gradients in hyporheic zones depend on fine sediment and associated respiration rates

David Piatka, Romy Wild, Jürgen Geist, Robin Kaule, Ben Gilfedder, Stefan Peiffer, and Johannes A. C. Barth

Dissolved oxygen (DO) in the hyporheic zone (HZ) is a crucial parameter for the survival of many stream organisms and is involved in a multitude of aerobic chemical reactions. However, HZ DO budgets are easily perturbed by climate change and anthropogenic processes that have caused increased deposition of fine sediments (< 2 mm) in many stream beds. The fine sediment fraction hampers exchange of DO-rich stream water with the HZ. In this study we performed a raster sampling approach (0.90 cm length x 1.50 cm width; 30 cm distance between sampling points) at sediment depths of 10 and 25 cm with a focus on DO and its stable isotopes (δ¹⁸O_DO). The aim was to analyze small-scale turnover patterns in a forested (site 1) and an anthropogenically influenced stream section (site 2) in a 3^rd order stream in southern Germany. Grain size analyses showed similar average fine sediment fractions at site 1 (42.5 ±13.7 %) and site 2 (46.3 ±10.8 %). They increased with depth at both sites (38.5 ± 6.3 %, 0-15 cm; 46.5 ± 17.4 %, 15-30 cm at site 1 and 40.6 ±4.5 %, 0-15 cm; 52.0 ±12.2 %, 15-30 cm at site 2). DO concentrations in the HZ ranged from 1.4 to 4.5 mg L^-1 (2.0 ±0.7 mg L^-1) and 1.5 to 1.8 mg L^-1 (1.7 ±0.1 mg L^-1) at site 1 and from 1.2 to 2.9 mg L^-1 (1.6 ±0.5) and 1.0 to 2.4 mg L^-1 (1.6 ±0.4) at site 2 at 10 and 25 cm depth, respectively. The low DO concentrations in the HZ suggest high DO consumption rates and reduced exchange with stream water. This is possibly a result of increased fine sediment proportions. However, other factors such as organic carbon contents and increased respiration rates may also influence DO gradients. In contrast, the stream water had an average DO concentration of 9.8 ±0.2 mg L^-1. Associated δ¹⁸O_DO values of the open water (23.4 ±0.1 ‰) differed from those of sediment waters that showed averages of +22.5 ±0.5 ‰ and +22.4 ±0.3 ‰ at site 1 and +22.5 ±0.4 ‰ and +22.3 ±0.2 ‰ at site 2 at 10 and 25 cm depth, respectively. These sedimentary values indicated dominant photosynthesis, even though due to absence of light in the subsurface this process seems unlikely. Therefore, kinetically-driven processes such as diffusion, interactions with Fe or unknown DO sources within the HZ might have caused such ¹⁶O-enriched values. Our findings suggest that the analyses of DO, δ¹⁸O_DO and fine sediment gradients in the HZ should be combined with stable carbon isotope measurements to further our understanding of hyporheic processes relevant for stream biota.

How to cite: Piatka, D., Wild, R., Geist, J., Kaule, R., Gilfedder, B., Peiffer, S., and Barth, J. A. C.: Dissolved oxygen gradients in hyporheic zones depend on fine sediment and associated respiration rates, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-13347, https://doi.org/10.5194/egusphere-egu21-13347, 2021.

09:29–09:31

EGU21-14334

ECS

Source-dependent variations in organic carbon degradation rates in lake sediments

Xingguo Han, Julie Tulo, Longhui Deng, Annika Fiskal, Carsten Schubert, Lenny Winkel, and Mark Lever

Lake sediments are globally important organic carbon (OC) sinks. Biomolecule chemical reactivity, adsorption and physical shielding have been suggested as important factors in controlling OC degradation rates in sediments. Yet, few studies have investigated the relative importance of these variables, or traced how OC from different organismal sources changes over time due to source-dependent variations in degradation rates.

We investigate the factors that control organic biomolecule degradation based on analyses of eukaryotic DNA, biomarkers, and (macro)molecule compositions (using pyrolysis-GC/MS) in sediments of five lakes in central Switzerland that differ in trophic state. We specifically target biomolecules of dominant phytoplankton groups (diatoms, green algae), and terrestrial vascular plants. We show that the decay rates of diatom DNA are significantly higher than those of diatom lipid biomarkers and (macro)molecules, consistent with the higher chemical reactivity of DNA. However, the decay rates of green algal DNA and vascular plant DNA are much slower than those of diatom DNA and similar in magnitude to their corresponding membrane lipids and (macro)molecules. In the case of vascular plant biomolecules (DNA, lignin, polyaromatic compounds), no significant biomolecule degradation was detected over the time scales studied (1-5 centuries).

Our results suggest that chemical reactivity and physical shielding, but not adsorption, are key variables controlling organic biomolecule decay in the lakes studied. In the case of green algae and vascular plants, greater chemical resistance of cell wall structural components to microbial attack appears to facilitate long-term preservation of even highly reactive, intramolecular compounds, such as DNA. These findings have important implications for the use of sedimentary eukaryotic DNA records to reconstruct past environmental changes.

How to cite: Han, X., Tulo, J., Deng, L., Fiskal, A., Schubert, C., Winkel, L., and Lever, M.: Source-dependent variations in organic carbon degradation rates in lake sediments, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-14334, https://doi.org/10.5194/egusphere-egu21-14334, 2021.

09:31–09:33

EGU21-15473

ECS

Nutrient cycling and productivity of a Himalayan Glacier Surface

Monica sharma shamurailatpam, Chris yates, Jon telling, Jemma l. wadham, and Ramanathan al

Nutrients deposited and cycled on the glacier surfaces are important not only because of their role in the global biogeochemical cycling in the downstream environments, but also because of their importance as a primary food source for microbes inhabiting glacial surfaces, their ice surface darkening properties, and the consequent potential for enhancing glacier melt. The present study focuses on the Chhota Shigri (CS) Glacier in the North-Western (NW) Indian Himalayan region. The dissolved organic carbon (DOC) concentration in the bare ice is relatively higher than the other aspects studied in the glacial environment of CS indicating that much of the active microbial activity occurring in the bare ice. Total inorganic nitrogen (TIN) is typically concentrated in the snow, which is the major contributor of NO₃^-. The rapid declining of TIN in the bare ice as compare to other aspects and its enrichment in DOC suggests for a more active microbial activity occurrence in the exposed ice rather than in isolated cryoconites holes in the Chhota Shigri Glacier. The net biological productivity indicates the dominance of net gross photosynthesis over respiration, suggesting a net autotrophic production in CS.

How to cite: sharma shamurailatpam, M., yates, C., telling, J., l. wadham, J., and al, R.: Nutrient cycling and productivity of a Himalayan Glacier Surface, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-15473, https://doi.org/10.5194/egusphere-egu21-15473, 2021.

09:33–09:35

EGU21-16201

ECS

Emerging controls of in-stream uptake at the catchment scale

Joni Dehaspe and Andreas Musolff

09:35–10:30

Meet the authors in their breakout text chats