BG4.2

Coastal environments have experienced a rapid transformation due to the expansion of tourism. This growth may enhance problems as over-saturation of spaces or environmental pollution. One of the main problems is associated with the collapse of environmental infrastructures, which may become saturated during high seasons. Indeed, wastewater treatment plants (WWTP) can be located in coastal areas delivering high concentrations of nutrient effluents into the marine environment. Alternatively, WWTP effluents are introduced into coastal aquifers via injection wells, given that the geological matrix is used to filter naturally the transported effluent solutes. However, the injection of significant amounts of WWTP effluents can modify the hydrogeological dynamics and enrich substantially the solute concentrations in groundwaters. Zones with a hydraulic connection between the coastal aquifer and the sea, these contaminated groundwaters may be transferred to coastal environments via Submarine Groundwater Discharge (SGD). Thus, SGD may act as a pathway delivering part of the WWTP-derived nutrients and pollutants into the marine environment, which may lead to eutrophication or harmful algal blooms. More importantly, such process may become threatening for society when the discharge occurs into bathing waters, affecting the ecosystem and perception of stakeholders.

In this study, we evaluate the role of SGD as a conveyor of nutrients from a karstic coastal aquifer affected by the injection of WWTP effluents to the Deià cove in Mallorca (Balearic Islands). Results show that the tourism seasonality changes the coastal aquifer natural dynamics during the dry season, delivering via SGD, nutrients concentrations above the maximum limits established by the Spanish and European water framework directives. Due to those enriched nutrient fluxes, the coastal water ecosystem has registered the highest values of ∂¹⁵N in Posidonia oceanica in the Balearic Islands and suffers periodic algal blooms, creating a conflict among stakeholders.

How to cite: Alorda-Kleinglass, A., Ruiz-Mallen, I., Diego-Feliu, M., Rodellas, V., and Gracia-Orellana, J.: Submarine Groundwater Discharge: The invisible mechanism that degrades the quality of crystalline bathing water in the Balearic Islands, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-15714, https://doi.org/10.5194/egusphere-egu21-15714, 2021.

Submarine groundwater discharge (SGD) is an important connection between fresh groundwater and the marine ecosystem. The scientific interest in SGD has grown considerably during the last decades due to the recognition of SGD in coastal environments as a significant source of nutrients and pollutants.  The Sahlenburg area (Northern Germany) is known by its highly permeable sediments and high rainfall precipitation that produces a large reservoir of groundwater.  Such characteristics are essential for industry, agriculture and drinking water supply with a large regional importance. In addition, this groundwater discharges in the form of highly productive springs directly into the adjacent tidal flats, with so far unknown effects on the local biogeochemistry.  The aim of this study was to characterize the spatial distribution of salinity, fluorescence dissolved organic matter (FDOM), dissolved organic matter (DOC) and total dissolved nitrogen (TDN) of the springs of Sahlenburg tidal flat area in Cuxhaven, Germany. We hypothesize that the SGD composition is changing on its way through the tidal flat due to biogeochemical factors. This may affect the composition of the water in the final part of the pathway with more influence of seawater. Porewater springs were sampled in February 2019 during low tide in three different types of locations in the tidal flat area: nearshore where the springs are located close to the vegetated shoreline (salt marsh), offshore approximately 70 meters from the vegetation and in the middle from both locations. In addition, porewater from a nearby sandy beach (around 500 meters away from the area of spring sampling), and surface samples from a nearby lake and seawater, were obtained. Salinity and FDOM were measured in situ, and DOC and TDN in the laboratory.  The preliminary data showed low average values for salinity in all springs (0.2-1.4), as well as in beach porewater, indicating strong influence of fresh groundwater in the whole area. When comparing the three spring location types, the lowest salinities were found offshore, and the highest nearshore. This difference could be due to the size of the springs, since nearshore springs usually were smaller when compared to offshore springs. Furthermore, depressions in the tidal flat relief close to nearshore springs favored seawater retention in pools during low tide. Additionally, we found higher average values for DOC and FDOM in the nearshore when compared with the other spring areas, but lower compared to the lake, beach porewater and seawater. The average values for TDN (272-452 µmol L^-1) in the groundwater springs were higher when compared to all other sample types (beach porewater, seawater, and lake water) in this study. These values suggest an anthropogenic input (e.g., agriculture influence) in the surrounding watershed and might stimulate primary productivity in the tidal flat. We conclude that groundwater springs in Sahlenburg tidal flat differ locally in their biogeochemistry due to different residence times, heterogeneity of sediment layers, and size of the springs.

How to cite: Carvalho da Silva, R., Waska, H., Schwalfenberg, K., and Dittmar, T.: Submarine groundwater discharge (SGD) in the springs of Sahlenburg tidal flat, Germany: a geochemical approach., EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-10833, https://doi.org/10.5194/egusphere-egu21-10833, 2021.

The characteristics of waves breaking on a beach can have significant impacts on how water infiltrates and influences coastal groundwater flows. The effects of continuous wave action on groundwater in coastal aquifers is important to understand to predict subsurface flows in beaches. This investigation will study how coastal wave dynamics in the swash zone impact the groundwater table using physical laboratory modelling and detailed image analysis that allows for high density spatial and temporal resolution degree of saturation measurements using unsaturated transparent soil as illustrated in Figure 1. Transparent soil methods will be applied to observe simulated wavedriven subsurface flows in a cross-section of a sandy beach. The objective of this study is to extend the current knowledge of how waves drive groundwater fluctuations by experimentally quantifying the time and length scales of flow within a beach.

