Description: Benthic fluxes of dissolved nutrients and oxygen measured in the southern North Sea using ex situ incubation chambers indicate a prominent annual cycle characterized by low level from mid-autumn (Oct) to early spring (Mar) and enhanced values from mid-spring (Apr) to early autumn (Sep) with peak in late summer (late Aug/early Sep). The same cycle is also shown in the budget of total organic carbon (TOC) and macrobenthic biomass in surface sediments. The significant positive correlations between the benthic nutrient fluxes, oxygen, sedimentary TOC and macrobenthos suggest that their variation might respond to a common source, i.e. the primary production. However, the linkages between these quantities and pelagic primary production, which exhibits a dominant bloom in early spring (Mar/Apr) and a secondary bloom in early summer (Jun/Jul) in the study area, is not straightforward. We present a numerical study to unravel the complex linkages. A 3-D coupled hydrodynamic-biogeochemical model (ECOSMO) was used to provide benthic boundary conditions for a 1-D biogeochemical model in the sediment (TOCMAIM) that mechanistically resolves the interaction between macrobenthos and organic matter through bioturbation. Simulation results based on a satisfactory hindcast from 1948 to 2015 reveal that although the spring algal bloom normally starts in late winter (Feb) and peaks in early spring (Mar/Apr), deposition of labile OC to seafloor is limited in this period due to energetic hydrodynamic conditions. Sedimentation and accumulation of labile OC (originated from fresh planktonic detritus) in seafloor surface sediments are facilitated in summer when wind-waves become weak enough. This drives the blooming of macrobenthos, with peak of biomass in late summer (Aug). Bioturbation intensity, which is dependent upon macrobenthic biomass, community structure as well as local food resource, peaks also in later summer. Enhanced bioturbation and benthic metabolism result in an increased oxygen flux into sediments, promoting remineralization of OC and release of nutrients. The following period (late Sep/Oct) is characterized by low level of pelagic primary production in combination with enhanced wind-waves, which not only reduce the input of labile OC into sediments substantially but also remobilize surface material (sediments and OC) on a major part of the shallow coastal seafloor. Depletion of labile OC in the uppermost centimeters of sediments by a combined effect of erosion, macrobenthic uptake and downward mixing (through bioturbation) accounts for the rapid decline of benthic nutrient fluxes in Oct, which remain low through the stormy winter until the next spring.

Description: The importance of macrobenthos in benthic‐pelagic coupling and early diagenesis of organic carbon (OC) has long been recognized but has not been quantified at a regional scale. By using the southern North Sea as an exemplary area we present a modelling attempt to quantify the budget of total organic carbon (TOC) reworked by macrobenthos in seafloor surface sediments. Vertical profiles in sediments collected in the field indicate a significant but nonlinear correlation between TOC and macrobenthic biomass. A mechanistic model is used to resolve the bi‐directional interaction between TOC and macrobenthos. A novelty of this model is that bioturbation is resolved dynamically depending on variations in local food resource and macrobenthic biomass. The model is coupled to 3D hydrodynamic‐biogeochemical simulations to hindcast the mutual dependence between sedimentary TOC and macrobenthos from 1948 to 2015. Agreement with field data reveals a satisfactory model performance. Our simulations show that the preservation of TOC in the North Sea sediments is not only determined by pelagic conditions (hydrodynamic regime and primary production) but also by the vertical distribution of TOC, bioturbation intensity, and the vertical positioning of macrobenthos. Macrobenthos annually ingest 20%–35% and in addition vertically diffuse 11%–22% of the total budget of TOC in the upper‐most 30 cm sediments in the southern North Sea. This result indicates a central role of benthic animals in modulating the OC cycling at the sediment‐water interface of continental margins.

Description: Field data collected for the North Sea indicate a prominent seasonal variation in the vertical distribution of total organic carbon (TOC) and macrobenthic biomass in sediments. The vertical TOC profiles classify into three modes, with maximum at surface, middle and deep part of sediments, respectively. We here present a mechanistic model to quantify, for the first time, the dynamic interaction between sedimentary TOC and benthic fauna. The major model principles include that (i) the vertical distribution of macrobenthic biomass is a trade-off between nutritional benefit (quantity and quality of TOC) and the costs of burial (respiration) and mortality, and (ii) the vertical transport of TOC is in turn modulated by macrobenthos through bioturbation. A novelty of our model is that bioturbation is resolved dynamically depending on variation of local food resources and macrobenthic biomass. This allows capturing of the benthic response to both depositional and erosional conditions and improving estimates of the material exchange flux at the sediment-water interface. The coupling of the TOC-benthos model with 3D hydrodynamic-ecological simulations reveals that the three profile modes of sedimentary TOC (in both quantify and quality) can be explained as a combined response to pelagic conditions (shear stress and primary production) and the synergy between bioturbation, vertical redistribution of higher quality TOC and vertical positioning of benthic organisms. A model reconstruction of the benthic status in the North Sea from 1950s to 2010s indicates that despite a relatively stable pattern at decadal and regional scales, significant variations exist at smaller scales characterized by seasons and local areas. In addition, inter-annual and multi-year cycle-like variations are also prominent especially in coastal areas.

