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Salt mixing enables the transport of water between the inflow and outflow layers of estuarine circulation and therefore closes the circulation by driving a diahaline exchange flow. A recently derived universal law links the salt mixing inside an estuarine volume bounded by an isohaline surface to freshwater discharge: it states that on long-term average, the area-integrated mixing across the bounding isohaline is directly proportional to the freshwater discharge entering the estuary. However, even though numerous studies predict that periods of extreme discharge will become more frequent with climate change, the direct impact of such periods on estuarine mixing and circulation has yet to be investigated. Therefore, this numerical modeling study focuses on salinity mixing and diahaline exchange flows during a low-discharge and an extreme high-discharge period. To this end, we apply a realistic numerical setup of the Elbe estuary in northern Germany, using curvilinear coordinates that follow the navigational channel. This is the first time the direct relationship between diahaline exchange flow and salt mixing as well as the spatial distribution of the diahaline exchange flow is shown in a realistic tidal setup. The spatial distribution is highly correlated with the local mixing gradient for salinity, such that inflow occurs near the bottom at the upstream end of the isohaline. Meanwhile, outflow occurs near the surface at its downstream end. Last, increased vertical stratification occurs within the estuary during the high-discharge period, while estuarine-wide mixing strongly converges to the universal law for averaging periods of the discharge event time scale. Significance Statement: Inside estuaries, such as river mouths, terrestrial freshwater is mixed with salty ocean water. This is accompanied by an estuarine circulation with inflow of saltwater into the estuary and outflow of brackish water toward the ocean. Here, we aim to better understand how salt mixing and estuarine circulation in a tidal estuary react to periods of extreme freshwater discharge. We find that even during extremely high or low discharge, salt mixing follows the freshwater discharge on time scales as short as days, and that estuarine circulation patterns are largely explained by the local distribution of mixing. As extreme runoff events are likely to occur more often with climate change, these findings may help to understand the dynamics inside future estuaries. [ABSTRACT FROM AUTHOR]

The present study aims to estimate effective diahaline turbulent salinity fluxes and diffusivities in numerical model simulations of estuarine scenarios. The underlying method is based on a quantification of salinity mixing per salinity class, which is shown to be twice the turbulent salinity transport across the respective isohaline. Using this relation, the recently derived universal law of estuarine mixing, predicting that average mixing per salinity class is twice the respective salinity times the river run‐off, can be directly derived. The turbulent salinity transport is accurately decomposed into physical (due to the turbulence closure) and numerical (due to truncation errors of the salinity advection scheme) contributions. The effective diahaline diffusivity representative for a salinity class and an estuarine region results as the ratio of the diahaline turbulent salinity transport and the respective (negative) salinity gradient, both integrated over the isohaline area in that region and averaged over a specified period. With this approach, the physical (or numerical) diffusivities are calculated as half of the product of physical (or numerical) mixing and the isohaline volume, divided by the square of the isohaline area. The method for accurately calculating physical and numerical diahaline diffusivities is tested and demonstrated for a three‐dimensional idealized exponential estuary. As a major product of this study, maps of the spatial distribution of the effective diahaline diffusivities are shown for the model estuary. Plain Language Summary: Eddy diffusivity determines how intensively concentrations in a fluid are spreading due to turbulent motion. Here, we analyze the diffusivity that spreads salt concentration (i.e., salinity) across a surface of constant salinity (the isohalines), also called effective diahaline diffusivity. A new method is presented that calculates effective diahaline diffusivities based on the specific volume between two specified isohalines, on the salinity mixing within this volume as well as on the surface area of the isohalines. We define mixing as the rate of destruction of salinity variance per unit volume due to turbulent mixing processes. The method applies to computer models of ocean dynamics on scales ranging from coastal to global. In such models, the mixing is determined by statistical mathematical equations of turbulent processes, which is the so‐called physical mixing. In models, additional (numerical) mixing occurs due to numerical inaccuracies of algorithms that move around water masses passively with the currents, a process called advection. Using our method, the total effective diffusivity determined for each isohaline surface can be accurately separated into contributions from physical mixing and numerical mixing. We demonstrate the functioning of the new method for an idealized model simulation of an estuary. Key Points: Mixing and volume per salinity class determine effective diahaline diffusivityEffective diahaline diffusivity is split into physical and numerical contributionsIn an idealized estuary, largest effective diffusivities are found in the brackish waters of the navigational channel [ABSTRACT FROM AUTHOR]

We analyzed changes in mean sea level (MSL) for the period 1950–2015 using a regional ocean model for the Baltic Sea. Sensitivity experiments allowed us to separate external from local drivers and to investigate individual forcing agents triggering basin-internal spatial variations. The model reveals a basin-average MSL rise (MSLR) of 2.08 ± 0.49 mm yr−1, a value that is slightly larger than the simultaneous global average of 1.63 ± 0.32 mm yr−1. This MSLR is, however, spatially highly nonuniform with lower than average increases in the southwestern part (1.71 ± 0.51 mm yr−1) and higher than average rates in the northeastern parts (2.34 ± 1.05 mm yr−1). While 75% of the basin-average MSL externally enters the Baltic basin as a mass signal from the adjacent North Sea, intensified westerly winds and a poleward shift of low pressure systems explain the majority of the spatial variations in the rates. Minor contributions stem from local changes in baroclinicity leading to a basin-internal redistribution of water masses. An observed increase in local ocean temperature further adds to the total basinwide MSLR through thermal expansion but has little effect on the spatial pattern. To test the robustness of these results, we further assessed the sensitivity to six different atmospheric surface forcing reanalysis products over their common period from 1980 to 2005. The ensemble runs indicated that there are significant differences between individual ensemble members increasing the total trend uncertainty for the basin average by 0.22 mm yr−1 (95% confidence intervals). Locally the uncertainty varies from 0.05 mm yr−1 in the central part to up to 0.4 mm yr−1 along the coasts. [ABSTRACT FROM AUTHOR]