Figure 1.

Transparent soil can be used to experimentally measure subsurface fluid/air flow and is used to quantify spatial and temporal saturation conditions every few seconds during an experiment. Digital image analysis allows for millimeter spatial resolution throughout the domain. Transparent soil is formed are applied by using crushed quartz rock in place of sand and an oil mixture with an identical refractive index to the grains (Peters et al. 2011). When the pores of the crushed quartz are saturated with the oil, the mixture appears transparent. When dry, the crushed quartz appears opaque. Change in colour is quantified, through digital image analysis, to measure degree of saturation throughout the domain (Sills et al. 2017)

This study applied transparent soil techniques in its first application to understand coastal processes by observing how incident waves infiltrate beaches and induce groundwater table fluctuations. Four tests are reported with variations in beach slope and wave properties, and the images are processed to quantify spatial and temporal degree of saturation variation. In each test, the swash interacted with the groundwater table by forming a partially saturated zone above the saturated zone of the beach. This partially saturated mound followed a consistent shape, that varied in size and rate of change primarily due to beach slope. The partially saturated zone is formed by a combination of capillary forces and downward infiltration, forming a continuously dampened zone in the beach. Finally, the results show a strong inverse correlation between the wetting front formed in a beach and the slope of the swash zone. Steeper slopes displayed much larger partially saturated mounds and were observed to do so in a slower manner compared to flatter slopes.

Peters, S. B., Siemens, G., and Take, W. A. (2011). “Characterization of Transparent Soil for Unsaturated Applications.” Geotechnical Testing Journal, 34(5).

Sills, L.-A. K., Mumford, K. G., and Siemens, G. A. (2017). “Quantification of Fluid Saturations in Transparent Porous Media.” Vadose Zone Journal, 16(2), 1–9.

How to cite: Siemens, G., Mulligan, R., and Benoit, D.: Physical Modelling of Wave-Driven Groundwater Flows in a Sand Beach Using Transparent Soil, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-13923, https://doi.org/10.5194/egusphere-egu21-13923, 2021.

The benthic oxygen consumption rate (OCR) has been widely used to measure the total benthic organic carbon degradation rate, while the oxygen distribution provides the general biogeochemistry state of marine sediments. In shallow coastal environments the light driven photosynthesis by benthic microalgae, resulting in large diurnal oscillations of oxygen concentration, further affects the oxygenation of the sediment. Yet, for permeable sediments, studies incorporating pore water advection driven by physical forces into oxygen consumption and distribution measurements are  still limited. Here we examine the combined effect of benthic oxygen production and advective oxygen transport on oxygen dynamics and consumption rate in a microphytobenthos-dominated sediment (permeability k =2 x 10^-11 to 5 x 10^-11 m²) in a laboratory simulation with stirred benthic chambers at 40 rpm. Under alternating light (50 μE m⁻² s^-1) and flow regimes, oxygen concentration, penetration depth and consumption rates were monitored by means of micro-profiling and planar optode measurements. In all cases, we found that oxygen penetration depth increased up to a factor of 2 with pore water flow simulation. On the other hand, advective transport was found to reduce maximum oxygen concentration in the sediment by up to 30 %.  The OCR were up to 2-times higher with only light (28 ± 3.5 µM/min) compared to combined light and flow simulation, however the total oxygen uptake was generally uniform in all chambers (41.83 ± 5.9 mmol/m² d^-1), suggesting the local redistribution of oxygen with flow without marked overall changes in O₂ consumption. Our result emphasized the importance of advective transport controlling benthic oxygenation in photic permeable sediment.

How to cite: Werna, W. and Forster, S.: Benthic Oxygen dynamics: Influence of pore water advection and microphytobenthos in a permeable sediment, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-1239, https://doi.org/10.5194/egusphere-egu21-1239, 2021.

The availability of major nutrients, nitrogen (N) and phosphorus (P), largely controls primary productivity in eastern boundary upwelling systems. The oxygen minimum zone (OMZ) on the Namibian shelf is characterized by high productivity and extraordinarily high particulate organic carbon (POC) contents (up to 19 % dry weight) in the surface sediments. The anaerobic degradation of POC by bacterial sulfate reduction leads to the production of hydrogen sulfide (H₂S) that supports extensive communities of large sulfur bacteria Thiomargarita namibiensis in surface shelf sediments. These bacteria oxidize sulfide by reducing nitrate (NO₃^-) to either ammonium (NH₄⁺) or dinitrogen (N₂). Thiomargarita also affect phosphorus cycling by intracellular incorporation of polyphosphates and extracellular formation of hydroxyapatites. In order to understand and quantify the complexity of the coupled benthic cycles of C, N, P, S, Fe in the Benguela Upwelling System, a reaction-transport model (RTM) was used to simulate sediment biogeochemical data collected from the RV Meteor cruise (M157, August 4th-September 16th 2019) off Namibia. This allowed deeper insights into the role of sulfur-oxidizing bacteria on P and N fluxes across the sediment surface. Results are presented that point toward potentially strong feedbacks by Thiomargarita on primary production in response to ongoing global warming and ocean deoxygenation.

How to cite: Chuang, P.-C., Zabel, M., Sommer, S., Scholz, F., Vosteen, P., and Dale, A. W.: Impact of Thiomargarita on the rates of N, S and P turnover in mudbelt sediments from the Benguela Upwelling System: a model study, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-1837, https://doi.org/10.5194/egusphere-egu21-1837, 2021.