Description: Trait based indices constitute a versatile tool for the prediction of ecosystem functioning over large spatial scales and represent a promising approach to meet societal, political and regulatory demands. Here we investigate for the first time the ability of different trait based indices to predict nutrient fluxes of ammonium, nitrate, nitrite, silicate and phosphate under different environmental conditions. We hypothesize that irrigation traits, as applied in the newly proposed index “Community Irrigation Potential” (IPc), will increase the predictability of macrofaunal impact on nutrient fluxes compared to commonly used sediment reworking traits, as in the index “Community Bioturbation Potential” BPc. We correlate IPc and BPc with experimental nutrient flux data measured under different environmental conditions. Both trait based indices and environmental conditions significantly affected all analysed nutrient fluxes. We therefore conclude that neither the trait based indices nor the environmental conditions suffice for quantitative modelling of sediment biogeochemical turnover. Accordingly, information on of macrofaunal activity is needed to reliably predict biogeochemical turnover. Our results further demonstrate that generally nutrient fluxes of ammonium, nitrate, nitrite, silicate and phosphate are more closely linked to irrigation traits than to sediment reworking traits. In conclusion, linking macrofaunal bioirrigation to important environmental factors such as permeability, changing nutrient gradients in the water column and organic matter concentrations may strongly enhance performance of ecosystem models.

Description: This study shows that macrofaunal irrigation traits constitute a valuable complement to sediment reworking traits in estimating macrofaunal impact on nutrient fluxes across the sediment-water interface. We correlated density, biomass, community bioturbation potential (BPc, an index based on reworking traits, body mass and density) and community irrigation potential (IPc, an index based on irrigation traits, body mass and density) with nitrite, nitrate, ammonium, silicate and phosphate flux data under different environmental conditions. Generalized linear models performed best with a combination of environmental conditions and irrigation trait-based indices. This was not only a direct effect of the irrigation traits, but also of the scaling factor 0.75 employed in IPc to infer metabolic activity from body mass. Accordingly, predictive models of nutrient flux across the sediment-water interface will profit greatly from incorporating macrofaunal irrigation behaviour by means of trait-based indices.

Description: Coastal morphodynamic and hydrodynamic processes impose a first-order control on biogeochemical cycling and distribution of benthic habitats. However, little is known on how biota in turn affect sediment transport, biogeochemical cycling and morphodynamics at a regional scale and long-term. Here we firstly present a mini-review of recent progress on numerical modeling of interactions between benthic biota, biogeochemical cycling and coastal morphodynamics. Special focus is laid on 1) the synergetic effects of dominant benthic functional groups on sedimentation process, and 2) the macrobenthic response to changing morphological, hydrodynamic and biogeochemical conditions. In a second part we present preliminary results from a case study area (Jade Bay, Wadden Sea) based on simulations using a coupled hydrodynamic-biogeochemical-morphodynamic model and comparison with biogeochemical and taxonomic field monitoring data. The aim is to understand benthic ecosystem functioning and to identify the main drivers of coastal ecosystem changes for better prediction of their future development.

Description: Loss of macrofaunal bioturbation and bioirrigation activity may strongly reduce benthic biogeochemical cycling and thus ecosystem functioning. The identification of bioturbating key species in the marine benthic realm is therefore of high importance for ecosystem management purposes. In this study top bioturbators in the German Bight were identified by mapping the trait based bioturbation potential (BPc) for 423 North Sea stations. BPc mapping highlighted the importance of Amphiura filiformis, Echinocardium cordatum and Nucula nitidosa as major bioturbating species in the German Bight. Effects of their bioturbation and bioirrigation activity on silicate, ammonium and nitrate flux were investigated in laboratory experiments. While E. cordatum significantly influenced biogeochemical cycling, effects of A. filiformis remained inconclusive probably due to arm regeneration. N. nitidosa showed little impact on biogeochemical cycling, although the bivalve was found to be an important bioturbator. E. cordatum may thus be considered an essential mediator of biogeochemical cycling in the sediment water interface as well as one of the most important bioturbators in the German Bight.