The influence of a summer storm event in 2007 on the North Sea and its effects on the ocean stratification are investigated using a regional coupled ocean (Regional Ocean Modeling System, ROMS)-atmosphere (Weather Research & Forecasting model, WRF) modeling system. An analysis of potential energy anomaly (PEA, Φ) and its temporal development reveals that the loss of stratification due to the storm event is dominated by vertical mixing in almost the entire North Sea. For specific regions, however, a considerable contribution of depth-mean straining is observed. Vertical mixing is highly correlated with wind induced surface stresses. However, peak mixing values are observed in combination with incoming flood currents. Depending on the phase between winds and tides, the loss of stratification differs strongly over the North Sea. To study the effects of interactive ocean-atmosphere exchange, a fully coupled simulation is compared with two uncoupled ones for the same vertical mixing parameters to identify the impact of spatial resolution as well as of SST feedback. While the resulting new mixed layer depth after the storm event in the uncoupled simulation with lower spatial and temporal resolution of the surface forcing data can still be located in the euphotic zone, the coupled simulation is capable to mix the entire water column and the vertical mixing in the uncoupled simulation with higher resolution of the surface forcing data is strongly amplified. These differences might have notable implications for ecosystem modeling since it could determine the development of new phytoplankton blooms after the storm and for sediment modeling in terms of sediment mobilization. An investigation of restratification after the extreme event illustrates the persistent effect of this summer storm. [ABSTRACT FROM AUTHOR]

Due to their choked geometry, coastal lagoons can attenuate extreme water levels compared to the open sea. However, this protective property is expected to decrease due to sea‐level rise. By studying idealized lagoons in a non‐dimensional parameter space, this study describes non‐linear interactions between tides, storm surges, freshwater fluxes into the lagoon, and sea‐level rise. The non‐dimensional numbers include lagoon geometry and forcing scales. The main objective is to provide an overview of potentially affected lagoons and to highlight the importance of attenuation changes due to sea‐level rise. Tidal and storm surge induced maximum water levels inside lagoons rise faster than sea‐level rise for most of the parameter space. Maximum water levels due to freshwater fluxes rise slower than sea‐level rise for strongly choked lagoons. For compound events, the response between rising faster or slower than sea‐level rise depends strongly on the lagoon geometry. Plain Language Summary: Coastal lagoons are known to reduce extreme water levels compared to the open sea due to their narrow and shallow connections to the ocean. However, the rising mean sea level will decrease this protective property. In this study, we investigate lagoons using a simple box model to explore how combinations of tides, storm surges, river discharge, and sea‐level rise will change water levels inside lagoons. The study aims to provide an overview of potentially affected lagoons and to highlight the importance of these processes. Due to sea‐level rise, water from the ocean can more easily flow into the lagoon. Therefore, our results show that maximum water levels inside a lagoon can rise faster than sea‐level rise. On the other hand, river discharge can exit the lagoon more easily at the same time. For this case, water levels inside a lagoon rise slower than sea‐level rise. For events where tides, storm surges, and river discharge are all present, the maximum water level may rise either slower or faster than sea‐level rise, depending on the properties of the lagoon. Further studies are needed to understand the implications for coastal communities. Key Points: Tides, storm surges, freshwater fluxes, and sea‐level rise interact non‐linearly on the water level in choked coastal lagoonsMaximum water levels inside choked lagoons can increase faster than sea‐level rise due to non‐linear attenuation changesMaximum water levels due to freshwater fluxes increase slower than sea‐level rise [ABSTRACT FROM AUTHOR]

Shelf seas such as the North Sea and the Baltic Sea are characterised by spatially and temporally varying stratification that is highly relevant for their physical dynamics and the evolution of their ecosystems. Stratification may vary from unstably stratified (e.g., due to convective surface cooling) to strongly stratified with density jumps of up to 10 kg/m 3 per m (e.g., in overflows into the Baltic Sea). Stratification has a direct impact on vertical turbulent transports (e.g., of nutrients) and influences the entrainment rate of ambient water into dense bottom currents which in turn determine the stratification of and oxygen supply to, e.g., the central Baltic Sea. Moreover, the suppression of the vertical diffusivity at the summer thermocline is one of the limiting factors for the vertical exchange of nutrients in the North Sea. Due to limitations of computational resources and since the locations of such density jumps (either by salinity or temperature) are predicted by the model simulation itself, predefined vertical coordinates cannot always reliably resolve these features. Thus, all shelf sea models with a predefined vertical coordinate distribution are inherently subject to under-resolution of the density structure. To solve this problem, Burchard and Beckers (2004) and Hofmeister et al. (2010) developed the concept of vertically adaptive coordinates for ocean models, where zooming of vertical coordinates at locations of strong stratification (and shear) is imposed. This is achieved by solving a diffusion equation for the position of the coordinates (with the diffusivity being proportional to the stratification or shear frequencies). We will show for a coupled model system of the North Sea and the Baltic Sea (resolution ∼ 1.8 km) how numerical mixing is substantially reduced and model results become significantly more realistic when vertically adaptive coordinates are applied. We additionally demonstrate that vertically adaptive coordinates perform well in simulating the two dynamically different regions North Sea and Baltic Sea with a single parameter set. An analysis of the computational overhead of the adaptive coordinates indicates an increase of 5–8% in runtime. This is still less expensive than adding more sigma-layers to reduce spurious numerical mixing. [ABSTRACT FROM AUTHOR]