Excess bioavailable nitrogen (N) is the key driver of coastal eutrophication, thus knowledge on the fate of N in coastal systems is imperative for improving eutrophication mitigation measures. In the coastal Baltic Sea, benthic heterotrophic denitrification, the main process of bioavailable N-removal from a coastal system, has recently been suggested to be seasonally limited by labile organic carbon (OC) availability¹ - despite the system´s richness in labile organic matter from long-term eutrophication. This challenges our common understanding of the intrinsic link between C- and N-cycling, and highlights the need for a more advanced concept of OC availability. Hence, in this project, we (i) extensively characterized the biochemical composition of coastal OC beyond traditional descriptors of ‘lability’, applying techniques such as isotopic fingerprinting and Fourier transform ion cyclotron resonance mass spectrometry, and (ii) concurrently quantified benthic nitrate reduction rates both with and without addition of easily degradable OC (glucose), to ultimately confirm and understand proposed OC-limitation of denitrification in coastal sediments. All measurements were done in high temporal and spatial resolution at the southern coast of Finland, covering a three-month period from late winter to early summer that included the peak annual input of fresh organic matter to the benthic system by the phytoplankton spring bloom. First results will be presented and their implications for understanding seasonal N turnover and coastal eutrophication dynamics will be discussed.

¹Hellemann D, Tallberg P, Aalto SL, Bartoli M, Hietanen S (2020) Seasonal cycle of benthic denitrification and DNRA in the aphotic  coastal zone, northern Baltic Sea. Mar Ecol Prog Ser 637:15–28

How to cite: Hellemann, D., Aalto, S.-L., Asmala, E., Jilbert, T., Kiljunen, M., Koch, B., Norkko, J., and Norkko, A.: Unraveling seasonal carbon-limitation of benthic nitrate-reduction in the coastal Baltic Sea, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-2924, https://doi.org/10.5194/egusphere-egu21-2924, 2021.

Dissolved silica (DSi) is an important macronutrient in the marine environment, necessary for growth of many aquatic organisms. Yet, DSi marine cycle is still not fully recognized, especially in dynamic, coastal zones. Although DSi is mainly transported to the sea by rivers, benthic fluxes of DSi, which originate from dissolution of the siliceous remains in the sediments, can also represent an important source of bioavailable silicon in the ocean. Benthic DSi fluxes are mainly powered by diffusion, but their rates are strongly shaped by the benthic fauna. Still, the role of benthos in these processes is not fully recognized. The main goal of this study was to investigate how various environmental factors and benthic fauna may shape the coastal cycle of Si in coastal environments during different seasons.

Our study was conducted in the shallow coastal ecosystems of the southern Baltic Sea characterized by contrasting environmental conditions: shallow, brackish and enclosed Szczecin Lagoon (Oder river estuary), dynamic open waters near Łeba with relatively low anthropogenic influence, enclosed Puck Bay and Vistula prodelta. We investigated both shore ecosystems (app. 0.5 m depth) and deeper areas (from 6 up to 60 m depth). DSi concentrations in the bottom waters and environmental characteristics (T, S, O₂, sediment organic matter) were investigated at 6 stations, during three seasons (winter, spring and autumn) in years 2019-2020 with s/y Oceania (IOPAN) and directly from the shore. Additionally, samples from shore stations were collected during summer. DSi benthic fluxes were determined at each station by performing ex situ incubations of sediment cores (n = 4-5) with natural benthic assemblages. The benthic organisms in studied cores were collected, identified, counted, and weighed.

The lowest fluxes were measured at sandy stations while highest return fluxes were observed at muddy sites. High variability in DSi benthic fluxes along studied localities was observed, ranging from -1.11 mmol d^-1m^-2 in summer at shore station in the Puck Bay and up to 6.79 mmol d^-1m^-2 in Szczecin Lagoon in autumn. We used  Gaussian Generalized Linear Models (GLMs) to estimate the role of environmental conditions, benthic fauna characteristics  and interactions among them in the variability of DSi benthic flux across studied localities. The most important predictors for the fluxes were all pair-wise interactions of temperature, total organic carbon, the C/N molar ratio, and the density of benthic macrofauna. Both interaction terms that included C/N ratio, a measure of organic matter quality (i.e. low C/N ratio indicates higher quality), were associated with increased DSi uptake by the sediment. Further, the interaction term between T and benthic marcofauna density was also linked to negative benthic fluxes of DSi. In contrast, the interaction of T and TOC caused a strong increase in DSi return fluxes.

How to cite: Borawska, Z., Szymczycha, B., Silberberger, M. J., Szczepanek, M., Koziorowska-Makuch, K., and Kędra, M.: Spatial and seasonal variation in dissolved silica benthic fluxes in the shallow zones of the southern Baltic Sea, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-5163, https://doi.org/10.5194/egusphere-egu21-5163, 2021.

The Argentina Continental Margin represents a unique geologic setting to study interactions between bottom currents and sediment deposition as well as their impact on (bio)geochemical processes, particularly the cycling of iron (Fe). Our aim was to determine (1) how different depositional conditions control post-depositional (bio)geochemical processes and (2) how stable Fe isotopes (δ⁵⁶Fe) of pore water and solid phases are affected accordingly. Furthermore, we (3) evaluated the applicability of δ⁵⁶Fe of solid Fe pools as a proxy to trace past diagenetic alteration of Fe, which might be decoupled from current redox conditions. Sediments from two different depositional environments were sampled during RV SONNE expedition SO260: a site dominated by contouritic deposition on a terrace (Contourite Site) and the lower continental slope (Slope Site) dominated by hemipelagic sedimentation. Sequentially extracted sedimentary Fe [1] and δ⁵⁶Fe analyses of extracts and pore water [2,3] were combined with sedimentological, radioisotope, geochemical and magnetic data. Our study presents the first sedimentary δ⁵⁶Fe dataset at the Argentina Continental Margin.

