Displays

Benthic oxygen fluxes measured in the south-eastern North Sea indicate a prominent annual cycle characterized by a low level between mid-autumn (October) and early spring (March), a slow increase since mid-spring (April) till late summer (late August/early September), and a subsequent accelerated decrease in early autumn (September). A significant positive correlation between the benthic oxygen flux, total organic carbon (TOC) and macrobenthic biomass in surface sediments suggests their potential mutual dependence. To understand their interactions quantitatively, 3-D benthic-pelagic coupled modelling was used to reconstruct the benthic status. Simulation results based on a satisfactory agreement with field data reveal that the benthic oxygen flux is determined by not only pelagic drivers (hydrodynamics, temperature and primary production) but also internal dynamics associated with the interaction between organic carbon and benthic fauna, and bedform morphodynamics. The slow increase of benthic flux since mid-spring till late summer is a compound effect of several processes with dominant contribution by accumulation of labile OC and growth of macrobenthos in surface sediments. Bioturbation intensity peaks in late summer, resulting in highest oxygen flux into sediments and promoting remineralization of subsurface OC and release of nutrients. Shutdown of pelagic primary production in combination with enhanced wind-waves in early autumn cause a systematic shift of the benthic carbon pool from deposition to erosion within a few weeks, accounting for the accelerated decrease of benthic oxygen flux. Our results indicate a central role of macrobenthos in modulating the rate of both solute and solid fluxes across the sediment-water interface.

How to cite: Zhang, W., Neumann, A., Daewel, U., and Schrum, C.: The central importance of macrobenthos in benthic-pelagic coupling in coastal shelf seas , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-3361, https://doi.org/10.5194/egusphere-egu2020-3361, 2020.

The East Anglian salt marsh system (UK) has recently generated intriguing data with respect to sediment biogeochemistry. Neighbouring ponds in these salt marshes show two distinct regimes of redox cycling: the sediments are either iron-rich and bioturbated, or they are sulphide-rich and unbioturbated. No conclusive explanation has yet been given for this remarkable spatial co-occurrence.  Using pore-water analysis and solid-phase speciation, I will demonstrate that differences in solid-phase carbon and iron inputs are likely small between pond types, so these cannot act as the direct driver of the observed redox dichotomy. Instead, the results suggest that the presence of bioturbation is the driving force behind the transition from sulphur-dominated to iron-dominated sediments. The presence of burrowing fauna in marine sediments stimulates the mineralisation of organic matter, increases the iron cycling and limits the build-up of free sulphide. Subsequent early diagenetic modelling confirms that the observed regimes in pond geochemistry are caused by negligible differences in solid-phase inputs, which are amplified by positive feedbacks resulting from the impact of bioturbation on iron and sulphur cycling.

How to cite: van de Velde, S., Antler, G., and Meysman, F.: Burrowing fauna mediate alternative stable states in the redox cycling of salt marsh sediments, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-3840, https://doi.org/10.5194/egusphere-egu2020-3840, 2020.

Ferruginous tubular structures concretions are widely distributed over the seafloor surrounding the Gran Burato depression in the Transitional Zone (TZ) province of the Galician Continental Margin (NW Iberian Margin). These bioforms-like structures are created by iron oxides precipitations into the tube-dwelling macrozoobenthos as a result of Fe²⁺ upward diffusion and O₂ ventilation and diffusion acting in the water-sediment interphase in a non-steady state early diagenesis. X-ray diffraction analyses display that goethite is the main mineralogical component of these bioforms-like structures. Furthermore, non-steady state diagenesis has been identified by several oxidations fronts recognised in three piston cores, reflecting that the redoxcline has not achieved the deeper equilibrium in the study area. Afterwards, these ferruginous tubes were eroded, remobilised and redistributed over the seabed by bottom currents. Ocean-floor observations show erosion and sea-bottom current structures as ripples, grooves, erratic blocks, accumulations of pteropods and carbonate crusts associated with hardgrounds. Sedimentation rates calculated in a piston core display very low values for the last 30 cal ka BP (mean of 1.57 cm ky⁻¹) with a marked hiatus between 17.80 to 10.45 cal ka BP, meanwhile abraded surfaces have been identified by high-resolution seismic data confirming erosional processes in this area of the TZ province. We conclude that the ferruginous bioforms accumulation over the deep-ocean floor is indicative of a present-day vigorous seafloor current acting and eroding the sediments of the TZ province. This bottom current is a direct consequence of the general seafloor elevation of the TZ province that causes constriction of the water masses (MOW and LSW) that induces a general intensification of the bottom currents and greater erosional capacity. This erosional process causes the continuous oxygenation of the upper sediments, and it prevents to reach the steady-state diagenesis, playing this fact an essential role in the ferruginous formations and accumulations in the study area.