Globally coupled climate models are generally capable of reproducing the observed trends in the globally averaged atmospheric temperature. However, the global models do not perform as well on regional scales. Here, we present results from four 100-year, high-resolution ocean model experiments (resolution less than 1 km) for the western Baltic Sea. The forcing is taken from a regional atmospheric model and a regional ocean model, imbedded into two global greenhouse gas emission scenarios, A1B and B1, for the period of 2000 to 2100 with each two realisations. Two control runs from 1960 to 2000 are used for validation. For both scenarios, the results show a warming with an increase of 0.5-2.5 K at the sea surface and 0.7-2.8 K below 40 m. The simulations further indicate a decrease in salinity by 1.5-2 practical salinity units. The increase in water temperature leads to a prolongation of heat waves based on present-day thresholds. This amounts to a doubling or even tripling of the heat wave duration. The simulations show a decrease in inflow events (barotropic/baroclinic), which will affect the deepwater generation and ventilation of the central Baltic Sea. The high spatial resolution allows us to diagnose the inflow events and the mechanism that will cause future changes. The reduction in barotropic inflow events correlates well with the increase in westerly winds. The changes in the baroclinic inflows can be consistently explained by the reduction of calm wind periods and thus a weakening of the necessary stratification in the western Baltic Sea and the Danish Straits. [ABSTRACT FROM AUTHOR]

Globally-coupled climate models are generally capable of reproducing the observed trends in the globally averaged atmospheric temperature or mean sea level. However, the global models do not perform as well on regional/local scales. Here, we present results from four 100-year ocean model experiments for the Western Baltic Sea. In order to simulate storm surges in this region, we have used the General Estuarine Transport Model (GETM) as a high-resolution local model (spatial resolution ≈ 1 km), nested into a regional atmospheric and regional oceanic model in a fully baroclinic downscaling approach. The downscaling is based on the global model ECHAM5/MPI-OM. The projections are imbedded into two greenhouse-gas emission scenarios, A1B and B1, for the period 2000-2100, each with two realisations. Two control runs from 1960 to 2000 are used for validation. We use this modelling system to statistically reproduce the present distribution of surge extremes. The usage of the high-resolution local model leads to an improvement in surge heights of at least 10% compared to the driving model. To quantify uncertainties associated with climate projections, we investigate the impact of enhanced wind velocities and changes in mean sea levels. The analysis revealed a linear dependence of surge height and mean sea level, although the slope parameter is spatially varying. Furthermore, the modelling system is used to project possible changes within the next century. The results show that the sea level rise has greater potential to increase surge levels than does increased wind speed. The simulations further indicate that the changes in storm surge height in the scenarios can be consistently explained by the increase in mean sea level and variation in wind speed. [ABSTRACT FROM AUTHOR]

During the last decades, the Euler scheme was the common 'workhorse' in particle tracking, although it is the lowest-order approximation of the underlying stochastic differential equation. To convince the modelling community of the need for better methods, we have constructed a new test case that will show the shortcomings of the Euler scheme. We use an idealised shallow-water diffusivity profile that mimics the presence of a sharp pycnocline and thus a quasi-impermeable barrier to vertical diffusion. In this context, we study the transport of passive particles with or without negative buoyancy. A semi-analytic solutions is used to assess the performance of various numerical particle-tracking schemes (first- and second-order accuracy), to treat the variations in the diffusivity profile properly. We show that the commonly used Euler scheme exhibits a poor performance and that widely used particle-tracking codes shall be updated to either the Milstein scheme or second-order schemes. It is further seen that the order of convergence is not the only relevant factor, the absolute value of the error also is. [ABSTRACT FROM AUTHOR]

Abstract: Stochastic differential equations (SDEs) offer an attractively simple solution to complex transport-controlled problems, and have a wide range of physical, chemical, and biological applications, which are dominated by stochastic processes, such as diffusion. As for deterministic ordinary differential equations (ODEs), various numerical scheme exist for solving SDEs. In this paper various particle-tracking schemes are presented and tested for accuracy and efficiency (time vs. accuracy). To test the schemes, the particle tracking algorithms are implemented into a community wide used 1D water column model. Modelling individual particles allows a straightforward physical interpretation of the involved processes. Further, this approach is strictly mass conserving and does not suffer from the numerical diffusion that plagues grid-based methods. Moreover, the Lagrangian framework allows to assign properties to the individual particles, that can vary spatially and temporally. The movement of the particles is described by a stochastic differential equation, which is consistent with the advection–diffusion equation. Here, the concentration profile is represented by a set of independent moving particles, which are advected according to the velocity field, while the diffusive displacements of the particles are sampled from a random distribution, which is related to the eddy diffusivity field. The paper will show that especially the 2nd order schemes are accurate and highly efficient. At the same level of accuracy, the 2nd order scheme can be significantly faster than the 1st order counterpart. This gain in efficiency can be spent on a higher resolution for more accurate solutions at a lower cost. [Copyright &y& Elsevier]