The depositional conditions differed between and within both sites as evidenced by variable grain sizes, organic carbon contents and sedimentation rates. At the Contourite Site, non-steady state pore-water conditions and diagenetic overprint occurs in the post-oxic zone and the sulfate-methane transition (SMT). In contrast, pore-water profiles at the Slope Site suggest that currently steady-state conditions prevail, leading to a strong diagenetic overprint of Fe oxides at the SMT. Pore-water δ⁵⁶Fe values at the Slope Site are mostly negative, which is typical for on-going microbial Fe reduction. At the Contourite Site the pore-water δ⁵⁶Fe values are mostly positive and range between -0.35‰ to 1.82‰. Positive δ⁵⁶Fe values are related to high sulfate reduction rates that dominate over Fe reduction in the post-oxic zone. The HS^- liberated during organoclastic sulfate reduction or sulfate-mediated anaerobic oxidation of methane (AOM) reacts with Fe²⁺ to form Fe sulfides. Hereby, light Fe isotopes are preferentially removed from the dissolved pool. The isotopically light Fe sulfides drive the acetate-leached Fe pool towards negative values. Isotopic trends were absent in other extracted Fe pools, partly due to unintended dissolution of silicate Fe masking the composition of targeted Fe oxides. Significant amounts of reactive Fe phases are preserved below the SMT and are possibly available for reduction processes, such as Fe-mediated AOM [4]. Fe²⁺ in the methanic zone is isotopically light at both sites, which is indicative for a microbial Fe reduction process.

Our results demonstrate that depositional conditions exert a significant control on geochemical conditions and dominant (bio)geochemical processes in the sediments of both contrasting sites. We conclude that the applicability of sedimentary δ⁵⁶Fe signatures as a proxy to trace diagenetic Fe overprint is limited to distinct Fe pools. The development into a useful tool depends on the refining of extraction methods or other means to analyse δ⁵⁶Fe in specific sedimentary Fe phases.

References:

[1]Poulton and Canfield, 2005. Chemical Geology 214: 209-221.
[2]Henkel et al., 2016. Chemical Geology 421: 93-102.
[3]Homoky et al., 2013. Nature Communications 4: 1-10.
[4]Riedinger et al., 2014. Geobiology 12: 172-181.

How to cite: Melcher, A.-C., Henkel, S., Pape, T., Meixner, A., Kasemann, S. A., Köster, M., Volz, J., Wilckens, H., Miramontes, E., Geibert, W., Schwenk, T., Frederichs, T., Staubwasser, M., and Kasten, S.: The impact of depositional conditions on biogeochemical cycling of iron and stable iron signatures in sediments of the Argentina Continental Margin, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-8356, https://doi.org/10.5194/egusphere-egu21-8356, 2021.

The Arctic Ocean is currently experiencing rapid oceanographic shifts and significant sea-ice loss as a result of regional atmospheric and oceanic warming. The Barents Sea is a notable example of these phenomena, having seen a near 40% decline of its April sea-ice extent since 1979, and a progressive northward expansion of Atlantic Water (i.e., Atlantification). Such changes affect primary productivity and nutrient cycling in ways that remain poorly understood. Longer ice-free periods and the inflow of warmer Atlantic Water are expected to lead to extended bloom seasons on short, near-future timescales and therefore increase nutrient uptake in upper water layers. The benthic recycling of nutrients is believed to play an important part in replenishing nutrient inventories in overlying waters thus maintaining high primary productivity over the continuously expanding growth season. Therefore, it is crucial to increase our understanding of nutrient dynamic controls in changing oceans to make more accurate predictions and decipher the complex feedbacks involved in these evolving environments. However, most efforts to constrain and quantify nutrient fluxes so far have been directed at silicon, nitrogen or iron. This study aims to provide specific insight into phosphorus (P) cycling through its response to OM fluctuations and coupling with iron cycling. An integrated data-model approach was used to investigate the dynamics of P cycling at the sediment-water interface across five locations along the 30°E meridian that were drilled in the framework of the ChAOS project in the Barents Sea. The model approach allowed to explore the sensitivity of P cycling to plausible ranges of reactive iron and OM inputs. Greater inputs of reactive iron were found to decrease benthic phosphate fluxes (J_PO4) whereas greater inputs of OM increased phosphate return to the water column. The quality of these inputs is equally significant: J_PO4 decreased when iron hydroxides were made more reactive and increased with more reactive OM. Our findings indicate that variation in climatically sensitive processes, such as burial of terrestrial sediments and iron cycling, could represent powerful feedbacks on J_PO4 through adsorption/desorption mechanisms. Results also reveal significant oceanographic controls on J_PO4, suggesting Atlantification of the Barents Sea will play into future phosphate availability.

How to cite: Lefebvre, C., S. Freitas, F., Hendry, K., and Arndt, S.: Benthic Cycling of Phosphate in the changing Arctic Ocean, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-8754, https://doi.org/10.5194/egusphere-egu21-8754, 2021.