Description: Increasing anthropogenic activities on land and at sea underline the demand for easily applicable indices to effectively predict human mediated changes in ecosystem functioning. Here, we propose a novel bioirrigation index (IPc) that is based on body mass, abundance, burrow type, feeding type and injection pocket depth of bottom dwelling animals. The index was validated with combined field (in situ communities) and manipulative (single species) experiments. Results from both community and single-species experimental incubations indicate that IPc is able to predict the bioirrigation rate in different sediment types (mud, fine sand, sand). The trait-based index thus demonstrates robustness in the prediction of animal-mediated functional processes that support biogeochemical functions under variable environmental conditions. Accordingly, we argue that trait-based indices provide a useful tool for the quantitative prediction of ecosystem processes as effect traits provide a direct link to the behavioral mechanisms that drive ecosystem functioning.

Description: Increasing anthropogenic activities on land and at sea underline the demand for easily applicable indices to effectively predict human mediated changes in ecosystem functioning. Here, we propose a novel bioirrigation index (IPc) that is based on body mass, abundance, burrow type, feeding type and injection pocket depth of bottom dwelling animals. Results from both community and single-species experimental incubations indicate that IPc is able to predict the bioirrigation rate in different sediment types (mud, fine sand, sand). Further, IPc increased the predictability of biogeochemical cycling (i.e. changing concentrations of phosphate, silicate, ammonium, nitrate and nitrite) under different environmental conditions (i.e. sediment type, temperature, faunal inventory, gradients across the sediment water interface), compared to trait based bioturbation potential (BPc). The trait-based index thus demonstrated robustness in the prediction of animal-mediated functional processes that support biogeochemical functions. Additionally our results confirm that biogeochemical cycling is more closely linked to irrigation traits than to sediment reworking traits. Based on these findings we argue that trait-based indices provide a useful tool for the prediction of ecosystem processes as effect traits provide a direct link to the behavioral mechanisms that drive ecosystem functioning.

Description: Macrofaunal bioturbation is an important mechanism for the enhancement of remineralization and biogeochemical cycling in marine sediments. Reduction of bioturbation activity may accordingly have far-reaching negative implications for general ecosystem performance. This is especially the case for shallow shelf seas, such as the German Bight, which account for 50% of global benthic remineralization, although they cover only 7% of the total sea surface. Increasing anthropogenic activities (e.g. wind farm construction) in these shallow shelf seas have intensified the need for reliable quantifications and predictions of macrofaunal bioturbation activities and resulting biogeochemical processes. The aim of this thesis was,thus, to develop easily applicable concepts that allow for the quantification of sediment reworking and the prediction of bioirrigation. In order to simplify the quantification of sediment reworking, which can so far only be assessed experimentally, I compared two of the most commonly applied methods (sediment profile imaging (SPI) and standard slicing technique (ST)). In addition, the time-saving and easily applicable SPI method was tested for its suitability to assess sediment reworking from cylindrical multi-corer samples (Manuscript I). The results suggested that SPI is suitable and even more accurate than ST for the investigation of sediment reworking activity. This omits the previously necessary need for timeconsuming slicing or the complex transfer into rectangular aquaria. These findings will facilitate studies on spatiotemporal patterns of sediment reworking activity in the German Bight. Such studies are of special interest as the bioturbation potential (BPc), which was previously often applied to estimate the potential of communities to rework the sediment, does not correlate with actual sediment reworking rates (Manuscript II). Surprisingly BPc, which includes sediment reworking traits (i.e. mobility and reworking mode) but no specific bioirrigation traits, rather correlated with bioirrigation activity and nutrient fluxes of silicate, ammonium, nitrate, and nitrite (Manuscript II). To overcome ambiguity of BPc, I developed the irrigation potential (IPc), as an adaptation from BPc (Manuscript III). By incorporation of bioirrigation effect traits (i.e. burrow type, feeding type, injection pocket depth), IPc was specifically designed to predict bioirrigation and its influence on biogeochemical processes (Manuskript III). I could demonstrate that, in contrast to BPc, the modified index provides an accurate quantitative measure of macrofaunal bioirrigation for both single species and entire communities of various infaunal species, if the index is calculated with ash free dry body mass. IPc provided better estimations of phosphate, silicate, ammonium, nitrate and nitrite fluxes than BPc (Manuscript IV). The estimation of silicate, ammonium, nitrate, and nitrite fluxes may be further increased if IPcis calculated in wet body mass instead of ash free dry body mass. Wet body mass thereby serves as a proxy of the irrigated sediment volume. In general, IPc could become a valuable tool to support ecosystem management and future investigations on the effects of anthropogenic activities on biogeochemical turnover in shallow shelf seas. Findings of Manuscript IV however also demonstrated that IPc is a crucial but insufficient parameter for the modelling of sediment biogeochemical processes because these also dependent on environmental conditions (e.g. temperature, sediment organic matter content, permeability). A newly proposed temperature term (IcT) (Manuscript III) may provide a tool to identify spatiotemporal variations in macrofaunal bioirrigation activity. There is however a need to determine further how IPcor IcT relate to biogeochemical cycling under different environmental conditions as well as how the respective macrofaunal traits are affected by environmental parameters.