Description: The samples were taken in June 2021 on the cruise AL557 in the Skagerrak and the central and southern North Sea. The samples were taken to analyze pore water concentrations of nutrients in order to monitor the nitrogen turnover processes taking place in the sediment. Pore water samples were taken with rhizon samplers (0.15 µm pore size, CSS; Rhizosphere, Netherlands) from intact sediment cores that were taken with a Multiple Corer. The samples were directly frozen until analysis on 7th and 8th March 2022. The nutrient concentrations (ammonia, silicate, phosphate, nitrite) were measured using a TECAN infinite 200 plate reader. Colorimetric methods according to Ringuet et al. (2011) for phosphate and silicate, Yu et al. (1994) for ammonia and Garcia-Robledo et al. (2014) were applied.

Description: Microbial composition and diversity in marine sediments are shaped by environmental, biological, and anthropogenic processes operating at different scales. However, our understanding of benthic microbial biogeography remains limited. Here, we used 16S rDNA amplicon sequencing to characterize benthic microbiota in the North Sea from the top centimeter of 339 sediment samples. We utilized spatially explicit statistical models, to disentangle the effects of the different predictors, including bottom trawling intensity, a prevalent industrial fishing practice which heavily impacts benthic ecosystems. Fitted models demonstrate how the geographic interplay of different environmental and anthropogenic drivers shapes the diversity, structure and potential metabolism of benthic microbial communities. Sediment properties were the primary determinants, with diversity increasing with sediment permeability but also with mud content, highlighting different underlying processes. Additionally, diversity and structure varied with total organic matter content, temperature, bottom shear stress and bottom trawling. Changes in diversity associated with bottom trawling intensity were accompanied by shifts in predicted energy metabolism. Specifically, with increasing trawling intensity, we observed a transition toward more aerobic heterotrophic and less denitrifying predicted metabolism. Our findings provide first insights into benthic microbial biogeographic patterns on a large spatial scale and illustrate how anthropogenic activity such as bottom trawling may influence the distribution and abundances of microbes and potential metabolism at macroecological scales.

Description: Benthic organisms and their bioturbation activities have a profound effect on a multitude of sediment properties. While many studies have already explored benthic impacts at small temporal and spatial scales, little is known on how the small-scale effects accumulate and interactively guide large-scale (km-scale) morphological evolution. Here we firstly summarize the most important processes of benthos affecting sediment stability and then explore existing biomorphodynamic modeling studies both at small- and large-scales. In general, microbenthos (body size 〈0.1 mm) mainly stabilizes sediments while meio- (0.1–1 mm) and macrobenthos (〉1 mm) may stabilize or destabilize sediments. Among all types of sediment, fine-grained fraction (silt and clay) is most sensitive to the impact of benthos. Benthic organisms have the capability to mediate sediment transport and sedimentation patterns beyond their habitats on the long-term and over a large-scale. However, so far, numerical models evaluating benthic impact are limited to explorative studies and have not reached a stage where they can be used for predictive modeling. The barriers hindering a further development of biomorphodynamic models include not only limited understanding of fundamental biological/bio-physical processes affecting morphological development and dynamic feedback loops among them but also a shortage of data for model calibration and confirmation of simulation results. On the other hand, thriving for higher model complexity does not necessarily lead to better performance. Before conducting biomorphodynamic modeling, researchers must figure out which questions can be answered in a meaningful sense with simulation results that can be compared with observations and which level of modeling complexity is sufficient for that purpose.

Description: The contribution of sediments to nutrient cycling of the coastal North Sea is strongly controlled by the intensity of fluxes across the sediment water interface. Pore-water advection is one major exchange mechanism that is well described by models, as it is determined by physical parameters. In contrast, biotransport (i.e., bioirrigation, bioturbation) as the other major transport mechanism is much more complex. Observational data reflecting biotransport, from the German Bight for example, is scarce. We sampled the major sediment provinces of the German Bight repeatedly over the years from 2013 to 2019. By employing ex situ whole core incubations, we established the seasonal and spatial variability of macrofauna-sustained benthic fluxes of oxygen and nutrients. A multivariate, partial least squares analysis identified faunal activity, in specifically bioturbation and bioirrigation, alongside temperature, as the most important drivers of oxygen and nutrient fluxes. Their combined effect explained 63% of the observed variability in oxygen fluxes, and 36–48% of variability in nutrient fluxes. Additional 10% of the observed variability of fluxes were explained by sediment type and the availability of plankton biomass. Based on our extrapolation by sediment provinces, we conclude that pore-water advection and macrofaunal activity contributed equally to the total benthic oxygen uptake in the German Bight.