Hervey Bay, a large coastal embayment situated off the central eastern coast of Australia, is a shallow tidal area (average depth = 15 m), close to the continental shelf. It shows features of an inverse estuary, due to the high evaporation rate (approx. 2 m/year), low precipitation (less than 1 m/year) and on average almost no freshwater input from rivers that drain into the bay. The hydro- and thermodynamical structures of Hervey Bay and their variability are presented here for the first time, using a combination of four-dimensional modelling and observations from field studies. The numerical studies are performed with the Coupled Hydrodynamical Ecological Model for Regional Shelf Seas (COHERENS). Due to the high tidal range (>3.5 m), the bay is considered as a vertically well-mixed system, and therefore, only horizontal fronts are likely. Recent field measurements, but also the numerical simulations, indicate characteristic features of an inverse/hypersaline estuary with low salinity (35.5 psu) in the open ocean and peak values (>39.0 psu) in the head water of the bay. The model further predicts a nearly persistent mean salinity gradient of 0.5 psu across the bay (with higher salinities close to the shore). The investigation further shows that air temperature, wind direction and tidal regime are mainly responsible for the stability of the inverse circulation and the strength of the salinity gradient across the bay. Due to an ongoing drying trend, the occurrence of severe droughts at the central east coast of Australia and, therefore, a reduction in freshwater supply, the salinity flux out of the bay has increased, and the inverse circulation has also strengthened. [ABSTRACT FROM AUTHOR]

The Baltic Sea is a semi-enclosed sea and consists of brackish water with a salinity of about 7 g/kg. It sporadically experiences salt water inflow events through the Danish straits with strong inflows occurring once per year on average. An increase in mean sea level, which is expected to rise similarly to the projected global mean sea level (GMSL), impacts the dynamics of these salt water inflow events. Therefore, we use a regional setup of the ocean circulation model GETM spanning over the western Baltic Sea and having a spatial resolution of 600 meter to investigate the response of GMSL rise to inflow events during the years 1980 to 2015. As case study, the processes of a recent inflow event in December 2014 are additionally simulated with a model setup, using a horizontal resolution of 200 meter. It is analyzed how transports of volume and salt would differ, if GMSL would be one meter higher than present day sea level. Hence, a simulation is performed with the same model setup as the reference simulation, only the bathymetry of the model setup is deepened by one meter.The results of the reference model simulations are evaluated by comparisons with observations. For example, salinity and temperature at two stations located at the Darss Sill and in the Arkona Basin agree well. In addition, the volume transport through the Öresund, computed from sea surface height in Viken and Klagshamn, have a correlation of 0.96 with observations during the years 2014 and 2015 (nudging of sea surface height is applied at the model boundaries). A detailed analysis of the change in the physical processes with one meter GMSL rise will be presented. However, first analyses of the inflow event in December 2014, simulated with the 600m GETM model setup, show that the duration of this event did not change. However, the volume of high saline water (> 17 g/kg) in the Arkona Basin increased by about 10.7% during the inflow event. In addition, results show similar relative changes in volume and salt transport through the Danish straits (at Darss Sill and Drodgen Sill), and a less than 2% higher relative salt transport than volume transport when considering only the transports where salinity is greater than 17 g/kg. These results suggest a quite linear increase in salt inflow with increasing volume transport through the Danish straits during this inflow event. [ABSTRACT FROM AUTHOR]

The impact of sea level rise (SLR) on the future morphological development of the Wadden Sea (North Sea) is investigated by means of extensive process-resolving numerical simulations. A new sediment and morphodynamic module was implemented in the well-established 3D circulation model GETM. A number of different validations are presented, ranging from an idealized 1D channel over a semi-idealized 2D Wadden Sea basin to a fully coupled realistic 40-year hindcast without morphological amplification of the Sylt-Rømøbight, a semi-enclosed subsystem of the Wadden Sea. Based on the results of the hindcast, four distinct future scenarios covering the period 2010-2100 are simulated. While these scenarios differ in the strength of SLR and wind forcing, they also account for an expected increase of tidal range over the coming century. The results of the future projections indicate a transition from a tidal-flat-dominated system toward a lagoon-like system, in which large fractions of the Sylt-Rømøbight will remain permanently covered by water. This has potentially dramatic implications for the unique ecosystem of the Wadden Sea. Although the simulations also predict an increased accumulation of sediment in the back-barrier basin, this accumulation is far too weak to compensate for the rise in mean sea level. [ABSTRACT FROM AUTHOR]

While advanced computational capabilities have enabled the development of complex ocean general circulation models (OGCMs) for marginal seas, systematic comparisons of regional ocean models and their setups are still rare. The Baltic Sea Model Intercomparison Project (BMIP), introduced herein, was therefore established as a platform for the scientific analysis and systematic comparison of Baltic Sea models. The inclusion of a physically consistent regional reanalysis data set for the period 1961–2018 allows for standardized meteorological forcing and river runoff. Protocols to harmonize model outputs and analyses are provided as well. An analysis of six simulations performed with four regional OGCMs differing in their resolution, grid coordinates, and numerical methods was carried out to explore intermodel differences despite harmonized forcing. Uncertainties in the modeled surface temperatures were shown to be larger at extreme than at moderate temperatures. In addition, a roughly linear increase in the temperature spread with increasing water depth was determined and indicated larger uncertainties in the near-bottom layer. On the seasonal scale, the model spread was larger in summer than in winter, likely due to differences in the models' thermocline dynamics. In winter, stronger air–sea heat fluxes and vigorous convective and wind mixing reduced the intermodel spread. Uncertainties were likewise reduced near the coasts, where the impact of meteorological forcing was stronger. The uncertainties were highest in the Bothnian Sea and Bothnian Bay, attributable to the differences between the models in the seasonal cycles of sea ice triggered by the ice–albedo feedback. However, despite the large spreads in the mean climatologies, high interannual correlations between the sea surface temperatures (SSTs) of all models and data derived from a satellite product were determined. The exceptions were the Bothnian Sea and Bothnian Bay, where the correlation dropped significantly, likely related to the effect of sea ice on air–sea heat exchange. The spread of water salinity across the models is generally larger compared to water temperature, which is most obvious in the long-term time series of deepwater salinity. The inflow dynamics of saline water from the North Sea is covered well by most models, but the magnitude, as inferred from salinity, differs as much as the simulated mean salinity of deepwater. Marine heat waves (MHWs), coastal upwelling, and stratification were also assessed. In all models, MHWs were more frequent in shallow areas and in regions with seasonal ice cover. An increase in the frequency (regionally varying between ∼50 % and 250 %) and duration (50 %–150 %) of MHWs during the last 3 decades in all models was found as well. The uncertainties were highest in the Bothnian Bay, likely due to the different trends in sea ice presence. All but one of the analyzed models overestimated upwelling frequencies along the Swedish coast, the Gulf of Finland, and around Gotland, while they underestimated upwelling in the Gulf of Riga. The onset and seasonal cycle of thermal stratification likewise differed among the models. Compared to observation-based estimates, in all models the thermocline in early spring was too deep, whereas a good match was obtained in June when the thermocline intensifies. [ABSTRACT FROM AUTHOR]

