Description: Estuaries regulate carbon cycling along the land-ocean continuum and thus influence carbon export to the ocean, and global carbon budgets. The Elbe Estuary in Germany has been altered by large anthropogenic perturbations, such as widespread heavy metal pollution, minimally treated wastewater before the 1980s, establishment of wastewater treatment plants after the 1990s, and an overall nutrient and pollutant load reduction in the last three decades. Based on an extensive evaluation of key ecosystem variables, and an analysis of the available inorganic and organic carbon records, this study has identified three ecosystem states in recent history: the polluted (1985–1990), transitional (1991–1996), and recovery (1997–2018) states. The polluted state was characterized by very high dissolved inorganic carbon (DIC) and ammonium concentrations, toxic heavy metal levels, dissolved oxygen undersaturation, and low pH. During the transitional state, heavy metal pollution decreased by 〉 50%, and primary production re-established in spring to summer, with weak seasonality in DIC. Since 1997, during the recovery state, DIC seasonality was driven by primary production, and DIC significantly increased by 〉 23 μmol L−1 yr−1 in the mid to lower estuary, indicating that, along with the improvement in water quality the ecosystem state is still changing. Large anthropogenic perturbations can therefore alter estuarine ecosystems (on the order of decades), as well as induce large and complex biogeochemical shifts and significant changes to carbon cycling.

Description: Estuaries are an important contributor to the global carbon budget, facilitating carbon removal, transfer, and transformation between land and the coastal ocean. Estuaries are susceptible to global climate change and anthropogenic perturbations. We find that a long-term significant mid-estuary increase in dissolved inorganic carbon (DIC) of 6–21 µmol kg−1 yr−1 (1997–2020) in a temperate estuary in Germany (Elbe Estuary) was driven by an increase in upper-estuary particulate organic carbon (POC) content of 8–14 µmol kg−1 yr−1. The temporal POC increase was due to an overall improvement in water quality observed in the form of high rates of primary production and a significant drop in biological oxygen demand. The magnitude of mid-estuary DIC gain was equivalent to the increased POC production in the upper estuary, suggesting that POC is effectively remineralized and retained as DIC in the mid-estuary, with the estuary acting as an efficient natural filter for POC. In the context of this significant long-term DIC increase, a recent extended drought period (2014–2020) significantly lowered the annual mean river discharge (468 ± 234 m3 s−1) compared to the long-term mean (690 ± 441 m3 s−1, 1960–2020), while the late spring internal DIC load in the estuary doubled. The drought induced a longer dry season, starting in May (earlier than normal), increased the residence time in the estuary and allowed for a more complete remineralization period of POC. Annually, 77 %–94 % of the total DIC export was laterally transported to the coastal waters, reaching 89 ± 4.8 Gmol C yr−1, and thus, between 1997 and 2020, only an estimated maximum of 23 % (10 Gmol C yr−1) was released via carbon dioxide (CO2) evasion. Export of DIC to coastal waters decreased significantly during the drought, on average by 24 % (2014–2020: 38 ± 5.4 Gmol C yr−1), compared to the non-drought period. In contrast, there was no change in the water–air CO2 flux during the drought. We have identified that seasonal changes in DIC processing in an estuary require consideration when estimating both the long-term and future changes in water–air CO2 flux and DIC export to coastal waters. Regional and global carbon budgets should therefore take into account carbon cycling estimates in estuaries, as well as their changes over time in relation to impacts of water quality changes and extreme hydrological events.

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: A series of multibeam bathymetry surveys revealed the emergence of a large pockmark field in the southeastern North Sea. Covering an area of around 915 km2, up to 1,200 pockmarks per square kilometer have been identified. The time of emergence can be confined to 3 months in autumn 2015, suggesting a very dynamic genesis. The gas source and the trigger for the simultaneous outbreak remain speculative. Subseafloor structures and high methane concentrations of up to 30 mmol/l in sediment pore water samples suggest a source of shallow biogenic methane from the decomposition of post-glacial deposits in a paleo river valley. Storm waves are suggested as the final trigger for the eruption of the gas. Due to the shallow water depths and energetic conditions at the presumed time of eruption, a large fraction of the released gas must have been emitted to the atmosphere. Conservative estimates amount to 5 kt of methane, equivalent to 67% of the annual release from the entire North Sea. These observations most probably describe a reoccurring phenomenon in shallow shelf seas, which may have been overlooked before because of the transient nature of shallow water bedforms and technology limitations of high resolution bathymetric mapping.

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: Bundesministerium für Bildung und Forschung http://dx.doi.org/10.13039/501100002347

Description: Denitrification on continental margins and in coastal sediments is a major sink of reactive N in the present nitrogen cycle and a major ecosystem service of eutrophied coastal waters. We analyzed the nitrate removal in surface sediments of the Elbe estuary, Wadden Sea, and adjacent German Bight (SE North Sea) during two seasons (spring and summer) along a eutrophication gradient ranging from a high riverine nitrate oncentrations at the Elbe Estuary to offshore areas with low nitrate concentrations. The gradient encompassed the full range of sediment types and organic carbon concentrations of the southern North Sea. Based on nitrate penetration depth and concentration gradient in the porewater we estimated benthic nitrate consumption rates assuming either diffusive transport in cohesive sediments or advective transport in permeable sediments. For the latter we derived a mechanistic model of porewater flow. During the peak nitrate discharge of the river Elbe in March, the highest rates of diffusive nitrate uptake were observed in muddy sediments (up to 2.8 mmol m−2 d−1). The highest advective uptake rate in that period was observed in permeable sediment and was tenfold higher (up to 32 mmol m−2 d−1). The intensity of both diffusive and advective nitrate consumption dropped with the nitrate availability and thus decreased from the Elbe estuary towards offshore stations, and were further decreased during late summer (minimum nitrate discharge) compared to late winter (maximum nitrate discharge). In summary, our rate measurements indicate that the permeable sediment accounts for up to 90% of the total benthic reactive nitrogen consumption in the study area due to the high efficiency of advective nitrate transport into permeable sediment. Extrapolating the averaged nitrate consumption of different sediment classes to the areas of Elbe Estuary, Wadden Sea and eastern German Bight amounts to an N-loss of 3.1 ∗ 106 mol N d−1 from impermeable, diffusion-controlled sediment, and 5.2 ∗ 107 mol N d−1 from permeable sediment with porewater advection.