How to cite: López-Pérez, Á. E., Rubio, B., Rey, D., and Pinheiro, L.: Ferruginous bioforms accumulations in deep marine environments: an approach to their origin and formation mechanisms in the Transitional Zone province of the Galician Continental Margin (NW Iberian Peninsula), EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-4165, https://doi.org/10.5194/egusphere-egu2020-4165, 2020.

Sand waves are dynamic, sinusoidal bedforms that have been thoroughly studied in the context of the physical and hydrodynamical processes dominating these environments. However, information about the ecological and biogeochemical characteristics within these bedform habitats have been far fewer in comparison. To address this knowledge gap, a field campaign was undertaken in the summer of 2017 to investigate the biogeomorphology of asymmetrical sand waves in the Dutch North Sea, near island Texel. The goal was specifically to address both the macrofaunal community composition and the associated biogeochemistry along the different sections of these sand waves.  Using a combination of several field sampling techniques and lab incubations on board the NIOZ RV-Pelagia, we collected a comprehensive dataset covering the macrofauna assemblage, nutrient flux, oxygen consumption, sediment grain size and permeability, as well as physical and environmental data, within a transect line (< 1 km) that covered several sand waves. Here, we show considerable variability in the species abundance, composition and biomass, which were all significantly higher on the steeper sides of the sand waves; the multivariate statistical analyses on the datasets showed a significant influence of the sand wave position on benthic composition. Correspondingly, measurements from the steep slopes also exhibited a higher concentration of chl-a and organic matter, higher O₂ consumption, more fine particles and lower sediment permeability. Despite the overall homogeneity (e.g., sandy sediment) of a well-developed bedform environment such as a sand wave field, it is clearly possible to find significant variations in the benthic community composition and biogeochemical activity on a small spatial scale.  Oftentimes, studies look at larger spatial scales to maximize the characterization of an entire region. However, given the diverse environmental gradients within the North Sea, our observations may not be sufficiently captured or even missed altogether when superimposed upon such large spatial scales.  Thus, a close examination of the interrelated parameters such as biology, biogeochemistry, sedimentology and morphology should also be considered, at a high resolution, over a small local scale for such seemingly uniform habitats.  We hope our results will contribute valuable insight into small-scale patterns of variability in dynamic bedform environments. 

How to cite: Cheng, C., Borsje, B., O'Flynn, S., Beauchard, O., Ysebaert, T., and Soetaert, K.: Macrobenthos richness and biomass preferentially geared towards one half of asymmetrical sand waves, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-7353, https://doi.org/10.5194/egusphere-egu2020-7353, 2020.

Organic-rich freshwater sediments display millimetric oxygen and nitrate penetration and are sources of methane to the water column and to the atmosphere via diffusion and ebullition. Radial oxygen loss by submersed aquatic plants and burrow irrigation with O₂ and NO₃^- enriched water by macrofauna can significantly alter the subsurface sediment volume where respiration processes alternative to methanogenesis occur. We tested this hypothesis in perifluvial organic sediments colonized by the submerged phanerogam Vallisneria spiralis and the oligochaete Sparganophilus tamesis. Gas ebullition and diffusive fluxes were measured in microcosms maintained under controlled laboratory conditions over a period of two weeks. Four conditions were reproduced: sediments alone, sediment with oligochaetes, sediment with plants and sediment with plants and oligochaetes. Microcosms with sediments alone released the largest methane volume whereas sediments with plants and macrofauna released the lowest amount. The presence of the oligochaete had comparatively a stronger effect than that of the macrophyte. Simultaneously, the bioturbation activity of the oligochaete enhanced the production of N₂ and the consumption of oxygen and nitrate, suggesting increased rates of aerobic respiration and of denitrification. The presence of plants attenuated net N₂ losses from the benthic system likely due to the competition between assimilative and dissimilative N-related processes.