On the seabed of oxygen minimum zones (OMZ), embedded in organic-rich sediments, large sulfur bacteria (LSB) fulfil an important ecological role by detoxifying the overlying bottom waters. Thiomargarita Namibiensis and Beggiatoa spp. are chemoautotrophic microorganisms that reduce sulfur compounds to create biomass and link by doing so the carbon, sulfur, oxygen and nitrate cycle very efficiently. This particular ability make life in suboxic and hypoxic coastal waters feasible. Nevertheless, due to the complexity of sulfur oxidation and its various pathways the quantification of such activity is of great complexity. Hereby, we describe a model framework of LSB activity to implement intrinsic properties of the bacteria based on field observations and numerical modelling validations, linking the stoichiometry and energy conservation efficiency of LSB while counting for the reduced sulfur pools and its partitioning sub-products.

How to cite: Herran, N., Schmidt, M., Mohrholz, V., and Schulz-Vogt, H.: The role of large benthic sulfur bacteria in biogeochemical cycles - a model approach., EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-13368, https://doi.org/10.5194/egusphere-egu21-13368, 2021.

Benthic nitrogen fixation by heterotrophic non-cyanobacterial diazotrophs (NCDs) is common in anoxic marine sediments, however, it is currently unclear how resuspension of sediments affects this activity. Moreover, physical mixing processes are strongest in shallow coastal waters where permeable sediments prevail and anoxic conditions rarely occur. It is therefore of interest to understand whether such coastal sites provide ecological niches for NCDs. In order to gain insight into NCD nitrogen fixation during sediment resuspension, slurry incubations were carried out with nearshore sediments from stations along the Southern Baltic coastline and, for comparison, with anoxic sediments from the Gdansk Deep. Parallel to this, we carried out separate incubations treated with sodium molybdate, an inhibitor of sulfate reducing bacteria (SRB), to differentiate SRB activity from total NCD activity. Our data show low rates of nitrogen fixation by NCDs and indicate that SRBs (e.g. Desulfovibro) are actively fixing nitrogen. Nitrogen fixation rates varied greatly between locations, influenced by sediment grain size and POC-loading. Interestingly, nitrogen fixation took place despite of micromolar concentrations of inorganic nitrogen, which implies that NCDs may be more resilient towards N-stress than formerly expected. In conclusion, our experimental study supports previous findings of stimulation of nitrogen fixation by sediment resuspension, even in permeable sediments, however, at low rates.

How to cite: Liesirova, T., Aarenstrup-Launbjerg, T., Riemann, L., and Voss, M.: The impact of sediment resuspension on non-cyanobacterial nitrogen fixation along the Southern Baltic coastline, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-11032, https://doi.org/10.5194/egusphere-egu21-11032, 2021.

Inter-tidal wetlands at mangroves, salt marshes and sea grass bed are important carbon reservoirs that play a significant role in climate change mitigation. However, the lack of large-scale quantification and source identification of sediment organic carbon (SOC) in inter-tidal wetlands hampers the assessment of carbon storage potential in these systems. In this research, we hypothesized that SOC in the inter-tidal wetlands of Chinese East Coast were mainly from three potential sources (terrestrial soil, marine phytoplankton, local C₃ and C₄ plants as mangrove and salt marsh plants ). Based on elemental ratios and stable carbon isotope of core sediment from the inter-tidal wetlands along the east coast of China, we quantified the contribution of organic carbon (OC) sources and explored the hydrological and plant drivers controlling the variations of OC source contribution among different coastal environmental settings. We found SOC in the large estuaries (river runoff more than 50 billion m³/a) originated predominantly from terrestrial soil OC (46±9%), while the primary OC source of the smaller estuaries was marine phytoplankton OC (61±14%). These results suggested that the contribution of terrestrial soil OC increased with river runoff, whereas the share of marine phytoplankton OC decreased with runoff. Moreover, while mangroves played a substantial role in carbon storage at the southern part of the coast, our estimates revealed a sharp decline in the contribution of mangrove OC since the 1980s. These findings indicate carbon storage in the inter-tidal wetlands varies among contrasting coastal environmental conditions and among wetlands with different ages, providing implications for inter-tidal wetlands as an important carbon sink in the global carbon budget.

How to cite: Ding, Y. and Wang, D.: Identification and Quantification of Sediment Organic Carbon in the Inter-tidal Wetlands of Eastern Coast of China, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-3699, https://doi.org/10.5194/egusphere-egu21-3699, 2021.

Deoxygenation in response to eutrophication and climate change in coastal systems is increasing worldwide. Low oxygen conditions cause the chemical transformation of redox-sensitive trace metals (e.g. molybdenum and uranium) in seawater, and their subsequent transport to the sediment. Sedimentary trace metal contents can therefore be used as a record of changes in bottom water oxygen conditions allowing the history of deoxygenation to be reconstructed. However, most trace metal studies have focused on strongly reducing and sulfidic settings, leaving mildly reducing and oxygenated (but eutrophic) settings vastly understudied. Currently, it is unknown to what extent existing trace metal redox proxies are applicable to reconstruct oxygen conditions in coastal zones experiencing mild deoxygenation, despite the fact that such areas occupy vast stretches of the coastal oceans. Here, we study trace metal enrichments in 13 European coastal marine sites with varying bottom water redox conditions and depositional environments. Our data demonstrates that sedimentary molybdenum and uranium contents are sensitive to deoxygenation across a range of settings, although the mechanisms of enrichment may vary. Improved understanding of molybdenum and uranium dynamics in mildly reducing coastal settings will facilitate the development of reliable and widely applicable molybdenum and uranium-based redox proxies.

How to cite: Paul, M., van Helmond, N. A. G. M., Slomp, C. P., Jokinen, S. A., Virtasalo, J. J., Filipsson, H. L., and Jilbert, T.: Sedimentary trace metals: improving proxies for deoxygenation in coastal European seas, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-5499, https://doi.org/10.5194/egusphere-egu21-5499, 2021.

