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.

Description: Highlights ● We developed a pH eddy covariance system to detect a sub-seafloor CO2 release. ● It detected CO2 emission to the water column at injection rates of 5.7–143 kg d − 1. ● It was also sensitive enough to quantify benthic biological CO2 production. ● Close to bubble streams, the kinetics of aqueous CO2 equilibration are important. ● This system can be used to detect, attribute, and quantify seafloor sources of CO2. We detected a controlled release of CO2 (g) with pH eddy covariance. We quantified CO2 emission using measurements of water velocity and pH in the plume of aqueous CO2 generated by the bubble streams, and using model predictions of vertical CO2 dissolution and its dispersion downstream. CO2 (g) was injected 3 m below the floor of the North Sea at rates of 5.7–143 kg d − 1. Instruments were 2.6 m from the center of the bubble streams. In the absence of injected CO2, pH eddy covariance quantified the proton flux due to naturally-occurring benthic organic matter mineralization (equivalent to a dissolved inorganic carbon flux of 7.6 ± 3.3 mmol m − 2 d − 1, s.e., n = 33). At the lowest injection rate, the proton flux due to CO2 dissolution was 20-fold greater than this. To accurately quantify emission, the kinetics of the carbonate system had to be accounted for. At the peak injection rate, 73 ± 13% (s.d.) of the injected CO2 was emitted, but when kinetics were neglected, the calculated CO2 emission was one-fifth of this. Our results demonstrate that geochemical techniques can detect and quantify very small seafloor sources of CO2 and attribute them to natural or abiotic origins.

Description: Highlights • A mechanistic explanation is provided for the observed CO2 loss in the sediments. • Reactions of CO2 with the sediment lead to significant heating. • The observations were modeled including reactions and losses due to lateral transport. • CO2 leakage will lead to very local effects. Abstract We investigated the effect of an artificial CO2 vent (0.0015−0.037 mol s−1), simulating a leak from a reservoir for carbon capture and storage (CCS), on the sediment geochemistry. CO2 was injected 3 m deep into the seafloor at 120 m depth. With increasing mass flow an increasing number of vents were observed, distributed over an area of approximately 3 m. In situ profiling with microsensors for pH, T, O2 and ORP showed the geochemical effects are localized in a small area around the vents and highly variable. In measurements remote from the vent, the pH reached a value of 7.6 at a depth of 0.06 m. In a CO2 venting channel, pH reduced to below 5. Steep temperature profiles were indicative of a heat source inside the sediment. Elevated total alkalinity and Ca2+ levels showed calcite dissolution. Venting decreased sulfate reduction rates, but not aerobic respiration. A transport-reaction model confirmed that a large fraction of the injected CO2 is transported laterally into the sediment and that the reactions between CO2 and sediment generate enough heat to elevate the temperature significantly. A CO2 leak will have only local consequences for sediment biogeochemistry, and only a small fraction of the escaped CO2 will reach the sediment surface.

Description: Subterranean estuaries are connective zones between inland aquifers and the open sea where terrestrial freshwater and circulating seawater mix and undergo major biogeochemical changes. They are biogeochemical reactors that modify groundwater chemistry prior to discharge into the sea. We propose that subterranean estuaries of high-energy beaches are particularly dynamic environments, where the effect of the dynamic boundary conditions propagates tens of meters into the subsurface, leading to strong spatio-temporal variability of geochemical conditions. We hypothesize that they form a unique habitat with an adapted microbial community unlike other typically more stable subsurface environments. So far, however, studies concerning subterranean estuaries of high-energy beaches have been rare and therefore their functioning, and their importance for coastal ecosystems, as well as for carbon, nutrient and trace element cycling, is little understood. We are addressing this knowledge gap within the interdisciplinary research project DynaDeep by studying the combined effect of surface (hydro- and morphodynamics) on subsurface processes (groundwater flow and transport, biogeochemical reactions, microbiology). A unique subterranean estuary observatory was established on the northern beach of the island of Spiekeroog facing the North Sea, serving as an exemplary high-energy research site and model system. It consists of fixed and permanent infrastructure such as a pole with measuring devices, multi-level groundwater wells and an electrode chain. This forms the base for autonomous measurements, regular repeated sampling, interdisciplinary field campaigns and experimental work, all of which are integrated via mathematical modelling to understand and quantify the functioning of the biogeochemical reactor. First results show that the DynaDeep observatory is collecting the intended spatially and temporally resolved morphological, sedimentological and biogeochemical data. Samples and data are further processed ex-situ and combined with experiments and modelling. Ultimately, DynaDeep aims at elucidating the global relevance of these common but overlooked environments.

Description: Aquatic eddy covariance oxygen flux was determined over two seagrass (Posidonia oceanica) meadows at Elba, Italy. The first meadow (open-water) was located 300 m from the southwest corner of the island, and was studied over two continuous days from 15 to 18 May 2016. The second meadow (nearshore) was located 60 m from the north shore of the island, and was studied over two discontinuous days on 13 and 25 May 2017. Both meadows were located at 13 m depth. Eddy covaraince instruments were mounted to a lightweight frame and positioned over seagrass meadows such that the measurement volume was approximately 0.3 m above the top of the canopy. Eddy covariance velocity data were collected at 16 Hz with an acoustic Doppler velocimeter (Vector, Nortek-AS, Norway). Measurements of turbulent fluctuations in oxygen concentration were made 2 cm outside the measuring volume of the Vector using an optode minisensor (O2 Minisensor, Pyroscience GmbH, Germany). The 90% response time of the minisensor was less than 0.3 s. Stable oxygen measurements above and within the canopy were determined with galvanic oxygen sensors (OxyGuard, RBR Ltd., Canada). Eddy covariance fluxes were calculated from the product of turbulent fluctuations in vertical water velocity and oxygen concentration according to standard techniques (see Berg et al., 2003 for details on the aquatic eddy covariance technique, doi:10.3354/meps261075). Further details on calculations of flux, and their correction for the nighttime depletion of oxygen within the seagrass canopy, are presented in the linked manuscript (Koopmans et al., 2020, doi:10.3389/fmars.2020.00118).