How to cite: Benelli, S. and Bartoli, M.: Macrofauna and roots reduce methane production and attenuate nutrient recycling in organic-rich fluvial sediments, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-11581, https://doi.org/10.5194/egusphere-egu2020-11581, 2020.

Bioturbation is an important process in the early diagenesis of soft marine sediment. Benthic infaunal activity, such as feeding, burrowing and ploughing redistributes particles within the topmost layers of the sediment. Recently deposited particles are mixed into deeper sediment depth layers and old material remains longer near the surface. A sediment layer thus contains an assemblage of particles from young to very old ages. Under certain assumptions, bioturbational mixing can be modelled as a diffusive process with the macroscopic mixing coefficient D_B. Here we model the age distribution of the bioturbated sedimentary record with a depth dependent mixing coefficient D_B(z). The potential age bias introduced by mixing is typically higher than multiples of the mean mixed layer residence time, which scales linearly with the ratio of mixed layer depth and sediment accumulation rate. Scaling the mixing intensity has only a minor effect, as most marine environments are mixing dominated.

The rate of organic matter degradation can been modelled empirically as an age dependent process, with recently deposited, fresh organic matter having higher reactivities than older and more refractory material. With insights into the age distribution, this allows to couple the degradation of organic matter with bioturbation and estimate the burial of carbon.

How to cite: Kuderer, M. and Middelburg, J. J.: Age distribution and organic matter degradation in bioturbated sediment , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-16655, https://doi.org/10.5194/egusphere-egu2020-16655, 2020.

Benthic macrofauna occupy most of the oxygenated seafloor, where they have a strong influence on microbial activity and are major regulators of carbon and other elemental cycles. To explore the yet-elusive relationships between faunal sediment alteration (bioturbation), microbial community structure, and microbial activity, we conducted aquarium incubations of Abarenicola pacifica and Nereis vexillosa in a seawater flow system and field manipulation experiments in a sandy intertidal zone. Microsensor and geochemical profiling show strong impacts of both worms on the pore-water concentrations of electron acceptors (O₂, NO₃^-, and SO₄^-) and metabolites (NH₄⁺, HS^-, and Fe₂⁺), and suggest the distinctly different advective and diffusive type of bioirrigations generated by A. pacifica and N. vexillosa, respectively, in sediment. Comprehensive analyses on microbial community structure and activity using amplicon sequencing and quantitative-(Reverse Transcription)-PCR of 16S rRNA and functional genes suggest that the metabolically active microbial community structure in intertidal sandy sediments is highly resilient to macrofaunal disturbance. This resilience likely stems from metabolic versatility that enables dominant microorganisms to switch between (micro)aerobic and anaerobic lifestyles under the fluctuating redox conditions in these environments. Significant changes of microbial community structure were only locally observed in the fecal pellet and feeding funnel of A. pacifica and mucus of N. vexillosa, likely due to the distinct organic matter composition and/or higher exposure time to oxygen in these microenvironments. Results from the field-based manipulation experiments further suggest that, in addition to macrofaunal bioturbation, conditions of temperature, tidal movement, and supply of photosynthetic organic matter also play important roles in controlling microbial activity and community structure in intertidal sediment.

How to cite: Deng, L., Fiskal, A., Bölsterli, D., and Lever, M.: Laboratory- and field-based investigation on macrofaunal control of microbial community structure and activity in intertidal sediment, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-18846, https://doi.org/10.5194/egusphere-egu2020-18846, 2020.