Tropical coral reefs are both biologically diverse and economically important ecosystems, yet are under threat globally, facing a multitude of stressors including global warming, ocean acidification, nutrient loading, over-fishing and sedimentation. Reef building corals precipitate an aragonite skeleton (CaCO₃), which forms the base of the coral reef ecosystem, but it is this skeleton, which makes them sensitive to changes in ocean pH. To precipitate their skeletons, corals raise their internal pH, as seawater pH decreases this increases the energy demands needed to facilitate calcification. Furthermore, reductions in coral calcification has significant implications for reef health, potentially altering community structure with reef-wide consequences. Global ocean pH is decreasing due to rising atmospheric concentrations of CO₂, however, dynamic ecosystems, alongside carbon and freshwater input from land, may result in coastal ocean pH being lower than is predicted by open ocean models. While it is predicted than ocean pH will decrease by 0.3 units by 2100 if emissions are not curbed, coral reefs, particularly those near major river outflow, may already be experiencing pH values similar to that of future scenarios.

Our aim was to determine the factors which influence pH in coastal reef systems and thus potentially mitigate or exacerbate atmospheric CO₂ mediated ocean acidification. This was achieved by contrasting reefs in distinct environmental settings and collecting data over a sufficient temporal resolution to permit the identification of pertinent drivers. To accomplish this we deployed fixed point observatories in the distinct reefs of Belize (fore and back reef sites), Fiji and Dominica. These custom-built platforms were equipped with a spectrophotometric pH sensor and a conductivity, temperature and dissolved oxygen (CT-DO) sensor from which data was logged at 30-120 minute intervals.

A strong diel cycle in pH, O₂ and temperature was observed at all reef sites in response to the changing balance of respiration and photosynthesis. However, the range of these changes varied between the different sites - Belize fore reef (pH 7.849­ – 8.000), Belize back reef (pH 7.897 – 8.039), Fiji (pH 7.951 – 8.0950) and Dominica (pH 7.843 – 8.144). Meteorological conditions, such as wind direction, affected the amplitude of diurnal pH variability and its relationship with other parameters, likely by influencing mixing and the spatial distribution of seawater and freshwater endmembers. The relationship between pH and O₂ varied between sites reflecting differences in ecosystem processes (e.g. calcification and primary production) and ecosystem composition (e.g. hard coral and algae cover, proximity to seagrass). Our data confirms that different reef sites are subject to varying degrees of ocean acidification and that controls on pH vary between environments. Furthermore, it highlights the need for widespread high-resolution monitoring to identify, and where possible enact protective measures, in vulnerable reef regions. As coral reefs continue to experience ocean acidification our data also serves to document baseline conditions against which future changes can be assessed.

How to cite: Cryer, S., Evans, C., Carvalho, F., Fowell, S., Martincic, U., Andrews, G., Rosado, S., Young, A., de Ramon, A., and Loucaides, S.: Ecosystem composition and environmental factors as drivers of pH on Barrier Reefs, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-12156, https://doi.org/10.5194/egusphere-egu21-12156, 2021.

There is strong evidence that the source of terrestrial carbon and iron geochemistry play an important role in organic carbon transport and preservation in coastal and marine sediments ^1,2,3. There is a global drive to increase forestry and Scotland is undergoing a period of afforestation⁴. A portion of this is being planted in sea loch (fjord) catchments; however the effect of this increase in forestry on coastal carbon transport and storage is poorly understood. Fjord systems have recently been identified as significant terrestrial carbon stores⁵ therefore understanding how afforestation of these catchments changes the carbon dynamics from source to sea, is key.

In this study Mossbauer spectroscopy, XRD and XRF are used to examine how iron concentration and speciation differs within Scottish fjord sediments. This preliminary data provides insight of the variation in iron speciation in fjord systems, processes controlling iron transport and speciation and potential mineral binding mechanisms in coastal sediments. This enables us to start addressing key knowledge gaps in the transport of organic carbon and iron from land (forested source areas) to sea (fjords). Thus, contributing to our overarching aim of tracing the movement and interactions of organic carbon across the terrestrial - aquatic interface.

Through this project, further analytical techniques such as biomarker analysis, isotopic analysis and SEM, will be used to improve our understanding of source to sea processes in fjord systems throughout the northern hemisphere. This will hopefully enable improved understanding and quantification of local and national carbon stocks. Further insights into carbon and iron burial mechanisms may allow us to tailor land use and management around fjord environments to maximise natural carbon storage.

How to cite: Kellock, C., Smeaton, C., Shah, N., Austin, W., and Schroeder, C.: Source to Sea: Transport of organic carbon and iron from forested environments to coastal waters and sediments, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-12008, https://doi.org/10.5194/egusphere-egu21-12008, 2021.