The relationship between the salinity mixing, the diffusive salt transport, and the diahaline exchange flow is examined using salinity coordinates. The diahaline inflow and outflow volume transports are defined in this study as the integral of positive and negative values of the diahaline velocity. A numerical model of the Pearl River Estuary (PRE) shows that this diahaline exchange flow is analogous to the classical concept of estuarine exchange flow with inflow in the bottom layers and outflow at the surface. The inflow and outflow magnitudes increase with salinity, while the net transport equals the freshwater discharge Qr after sufficiently long temporal averaging. In summer, intensified salinity mixing mainly occurs in the surface layers and around the islands. The patchy distribution of intensified diahaline velocity suggests that the water exchange through an isohaline surface can be highly variable in space. In winter, the zones of intensification of salinity mixing occur mainly in deep channels. Apart from the impact of freshwater transport from rivers, the transient mixing is also controlled by an unsteadiness term due to estuarine storage of salt and water volume. In the PRE, the salinity mixing and exchange flow show substantial spring–neap variation, while the universal law of estuarine mixing m = 2SQr (with m being the sum of physical and numerical mixing per salinity class S) holds over longer averaging period (spring–neap cycle). The correlation between the patterns of surface mixing, the vorticity, and the salinity gradients indicates a substantial influence of islands on estuarine mixing in the PRE. [ABSTRACT FROM AUTHOR]

The dependency of the estuarine circulation on the depth-to-width ratio of a periodically, weakly stratified tidal estuary is systematically investigated here for the first time. Currents, salinity, and other properties are simulated by means of the General Estuarine Transport Model (GETM) in cross-sectional slice mode, applying a symmetric Gaussian-shaped depth profile. The width is varied over four orders of magnitude. The individual along-channel circulation contributions from tidal straining, gravitation, advection, etc., are calculated and the impact of the depth-to-width ratio on their intensity is presented and elucidated. It is found that the estuarine circulation exhibits a distinct maximum in medium-wide channels (intermediate depth-to-width ratio depending on various parameters), which is caused by a maximum of the tidal straining contribution. This maximum is related to a strong tidal asymmetry of eddy viscosity and shear created by secondary strain-induced periodic stratification (2SIPS): in medium channels, transverse circulation generated by lateral density gradients due to laterally differential longitudinal advection induces stable stratification at the end of the flood phase, which is further increased during ebb by longitudinal straining (SIPS). Thus, eddy viscosity is low and shear is strong in the entire ebb phase. During flood, SIPS decreases the stratification so that eddy viscosity is high and shear is weak. The circulation resulting from this viscosity-shear correlation, the tidal straining circulation, is oriented like the classical, gravitational circulation, with riverine outflow at the surface and oceanic inflow close to the bottom. In medium channels, it is about 5 times as strong as in wide (quasi one-dimensional) channels, in which 2SIPS is negligible. [ABSTRACT FROM AUTHOR]

This study explores the spatiotemporal variability in the residual circulation and its dependence on external forces in an interconnected system of intertidal basins. We focus on the Dutch Wadden Sea (DWS), where winds play a major role in water movements and storms significantly affect its short-term characteristics. We make use of realistic three-dimensional high-resolution numerical simulations to model the hydrodynamics in the DWS for the years 2005–2015. First, the 'empirical orthogonal function' method is applied to analyze systematically the relative contributions of various forcing mechanisms on the residual (i.e., tidally averaged) volume flow through the inlets and the shallow watersheds delimiting the system and each tidal basin. Then, we compute the tidally averaged flushing frequency of fresh water over the entire period of analysis, and we study its variations at short and long time scales. It is found that over 98% of the residual flow's spatiotemporal variations can be explained by the first three EOF modes, which are highly correlated with the cubic power of the vectorial wind components' speed and the amount of fresh water discharged into the back-barrier system. This study reveals that incidental short-duration events (e.g., storms) occurring at time scales of hours to days, albeit episodic and highly variable, strongly influence the residual transport's inter-annual variability and its long-term typical value in multiple-inlet coastal systems. • Yearly median residual flows are highly variable in multiple-inlet coastal systems due to the occurrence of episodic events. • Storms and extreme events leave a mark on the long-term typical residual transport. • The Wadden Sea is strongly event-driven, thus incidental episodes affect its short- and long-term exchange with the North Sea. [ABSTRACT FROM AUTHOR]