Description: During the PS118 research cruise with the German research vessel RV POLARSTERN (Feb 2019- April 2019), sediments were collected with a multicorer from 7 stations along a 400 mile transect from the eastern shelf of the Antarctic Peninsula to the West of the South Orkney Islands. A total of 69 high-resolution O2 profiles were measured in 23 cores in the upper sediment layer using optical microsensors (Optodes, Pyroscience) in order to determine oxygen penetration depths and diffusive oxygen uptake (DOU). Pore-water samples were taken at depth resolutions of 1 cm from 0-10 cm and below 10 cm with a resolution of 2 cm down to a maximum depth of 30 cm. Sediments of parallel cores were cut at the same depth resolution for solid-phase contents. Pore-water analyses of trace element such as dissolved iron (DFe) and manganese (DMn), dissolved inorganic carbon (DIC), hydrogen sulfide (H2S) and nutrients such as ammonium (NH4+), phosphate (PO4³), nitrate (NO3-), nitrite (NO2-) and silicate (SiO3²−) were measured. For solid-phase analyses, freeze-dried and ground sediment samples were measured for TOC and TN. At 5 stations, Al, Fe, Mn, P and S content was measured after total acid digestion. At the same 5 stations, excess 210Pb was measured in freeze-dried and homogenized sediments.

Description: During the PS118 research cruise with the German research vessel RV POLARSTERN (Feb 2019- April 2019), sediments were collected with a multicorer from 7 stations along a 400 mile transect from the eastern shelf of the Antarctic Peninsula to the West of the South Orkney Islands. A total of 69 high-resolution O2 profiles were measured in 23 cores in the upper sediment layer using optical microsensors (Optodes, Pyroscience) in order to determine oxygen penetration depths and diffusive oxygen uptake (DOU). Pore-water samples were taken at depth resolutions of 1 cm from 0-10 cm and below 10 cm with a resolution of 2 cm down to a maximum depth of 30 cm. Sediments of parallel cores were cut at the same depth resolution for solid-phase contents. Pore-water analyses of trace element such as dissolved iron (DFe) and manganese (DMn), dissolved inorganic carbon (DIC), hydrogen sulfide (H2S) and nutrients such as ammonium (NH4+), phosphate (PO4³), nitrate (NO3-), nitrite (NO2-) and silicate (SiO3²−) were measured. For solid-phase analyses, freeze-dried and ground sediment samples were measured for TOC and TN. At 5 stations, Al, Fe, Mn, P and S content was measured after total acid digestion. At the same 5 stations, excess 210Pb was measured in freeze-dried and homogenized sediments.

Description: During the PS118 research cruise with the German research vessel RV POLARSTERN (Feb 2019- April 2019), sediments were collected with a multicorer from 7 stations along a 400 mile transect from the eastern shelf of the Antarctic Peninsula to the West of the South Orkney Islands. A total of 69 high-resolution O2 profiles were measured in 23 cores in the upper sediment layer using optical microsensors (Optodes, Pyroscience) in order to determine oxygen penetration depths and diffusive oxygen uptake (DOU). Pore-water samples were taken at depth resolutions of 1 cm from 0-10 cm and below 10 cm with a resolution of 2 cm down to a maximum depth of 30 cm. Sediments of parallel cores were cut at the same depth resolution for solid-phase contents. Pore-water analyses of trace element such as dissolved iron (DFe) and manganese (DMn), dissolved inorganic carbon (DIC), hydrogen sulfide (H2S) and nutrients such as ammonium (NH4+), phosphate (PO4³), nitrate (NO3-), nitrite (NO2-) and silicate (SiO3²−) were measured. For solid-phase analyses, freeze-dried and ground sediment samples were measured for TOC and TN. At 5 stations, Al, Fe, Mn, P and S content was measured after total acid digestion. At the same 5 stations, excess 210Pb was measured in freeze-dried and homogenized sediments.

Description: During the PS118 research cruise with the German research vessel RV POLARSTERN (Feb 2019- April 2019), sediments were collected with a multicorer from 7 stations along a 400 mile transect from the eastern shelf of the Antarctic Peninsula to the West of the South Orkney Islands. A total of 69 high-resolution O2 profiles were measured in 23 cores in the upper sediment layer using optical microsensors (Optodes, Pyroscience) in order to determine oxygen penetration depths and diffusive oxygen uptake (DOU). Pore-water samples were taken at depth resolutions of 1 cm from 0-10 cm and below 10 cm with a resolution of 2 cm down to a maximum depth of 30 cm. Sediments of parallel cores were cut at the same depth resolution for solid-phase contents. Pore-water analyses of trace element such as dissolved iron (DFe) and manganese (DMn), dissolved inorganic carbon (DIC), hydrogen sulfide (H2S) and nutrients such as ammonium (NH4+), phosphate (PO4³), nitrate (NO3-), nitrite (NO2-) and silicate (SiO3²−) were measured. For solid-phase analyses, freeze-dried and ground sediment samples were measured for TOC and TN. At 5 stations, Al, Fe, Mn, P and S content was measured after total acid digestion. At the same 5 stations, excess 210Pb was measured in freeze-dried and homogenized sediments.