The flux of terrigenous organic carbon across estuaries is an important and changing component of the global carbon cycle, but it is poorly understood. It has been proposed that estuaries can act either as a transporter of terrestrial dissolved organic carbon (DOC) to the ocean or as a reactor system in which DOC can be buried or transformed into carbon dioxide and released to the atmosphere. However, there is no clear understanding of the factors that drive estuaries to behave in one way or the other. Here we present the results from a study conducted in thirteen British estuaries which drain catchments of diverse land-uses under different hydrological conditions. Our data show that land-use influences the composition of the dissolved organic matter (DOM), the mixing dynamics of DOC and the quantity of DOC exported off the estuaries. Estuaries, whose catchments are less intensively managed and represent more natural ecosystems (average proportion of arable and (sub)-urban land-use ~12 %), contain a higher proportion of biologically-refractory “humic-like” DOM, which is transported conservatively across the salinity gradient. In contrast, estuaries whose catchments are more intensively managed (average proportion of arable and (sub)-urban land-use ~32 %) contain a high fraction of “protein-like” DOM which is transported non-conservatively, and thus suggest the existence of additions and removal processes across the salinity gradient. Furthermore, estuaries with more intensively managed catchments tend to export more DOC to coastal areas than they receive from rivers. Our results indicate that future changes in land-use have the potential to alter aquatic fluxes of terrigenous DOM and the fate of the constituent carbon.

How to cite: Garcia-Martin, E. E., Sanders, R., Evans, C. D., Kitidis, V., Lapworth, D. J., Rees, A. P., Spears, B., Tye, A., Williamson, J. L., and Mayor, D. J.: Influence of land-use on the dynamics, quantity and composition of the organic matter transported across estuaries, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-1298, https://doi.org/10.5194/egusphere-egu21-1298, 2021.

Marine phytoplankton are crucial for ecosystem function and responsible for almost half of world’s primary production. In order to grow and reproduce phytoplankton need sufficient amount of macro and micro nutrients. Nutrient concentrations are changeable in different water mases and dependable on different natural and anthropogenic sources such as terrestrial water inputs, recycling by sloppy feeding, remineralization with bacteria and atmospheric deposition. High nutrient input to oligotrophic regions raises phytoplankton biomass that leads to higher organic matter production and heterotrophs` development.  Anthropogenic nutrient inputs are considered as the main cause of coastal eutrophication. Marine lipids, dominantly produced by phytoplankton, are good biogeochemical traces of organic matter origin and processing in marine environment and phytoplankton adaptation to environmental perturbations. They are important for multiple cell mechanisms functioning.

The goal of this research was to investigate the influence of a point source of nutrients on organic matter production and lipid composition as a consequence of phytoplankton acclimation to different nutrient loads. We sampled at two geographically close stations in the Krka River Estuary mouth, oligo- to mesotrophic Martinska station and station in vicinity of the town of Šibenik that is under high anthropogenic influence. Samples were taken from three depths (above, on and below halocline) and in four different seasons covering annual cycle. Lipid classes were characterized by thin–layer chromatography–flame ionization detection. Data are supported by hydrographic, dissolved organic carbon and particulate organic carbon parameters. We will discuss the changes of organic matter accumulation and estuarine lipid biogeochemistry caused by human activity.

Acknowledgement

This research was financed by the Croatian Science Foundation project BiREADI (IP-2018-01-3105).

How to cite: Novak, T., Gašparović, B., Vrana Špoljarić, I., and Čanković, M.: High local nutrient input influence on oligotrophic phytoplankton production and lipid biogeochemistry, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-15187, https://doi.org/10.5194/egusphere-egu21-15187, 2021.

Global warming increases the thawing rate of the permafrost in high northern latitudes. The Arctic soil organic carbon accounts for over 50% of global soil carbon which is roughly twice the amount present in the atmosphere. An increasing amount of the newly mobilized old organic carbon, and its associated compounds, originating from permafrost thaw, is expected to be delivered to the Arctic Ocean by rivers and groundwater discharges all along the Arctic coastline. Absorbance and fluorescence spectroscopy can be used to identify a specific optical signature of permafrost-derived solutes with the objective of studying their transport and transformation to coastal waters. Emission-excitation spectra (EEMs) from three sampling sites along the coastal area of the delta were assessed and parallel factor analysis (PARAFAC) was used to identify three different components characterizing the origin and the nature of the organic carbon present in various types of samples (massive ice, groundwater, seawater and water samples on top/bottom of slumps). This study suggests that the carbon originating from the thawing of the permafrost could indeed be traced along the coastal area of the Delta.

How to cite: Flamand, A., Chaillou, G., Kipp, L., and Whalen, D.: Characterization of an optical signature of the DOM of the coastal permafrost in the Mackenzie delta, by PARAFAC analysis, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-16506, https://doi.org/10.5194/egusphere-egu21-16506, 2021.

Past century increases in terrigenous N and P inputs to the ocean due to industrialization, agricultural practices and wastewater have been reported to have dramatic consequences for ecosystems in various coastal regions. Yet, the impacts of increased nutrient inputs through river transports and atmospheric deposition on the coastal and open ocean carbon cycle have yet to be quantitatively investigated at the global scale. To address this gap of knowledge, we enhanced the ocean biogeochemical model HAMOCC at a horizontal resolution of around 0.4° in order to improve the representation of temporal changes of riverine fluxes and of coastal ocean dynamics in the model. Through a series of simulations with differing model forcings, we isolated individual effects arising from (1) increasing atmospheric CO₂ levels, (2) a changing physical climate and (3) alterations in oceanic inputs of terrigenous P and N inputs, all over the 1905 to 2010 period. Our results indicate a strong response of the coastal ocean ecosystem to increased terrestrial nutrient inputs, which induce the global coastal Net Primary Production (NPP) to increase by 14% over the simulation time span. This eutrophication signal is, furthermore, partly exported to the open ocean, which undergoes an increase in NPP of 1.75 Pg C yr^-1, or 4 % in relative terms, in the simulations, owing to the cross-shelf export of 33-46% of the anthropogenic P and N inputs to the coastal ocean. As a whole, increased P and N inputs lead to an overall global ocean NPP rise of around 2.15 Pg C yr^-1, or 5% (combined coastal and open ocean). This net increase attributed to land-ocean couplings exceeds the simulated global oceanic NPP decrease of 4 % associated with stronger upper ocean thermal stratification over the time span, a feedback that been under stronger scrutiny in published literature. Our results suggest that increased riverine nutrient concentrations due to anthropogenic activities may also have substantial impacts for ecosystems in the open ocean, in contrary to what was assumed until now, although this is dependent on the rate of transfer of the nutrients from the coastal to the open ocean.