It is well known that in numerical models the advective transport relative to fixed or moving grids needs to be discretised with sufficient accuracy to minimise the spurious decay of tracer variance (spurious mixing). In this paper a general analysis of discrete variance decay (DVD) caused by advective and diffusive fluxes is established. Lacking a general closed derivation for the local DVD rate, two non-invasive methods to estimate local DVD during model runtime are discussed. Whereas the first was presented recently by Burchard and Rennau (2008), the second is a newly proposed alternative. This alternative analysis method is argued to have a more consistent foundation. In particular, it recovers a physically sound definition of discrete variance in a Finite-Volume cell. The diagnosed DVD can be separated into physical and numerical (spurious) contributions, with the latter originating from discretisation errors. Based on the DVD analysis, a 3D dissipation analysis is developed to quantify the physically and numerically induced loss of kinetic energy. This dissipation analysis provides a missing piece of information to assess the discrete energy conservation of an ocean model. Analyses are performed and evaluated for three test cases, with complexities ranging from idealised 1D advection to a realistic ocean modelling application to the Western Baltic Sea. In all test cases the proposed alternative DVD analysis method is demonstrated to provide a reliable diagnostic tool for the local quantification of physically and numerically induced dissipation and mixing. [ABSTRACT FROM AUTHOR]

The western Baltic Sea cod (WBC) stock is at historically low levels, mainly attributed to high fishing pressure and low recruitment. Stable stock assessment metrics suggested recovery potential, given appropriate fisheries management measures. However, changing environmental conditions violate stability assumptions, may negatively affect WBC, and challenge the resource management. The present study explored 42 years of changes in WBC biological parameters. WBC body condition gradually decreased over the last decades for juveniles and adults, with a rapid decrease in recent years when a single cohort dominated the overfished stock. The hepato-somatic index and the muscle weight decreased by 50% and 10% in the last 10 years, respectively, suggesting severely decreasing energy reserves and productivity. The changes in energy reserves were associated with changes in environmental conditions (increase in bottom water temperature, expansion of hypoxic areas during late summer/autumn), and changes in diet composition (less herring). A key bottleneck is the warming and longer-lasting summer period when WBC, trapped between warmed shallow waters and hypoxic deeper waters, have to mobilize energy reserves to account for reduced feeding opportunities and thermal stress. Our results suggest that stock recovery is unlikely to happen by fisheries management alone if environmental trajectories remain unchanged. [ABSTRACT FROM AUTHOR]

Abstract: In this study a model system consisting of the three-dimensional General Estuarine Transport Model (GETM) and the third generation wind wave model SWAN was developed. Both models were coupled in two-way mode. The effects of waves were included into the ocean model by implementing the depth-dependent Radiation stress formulation (RS) of Mellor (2011a) and the Vortex force formulation (VF) presented by Bennis et al. (2011). Thus, the developed model system offers a direct comparison of these two formulations. The enhancement of the vertical eddy viscosity due to the energy transfer by white capping and breaking waves was taken into account by means of injecting turbulent kinetic energy at the surface. Wave-current interaction inside the bottom boundary layer was considered as well. The implementation of both wave-averaged formulations was validated against three flume experiments. One of these experiments with long period surface waves (swell), had not been evaluated before. The validation showed the capability of the model system to reproduce the three-dimensional interaction of waves and currents. For the flume test cases the wave-induced water level changes (wave set-up and set-down) and the corresponding depth-integrated wave-averaged velocities were similar for RS and VF. Both formulations produced comparable velocity profiles for short period waves. However, for large period waves, VF overestimated the wave set-down near the main breaking points and RS showed artificial offshore-directed transport at the surface where wave shoaling was taking place. Finally the validated model system was applied to a realistic barred beach scenario. For RS and VF the resulting velocity profiles were similar after being significantly improved by a roller evolution method. Both wave-averaged formulations generally provided similar results, but some shortcomings were revealed. Although VF partly showed significant deviations from the measurements, its results were still physically reasonable. In contrast, RS showed unrealistic offshore-directed transport in the wave-shoaling regions and close to steep bathymetry. [Copyright &y& Elsevier]

The investigation of larval dispersal and retention, their variability and dependence on wind conditions, has become a major topic in fisheries research owing to potential effects on stock recruitment and stock structuring. The present study quantifies the wind-induced variability of larval retention of herring in a highly productive coastal lagoon of the Western Baltic Sea. This lagoon, the Greifswalder Bodden, represents the main spawning area of Western Baltic Spring-Spawning Herring, a stock that has recently undergone a continuous decline in recruitment. The study tests whether this decline was related to changes in larval retention, more precisely to changes in wind conditions, the main forcing of the lagoon's circulation. To answer this, a model approach was applied. Larvae were tracked as Lagrangian drifters under constant and variable wind conditions, examining the main drift patterns and reconstructing the incidents during the period of recruitment decline. For the latter, weekly cohorts of virtual larvae were released in the lagoon over the entire spawning period (April-June; >16 weeks). The fraction of retained larvae per cohort was related to observed larval abundances. On this basis, a new retention index was defined to evaluate the annual larval retention. The results presented cannot explain the observed recruitment decline but characterize the lagoon as an important larval retention area by virtue of unsteady wind conditions that prevent a steady outflow of larvae. [ABSTRACT FROM AUTHOR]

Highlights: [•] Efficient adaptive time stepping algorithm is developed for random walk models. [•] Its performance on spatially non-uniform turbulent mixing test cases is assessed. [•] For large vertical diffusivity gradient algorithm controls the pycnocline crossing. [•] Adaptive algorithm is more accurate and efficient than the fixed time step algorithm. [ABSTRACT FROM AUTHOR]