Chemosynthetic symbioses are widespread throughout marine benthic ecosystems. Through the combined metabolic activity of the symbionts and their animal hosts, they alter the sediment’s available nutrients, affecting the surrounding biological communities from microbes to seagrasses. The chemosymbiotic bivalve family Lucinidae is a remarkable example.  Lucinidae is one of the most species-rich animal families in the oceans today, with more than 400 species described. They can be found worldwide from the tropics to the poles, and can reach abundances of more than 4000 individuals per square meter of sediment. All Lucinids detoxify sediments of hydrogen sulfide and one particular species, Loripes orbiculatus, which associates with a sulfur-oxidizing symbiont, Candidatus Thiodiazotropha endoloripes, has been shown to release nitrogen compounds into the surrounding environment (Cardini et al., ISME J 2019). The symbionts of all Lucinidae, including Loripes orbiculatus, are acquired from their surrounding environment during the animal’s development, termed horizontal transmission. Although a substantial environmental population must be present for the symbiosis to persist across generations, we know very little about the environmental reservoir of horizontally transmitted symbionts, as surprisingly, symbionts are rarely, if ever, detected in surveys of sediment microbial communities. We hunted for the free-living symbionts in habitats surrounding the lucinid species Loripes orbiculatus and its symbiont, Candidatus Thiodiazotropha endoloripes, in Fetovaia Bay, Elba, Italy. Symbionts in the environment may have been previously overlooked in molecular surveys of bulk sediment, thus, we did targeted sampling of distinct environmental microhabitats including porewater and sediment. There was little evidence for an environmental symbiont population in Fetovaia Bay. Extensive 16S rRNA amplicon surveys of sediment samples found that less than 0.05% of the bacterial population belong to the Sedimeticolaceae family, which contains Candidatus Thiodiazotropha and other lucinid symbionts, and none of the 400,000 sequences we analyzed matched the symbiont’s 16S rRNA sequence. Given the absence of detectable symbionts in sediment, we considered it possible that lucinid clams engineer an environment to ‘farm’ symbionts through their sulfide mining and general burrowing activities. We therefore assessed the microbial communities in the mucus-lined burrow walls of the bivalves with molecular methods. In contrast to the surrounding sediment, ~10% of operational taxonomic units (OTUs) found in the mucus tubes created by lucinid clams were Sedimenticolaceae, and sequences matching the genus Candidatus Thiodiazotropha could be detected. The enrichment of Sedimenticolaceae in the mucus tubes created by Loripes orbiculatus suggests that the clams create an environment more suitable for Sedimenticolaceae than the ‘background’ surrounding sediment. This suggests that the population of available symbionts is environmental, but only detectable in lucinid-associated or modified environments such as the burrow walls. Considering their worldwide distribution and enormous abundance at some locations, lucinid clams and their chemosynthetic symbionts potentially have an enormous impact on structuring microbial communities in marine sediments globally, both indirectly by altering carbon, nitrogen and sulfur cycling, and directly by selecting for certain microbial groups.

How to cite: Osvatic, J., Windisch, J., Yuen, B., Hausl, B., Polzin, J., and Petersen, J.: Chemosymbiotic lucinid clams modify the physical, chemical and biological characteristics of marine sediments globally , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-19150, https://doi.org/10.5194/egusphere-egu2020-19150, 2020.

Sedimentary rocks, formed by the accumulation of mineral and organic particles, are important both for studies of the earth’s history as well as for being a source of fossil fuels. Over the course of the last decades, it has been demonstrated that scanning electron microscope (SEM)-based cathodoluminescence (CL) spectros­copy is a valuable technique for the characterisation of sedimentary rocks, complementary to other electron microscopy-based techniques, such as backscattered electron imaging (BSE/EBSD) and energy dispersive x-ray spectroscopy (EDS). Typically the CL yield is high enough for rapid scanning and, in some cases, even video-rate scanning, allowing fast in­spection of relatively large areas. It can be used to quantitatively map the quartz composition of the sample, for example, which enables the rigorous segmentation of granular and cemented material.