Description: During HE588, data were collected in five research areas in the south-eastern part of the German Bight as part of the DAM Pilotmission on the exclusion of mobile bottom-contact fishing in the North Sea (www.mgf-nordsee.de). The cruise started on October 24, 2021, and had a duration of twelve days at sea. The conducted tasks consisted of seafloor mapping with hydroacoustic devices, multicoring and grab sampling from the seafloor surface, lander deployments for the study of current characteristics, and video and diving surveys of benthic fauna. Despite the unstable weather conditions, all scientific tasks could be conducted successfully within the allocated time.

Description: Bioturbation is a central transport process for ecosystem functioning, especially in large soft sediment habitats like the Wadden Sea. The amphipod C. volutator is a dominant bioturbator in the Wadden Sea, due to its great abundance and almost continuous particle movement. Expedition or loss of its bioturbation activity could thus hold ramifications for ecosystem functioning within sediments, like carbon sequestration and nutrient recycling. Here we test the effect that temperature and organic enrichment have on the bioturbation of C. volutator; two prevalent abiotic factors in the Corophiid's habitat that have fluctuated over recent decades, and are expected to change in the future. In-situ experiments were conducted under 8 and 15 ◦C, with varying levels (0 g, 0.1 g, and 0.2 g) of powdered Ulva compressa enriching cores containing C. volutator. We found a significant interaction effect of temperature and organic enrichment on the bioturbation rate of the amphipod, with bioturbation only increasing with added organic enrichment at 15 ◦C. Further, a threshold within our experiments was also reached under 15 ◦C, where the amphipod ceased to expedite bioturbation under higher organic enrichment. This upper limit on this dominant bioturbation imposed with organic enrichment emphasizes the sensitivity of C. volutator. Our findings reveal bioturbation can be limited by temperature in colder months, and opposingly, limited by organic enrichment under warmer conditions. In future Wadden Sea scenarios where temperature is predicted to be warmer and winters milder, enhanced bioturbation activity by C. volutator could prove crucial in continued ecosystem functions.

Description: Sediment reworking by benthic infauna, namely bioturbation, is of pivotal importance in expansive soft-sediment environments such as the Wadden Sea. Bioturbating fauna facilitate ecosystem functions such as bentho-pelagic coupling and sediment nutrient remineralization capacities. Yet, these benthic fauna are expected to be profoundly affected by current observed rising sea temperatures. In order to predict future changes in ecosystem functioning in soft-sediment environments like the Wadden Sea, knowledge on the underlying processes such as sediment reworking, is crucial. Here, we tested how temperature affects bioturbation and its associated ecosystem processes, such as benthic nutrient fluxes and sediment oxygen consumption, using luminophore tracers and sediment incubation cores. We used a controlled mesocosm experiment set-up with key Wadden Sea benthos species: the burrowing polychaetes Arenicola marina and Hediste diversicolor, the bivalve Cerastoderma edule, and the tube-building polychaete Lanice conchilega. The highest bioturbation rates were observed from A. marina, reaching up to 375 cm2yr−1; followed by H. diversicolor, with 124 cm2yr−1 being the peak bioturbation rate for the ragworm. Additionally, the sediment reworking activity of A. marina facilitated nearly double the amount of silicate efflux compared to any other species. Arenicola marina and H. diversicolor accordingly facilitated stronger nutrient effluxes under a warmer temperature than L. conchilega and C. edule. The oxygen uptake of A. marina and H. diversicolor within the sediment incubation cores was correspondingly enhanced with a higher temperature. Thus, increases in sea temperatures may initially be beneficial to ecosystem functioning in the Wadden Sea as faunal bioturbation is definitely expedited, leading to a tighter coupling between the sediment and overlying water column. The enhanced bioturbation activity, oxygen consumption, and facilitated nutrient effluxes from these invertebrates themselves, will aid in the ongoing high levels of primary productivity and organic matter production.

Description: Bottom trawling represents the most widespread anthropogenic physical disturbance to shelf sea sediments. While trawling-induced mortality in benthic fauna has been extensively investigated, its impacts on ecosystem functioning and carbon cycling at regional scales remain unclear. Using the North Sea as an example, we address these issues by synthesizing a high-resolution dataset of bottom trawling impact on sediments, feeding this dataset into a 3-dimensional physical–biogeochemical model to estimate trawling-induced changes in biomass, bioturbation and sedimentary organic carbon, and assessing model results with field samples. Results suggest a trawling-induced net reduction in macrobenthic biomass by 10-27%. Trawling-induced resuspension and reduction of bioturbation jointly and accumulatively reduce the regional sedimentary organic carbon sequestration capacity by 21-67%, equivalent to 0.58-1.84 Mt CO2 yr-1. Our study emphasizes the need for proper management of trawling on muddy seabeds, if the natural capacity of shelf seas for carbon sequestration should be conserved and restored.