Abstract: Tourism around the Oder (Szczecin) Lagoon, at the southern Baltic coast, has a long tradition, is an important source of income and shall be further developed. Insufficient bathing water quality and frequent beach closings, especially in the Oder river mouth, hamper tourism development. Monitoring data gives only an incomplete picture of Escherichia coli (E. coli) bacteria sources, spatial transport patterns, risks and does neither support an efficient bathing water quality management nor decision making. We apply a 3D ocean model and a Lagrangian particle tracking model to analyse pollution events and to obtain spatial E. coli pollution maps based on scenario simulations. Model results suggests that insufficient sewage treatment in the city of Szczecin is the major source of faecal pollution, even for beaches 20km downstream. E. coli mortality rate and emission intensity are key parameters for concentration levels downstream. Wind and river discharge play a modifying role. Prevailing southwestern wind conditions cause E. coli transport along the eastern coast and favour high concentration levels at the beaches. Our simulations indicate that beach closings in 2006 would not have been necessary according to the new EU-Bathing Water Quality Directive (2006/7/EC). The implementation of the new directive will, very likely, reduce the number of beach closings, but not the risk for summer tourists. Model results suggest, that a full sewage treatment in Szczecin would allow the establishment of new beaches closer to the city (north of Dabie lake). [Copyright &y& Elsevier]

• Validation of geolocation models should be cost-effective and reported systematically. • Stratification, heterogeneous bathymetry and vertical movements improve geolocation. • The geolocation framework resulted in an uncertainty of less than 20 km on average. • Incorrectly modelled start and end positions are strong criteria to discard tracks. Geolocation models are essential tools to derive detailed information on the movement patterns of demersal fish equipped with data storage tags recording temperature and depth. However, modelled horizontal positions are accompanied by an often unknown error which needs to be assessed to support the appropriate interpretation. We conducted five validation experiments in the southern Baltic Sea comprising a) simulated tracks, b) stationary, nearshore moored tags, c) temperature data from an automated offshore measuring station, d) a temperature-depth probe mounted to an otter board of a commercial vessel and e) tags mounted to the conductivity-temperature-depth probe (CTD) and the otter board of a survey vessel to obtain temperature-depth data from known locations. Using depth-temperature data from each experiment, we calculated modelled positions using an adapted R-package HMMoce model framework. These experiments allowed us to compare true and modelled positions to quantify model performance and assess its potential efficacy for future geolocation studies on tagged cod. Correctly modelled start and end positions were a good indicator for all validation methods; modelled and true tracks were generally similar with differences in daily positions varying on average between 2 and 19 km. The performance of the geolocation model was improved when temperature-depth profiles indicated heterogeneity in the oceanographic environment, particularly in stratified water columns. The validation methods confirmed that the modified HMMoce framework could be used as a geolocation tool for Eastern Baltic cod when an uncertainty of less than 20 km from the true position is acceptable. Since all presented methods are cost-effective, transparent, open source and reproducible, they can be routinely applied to understand expected model uncertainties and thus make more robust inference about movements of tagged fish. [ABSTRACT FROM AUTHOR]

Eddies in the global and coastal ocean play a key role in mixing and transport processes. Here, we present an eddy census for the Baltic Sea covering the years 2008–2010. The eddy tracking was carried out with three different eddy detection methods (an Eulerian streamline based and two Lagrangian coherent structure-based methods). The near surface velocity fields were generated with GETM (General Estuarine Transport Model), an eddy-resolving ocean model (internal Rossby radius ≈ 5 km, grid resolution ≈ 0.6 km). The general shape of the statistics for the three detection algorithms agree well for lifetime and propagation distance. The most striking differences between the methods were visible for the eddy size, with much smaller eddy-radii detected by the Lagrangian methods. Although we only find minor variations in the monthly numbers of tracked eddies in total, we found large variations in the eddy core vorticity, with higher rotation and smaller radii during winter. This was caused by the changing stratification over the course of the year. Additionally, we found spatial changes in the ratio of cyclonic to anticyclonic rotating eddies and seasonal differences in the spatial distribution of eddies. [ABSTRACT FROM AUTHOR]

Quantitative sampling of sessile assemblages on temperate subtidal rocky reefs is expensive and severely time-limited by logistics. However, knowledge about distribution patterns of critical and endangered species and habitats at different spatial scales is needed for effective marine management strategies. To gain information of sessile community distribution on broader spatial scales (>1 km), visual imaging was employed for the first time on a reef complex in the south-western Baltic Sea. Analysis of 3000 images along 6 transects (in total 18 km long) from 10 to 40 m depth revealed high natural variation in reef physical structure, with well-defined changes in sessile species richness, cover and composition. Overall 14 morphological groups could be distinguished by imaging and 4 distinct community groups associated with specific habitat requirements were identified. Depth remained the best descriptor. However, data indicate that light intensity, concentration of organic carbon and suspended particulate matter have an effect on reef community distribution. Compared to fully marine conditions, the study revealed a unique zonation pattern in the circalittoral zone of the Fehmarnbelt brackish transition area, with an unexpected reef habitat in the trench. We conclude that towed camera platform imagery might help to close the information gap regarding rocky reefs in the temperate subtidal. It provides a valuable tool to assess the main distribution patterns of sessile assemblages on rough terrain, potentially applicable for management and conservation planning. [ABSTRACT FROM AUTHOR]

The Knudsen theorem for estuarine exchange flow, based on mass conservation of water and salt, and its generalization with resolution in salinity coordinates, the Total Exchange Flow (TEF) analysis framework, are reviewed here. The former had been developed, and applied to quantify exchange flow between the North Sea and the Baltic Sea, more than a century ago. In this paper, the underlying short research paper published in 1900 by Martin Knudsen, originally written in German, is translated into English. Reaching far beyond the Knudsen relation in particular, the extensive scientific achievements of Martin Knudsen on the salinity of seawater are reviewed. Using the Knudsen theorem and the TEF analysis framework, validated multi-decadal (years 1949–2013) model simulations are analyzed in terms of exchange flow through straits of the Western Baltic Sea. When comparing the model results to the original findings of Martin Knudsen, it is impressive to see how relevant his analysis of ratios of averaged inflowing and outflowing water masses still is today, given that they were based on just a few salinity observations. The model-based long-term Knudsen and TEF analyses of exchange flow in the Western Baltic Sea reproduces the Major Baltic Inflows (MBIs) that have occurred since the 1950s. For the complex inflow years 2002/2003, with two baroclinic summer inflows and one barotropic winter inflow in between, the strong underestimation of the exchange flow by Eulerian analysis as compared to TEF analysis is demonstrated. [ABSTRACT FROM AUTHOR]