Textulariid benthic foraminifers live on and in seafloor sediments and form shells of agglutinated sediment particles. They are very important biostratigraphic markers, and fossil agglutinated foraminifera are important archives for paleoceanographic reconstructions. Furthermore, living textulariids show a strong diversity, populating  a diverse range of marine habitats partly and can reach high living abundances, making them important for benthic ecosystems.

In this work, we show how CL spectroscopy can be employed to study agglutinated foraminifera using the species Liebusella goesi from the Swedish Gullmar Fjord as an example. Fast panchromatic imaging using a photomultiplier tube was performed over a large area of the foraminifera, which revealed textures and contrasts of interest in the shell (test). A high resolution SEM image was acquired simultaneously to provide spatial context. Such a dataset can be valuable in establishing the geological history as well as in identifying the chemical composition of the cement used for the agglutination of sediment particles. Both the composition of the agglutinated particles and the chemical composition of the cement might bear valuable information about the environmental conditions, when the test was formed. EDS measurements were performed, revealing the spatial distribution of elements such as potassium, calcium, sodium, silicon and oxygen, in the sediment particles of the shell. This was useful in indicating the presence of minerals such as quartz and feldspar, and hyperspectral CL imaging was performed to rigorously identify them, and to visualize intragranular features, not visible in the EDS data. Based on the CL spectral data, we were further able to identify different grades/types of quartz and feldspars. These results show that these foraminifera prefer different sediment materials with varying grain sizes, depending on the size of the newly formed chamber, to achieve the highest mechanical stability.

How to cite: Hari, S., Littmann, S., Glock, N., von Arx, J., Coenen, T., and Roy, A.-S.: Correlative cathodoluminescence and EDS imaging of the benthic agglutinated foraminifer Liebusella goesi, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-20478, https://doi.org/10.5194/egusphere-egu2020-20478, 2020.

Coastal sediments play an important role in the nutrient cycling, and the intensities of exchange processes between bottom water and pore water control the balance between sequestration and recycling of nutrients. Pore water advection as one major exchange mechanism is determined by physical parameters and thus well describable with models. By contrast, biotransport (bioirrigation, bioturbation) as the other major transport mechanism is much more complex and observational data are often scarce to quantify these processes.

We present ex-situ observations of oxygen and nutrient fluxes, sediment characteristics, and fauna composition over the past six years from all benthic provinces of the German Bight, which enable us to describe the spatial and seasonal variability of the benthic- pelagic coupling. We employ this dataset to detect environmental drivers of the observed variability and to test several proxies of faunal activity.

Our results show that abiotic parameters (sediment type, local primary production) explain the spatial variability while the dynamics of temperature and faunal activity explain the temporal variability. Effects of the complex benthic communities on benthic exchange rates can be parameterized by surprisingly simple proxies, which may help to improve benthic exchange models. By comparing in-situ measurements of pore water advection with ex situ observations, we conclude that biotransport approximately doubles the benthic- pelagic exchange rates in the German Bight.

How to cite: Neumann, A., van Beusekom, J., Eisele, A., Emeis, K.-C., Friedrich, J., Kröncke, I., Meyer, J., Schückel, U., and Wrede, A.: Proxies, drivers, and impact of macrofaunal transport in sediment of the southern North Sea., EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-20810, https://doi.org/10.5194/egusphere-egu2020-20810, 2020.

The organic matter (OM) concentration is one of the most important factors influencing benthic organism sediment reworking during bioturbation. This study was designed to evaluate differences in sediment reworking rate of Perinereis aibuhitensis based on quantification of its pellet production (PP) and OM transport rate from ambient sediment to the surface due to its feeding. The mesocosm experiment was conducted in acrylic container (15×1×20 cm) with two treatments (high OM treatment and low OM treatment) and each treatment had ten replicates. The pellets in each container were removed 2h before the beginning of the pellet collection, and then newly produced pellets were collected every 2 h during 24 h at each treatment. The mean grain size of pellets (5.1 ∅) was smaller than that of ambient sediment particles (5.9 ∅), and the mean OM concentration was much higher in pellet (0.69% for C and 0.06% for N) than in ambient sediment (0.46% for C and 0.05% for N). Since an organism cannot produce more organic matter than it ingests, production of organically enriched pellets by this species indicates selective ingestion. The overall OM transport rate was 0.7 g C m^-2 day^-1 in carbon and 0.06 g N m^-2 day^-1 in nitrogen, respectively. The daily PP was much higher in high OM treatment than that of low OM treatment with mean values of 0.007 and 0.002 g ind.^-1 h^-1, respectively. It is expected that Perinereis feeding activity strongly depended on OM concentrations. The overall sediment reworking rate based on the pellet production was much higher in high OM concentration (0.005 mm day^-1) than in low OM (0.001 mm day^-1) concentration.