How to cite: Lacroix, F., Ilyina, T., Mathis, M., Laruelle, G. G., and Regnier, P.: Past century increases of terrestrial nutrient inputs impact both the coastal and open ocean carbon cycle, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-9419, https://doi.org/10.5194/egusphere-egu21-9419, 2021.

The North Sea’s location has made it an object of oceanographic study for over 150 years. But within the more recent past, this shelf sea with its strong continental shelf pump system has captivated science’s attention for a different reason: artificial ocean alkalinization (AOA). Through the enhancement of metabolic pathways or the addition of dissolved alkaline minerals into the ocean to draw down atmospheric CO₂, AOA seems to offer a compelling option to mitigate climate change. But without judicious thought, human interaction could come at great cost. Alkalinity plays an integral role within a system’s biogeochemistry, and disruptions may result in large scale changes to small scale environments. But, within the North Sea, an extremely effective tracer of total alkalinity has been found, ²²⁸Ra. This isotope along with ²²⁶Ra, comprise the two long-lived isotopes of the Radium Quartet with half-lives of 1600yrs and 5.8yrs, respectively. As naturally occurring, sediment-derived radioisotopes, ²²⁸Ra and ²²⁶Ra have the potential to map the water-mass composition, the distribution patterns, and the associated timescales from shelf-sediment interaction to the open ocean. Furthermore, providing a possible method to shed light on the North Sea’s water-mass distributions, changing alkalinity patterns, and the potential effects of AOA. Over the course of this study, we will identify the basin’s key water-mass patterns and end members through the use of ²²⁶Ra and ²²⁸Ra, complemented by hydrological and carbonate parameters collected from the North Sea during the summers of 2018 and 2019 aboard the German research vessel Heincke. By employing inverse modelling techniques, we will investigate how AOA can cause changes on both a local and potentially global scale.

How to cite: Mears, C., Thomas, H., and Wolschke, H.: Using long-lived radium isotopes as water-mass tracers in the North Sea and investigating their use for tracking artificial ocean alkalinization, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-8232, https://doi.org/10.5194/egusphere-egu21-8232, 2021.

Eutrophication in coastal waters caused by non-treated urban discharges has been considered one of the most important effects of global change. At tropical latitudes, nutrient dynamics may be especially intense due to increased metabolic responses supported by high temperatures and solar incidence throughout the year. In addition, short-term variations, such as in rainfall and the tidal regime, may determine important changes in nutrient concentrations and the subsequent trophic status of coastal waters, which are still neglected especially during nocturnal periods due to common logistical constraints. Here, we assessed 24-h variations of water quality during the winter season in a tropical eutrophic bay that receives large inputs of nutrients from non-treated urban effluents (Guanabara Bay, RJ, Brazil). We measured concentrations of dissolved forms of nutrients (nitrate, nitrite, N-ammoniacal, phosphate, and silicate) and carbon (DOC), and oxygen (DO) associated with temperature, salinity, and pH in surface waters each 2h over two daily cycles (July and August 2018). Water samples for nutrients and DOC were preserved for later analysis, while other variables were measured in the field. A biomonitoring system with a submersible pump was used to collect surface coastal waters without bubbling, and along a 70 m pipe from the beach to the field lab. In turn, meteorological data were obtained from a city weather station located ~6 Km from the sampling area. The monthly accumulated precipitation with respect to the 24-h cycle in July was ~70% lower than in August (58 and 16 mm, respectively), although only that in July has showed a rainfall event during the sampling period. As a result, average DOC and N-ammoniacal concentrations in surface waters were ~50% lower, while nitrate, silicate and DO concentrations ~56, 164 and 50 % higher, respectively, during the 24-h cycle in August compared to July. Also, waters were slightly more basic and less saltier in August, contrasting with similar average values of phosphate concentrations and temperature between both sampling periods. Finally, DO concentrations indicated an intense metabolism, varying from a peak of supersaturation with high solar incidence to net autotrophy (2 pm) to undersaturation values as a proxy of net heterotrophy after the nocturnal period (6 am). In conclusion, this short-term study showed that higher monthly accumulated precipitation may dilute high DOC and N-ammoniacal concentrations in coastal aquatic ecosystems undergoing anthropogenic eutrophication. On the other hand, silicate and nitrate concentrations might be related to higher runoff inputs from the watershed. The event of precipitation in July also confirmed a drastic increase in nitrate concentrations, likely due to inputs from the watershed. Therefore, our findings reveal the complexity of accumulated and immediate effects of rainfall on nutrient levels in tropical coastal waters, which  highlight the importance of biomonitoring studies specially in urban areas.

How to cite: Fonseca, T., Bittencourt Peixoto, R., Pinho, L., Cotrim da Cunha, L., Pollery, R., and Marotta, H.: Short-term changes of anthropogenic eutrophication with precipitation in tropical coastal waters (Guanabara Bay, Rio de Janeiro, Brazil), EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-14046, https://doi.org/10.5194/egusphere-egu21-14046, 2021.