How to cite: Seo, J. and Koo, B. J.: The effects of organic matter concentration on sediment reworking of Perinereis aibuhitensis : a mesocosm study, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-20817, https://doi.org/10.5194/egusphere-egu2020-20817, 2020.

Ever-expanding human activities on land and at sea have amplified the need for easily applicable proxies to effectively predict human mediated changes in ecosystem functioning and biogeochemical cycling. Here we investigate the ability of different proxies to predict macrofaunal impact on nutrient fluxes of ammonium, nitrate, nitrite, silicate and phosphate under different environmental conditions. As proxies we chose simple community descriptors (i.e. density, wet biomass, ash free dry mass) as well as two trait-based indices that were created to describe macrofauna-sediment interactions (i.e. community bioturbation potential (BP_c) and community irrigation potential (IP_c)). We hypothesize that trait based indices, will increase the predictability of macrofaunal impact on nutrient fluxes compared the more simple community descriptors. We correlate all proxies with experimental nutrient flux data measured under different environmental conditions using generalized linear models. Generally environmental conditions significantly affected all analysed nutrient fluxes and mostly provided better predictions than any of the proxies for macrofaunal impact by itself. Yet a combination of the proxies and the environmental conditions always increased prediction accuracy. Hereby the irrigation trait based indices enhanced the predictability of the nutrient fluxes of ammonium, nitrate, nitrite, silicate and phosphate most.

How to cite: Wrede, A., Andresen, H., Asmus, R., Wiltshire, K. H., and Brey, T.: Describing macrofaunal impact on nutrient flux – what is the potential of trait based approaches?, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-21590, https://doi.org/10.5194/egusphere-egu2020-21590, 2020.

Seagrass meadows are among the most productive ecosystems in the world: they store a large amount of carbon and host highly diverse macrobenthic communities. They also play a key role in biogeochemistry at the sediment-water interface. The light requirements of seagrasses limit their development to shallow coastal areas where they are facing various natural and anthropogenic disturbances, which has induced a global loss of these ecosystems over the last decades. Nutrient enrichment of coastal waters, resulting from anthropogenic activities is one of the leading causes of this decline. Subpolar seagrass meadows present a strong seasonal dynamic, with a long winter when seagrasses rely on carbon reserves that they build up during the short growing season (limited to two to three months during summer time). Hence, it has been hypothesized that the effects of nutrient enrichment on seagrass ecosystem functioning depend on seasonal dynamics. In this study, we performed a series of mesocosm experiments over a month period to investigate the effects of the timing, duration and intensity of disturbance on macrofauna bioturbation, oxygen and nutrients porewater concentration profiles and benthic fluxes using three levels (including control) of realistic nutrient enrichments at the beginning (June) and at the end (August) of the growing season. In May, effects of intermediate level of nutrient enrichment were only visible on total oxygen uptake by the sediment at day 30 of disturbance while it affected oxygen and nutrients benthic fluxes at day 15 in August. The highest level of nutrient enrichment affected oxygen and nutrients benthic fluxes in May and August. Overall, our results highlight the importance of considering the time (period and duration) in the assessment of the functional consequences of disturbances.

How to cite: Pascal, L., Chaillou, G., Bernatchez, P., Nozais, C., and Archambault, P.: Influence of nutrients enrichment on ecosystem functioning in a subpolar seagrass meadow, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-22581, https://doi.org/10.5194/egusphere-egu2020-22581, 2020.