Major objectives were to provide a comprehensive dataset on beach macro-litter for parts of the southern Baltic Sea and to analyse if the methodology is fully applicable and a suitable monitoring method in the Baltic. We carried out a regular macro litter beach monitoring (OSPAR methodology, 4 time a year) on 35 beaches along the German and Lithuanian Baltic coast over 2–5 years. Additional experiments addressed the subjectivity of the field surveys and spatio-temporal variability on different scales. We observed no seasonality of the data and a monthly compared to a 3-monthly sampling resulted in 3 times higher annual item numbers. Along the Lithuanian coast, the average number of items per survey varied between 138 and 340 and along the German Baltic coast between 7 and 404, with a median value of 47. All data showed a very high spatio-temporal variability. Using the Matrix Scoring Technique we assessed beach litter sources. With 50% tourism and recreation was the most important source. 3D–transport simulations helped to explain the minor role of shipping as a source and, compared to the North Sea, the low numbers of items on German Baltic beaches. Floating litter had a short duration time in the western Baltic Sea and offshore drift dominated. Further, the common regular beach cleanings reduced the potential for local litter accumulation and translocation. We suggest a monitoring system on 14 Baltic beaches in Germany and 2 in Lithuania and provide cost calculations. The analysis of macro-litter in cormorant nesting material and the search for beached dead animals did not show any result. We can conclude that the macro-litter beach monitoring method is less suitable for Baltic beaches and should only serve as a complementary method in combination with others. [ABSTRACT FROM AUTHOR]

Abstract: We report on methane enrichments that were observed during summer in the upper water column of the Gotland Basin, central Baltic Sea. In the eastern part of the basin, methane concentrations just below the thermocline varied between 15 nM and 77 nM, in contrast to the western part where no methane enrichments could be detected. Stable carbon isotope ratios of methane (δ13C‐CH4 of −67.6‰) indicated its in situ biogenic origin from CO2 reduction, which was supported by clonal sequences that clustered with Methanomicrobiaceae, a family of methanogenic Archaea. Incubation experiments with a Temora longicornis dominated seston fraction obtained from the relevant depth showed a positive correlation between seston concentration and methane production rates. Our results, in combination with previous literature outcomes, suggest that the methane enrichment in the eastern basin might be sustained by a diet‐consumer relationship between the dinoflagellate Dinophysis norvegica and the copepod T. longicornis. However, our mass balance indicates that a local methane production of 110 pmol L−1 d−1 was needed to maintain the methane enrichment, and that the estimated production rate from our incubation experiments of 0.3 pmol CH4 d−1 per adult T. longicornis (about 1 pmol L−1 d−1) was too low to maintain the methane enrichment by zooplankton associated methane production only. These calculations also showed that methane was consumed below the thermocline and not transported into the upper‐ocean, suggesting that other sources in the mixed layer in the range of 95 pmol L−1 d−1 are needed to maintain the observed methane air–sea flux. [ABSTRACT FROM AUTHOR]

Simulations of the North Sea circulation by the global ocean model MPI-OM and the regional ocean models GETM, HAMSOM, NEMO, TRIM are compared against each other and with observational data for the period 1998–2009. The aim of the study is to evaluate the quality of the simulations in particular with respect to their suitability to drive biogeochemical shelf sea models. Our results demonstrate the benefit of the global model to avoid the specification of lateral open boundary conditions. Due to its stretched grid configuration, which provides a higher grid resolution at the Northwest European Shelf, the global model is able to reproduce the large-scale features, such as the water mass distribution and the thermal stratification in the central and northern North Sea, qualitatively similar to the regional models. The simulation of temperature and salinity near the coast however, shows large biases in almost all models because of the coarse meteorological forcing and too coarse vertical resolutions. The simulation of the Baltic Sea exchange and the spread of freshwater along the Norwegian coast proved difficult for all models except GETM, which reproduces impacts of the Baltic Sea outflow reasonably well. [ABSTRACT FROM AUTHOR]

This study aims to estimate, model and map the spatial differences of ecosystem functioning expressed by community bioturbation potential (BPc), an indicator of benthic faunal function based on bioturbation, in the German part of the Baltic Sea. The usefulness of bioturbation potential calculations was justified by its moderate but significant correlation with estimates of bioturbation rates derived from diagenetic models fitted to the in situ measured sediment depth profiles of naturally-occurring chlorophyll-a tracer. Seasonal and interannual variations of BPc were assessed and key species contributing to bioturbation in the study area were identified. To generate the most accurate map of BPc and to investigate its predictability based on abiotic parameters, we have tested 3 different methodological approaches: i) benthic macrofauna community bioturbation potential was initially calculated per station and treated as response variable for species distribution modelling technique (RandomForest, RF) with relevant available environmental layers used as predictors, ii) 35 key species were selected as the most contributing to BPc (responsible for 90% of total BPc), their population bioturbation potential (BPp) estimates were used as response variables and RF models were fitted on each of them to predict their full coverage distributions, that were subsequently summed up to the BPc, iii) BPc values at stations were interpolated to a raster surface using a natural neighbour technique. The comparison with observed values of BPc indicated that map derived by natural neighbour interpolation was the most accurate one given the considered resolution of 1 × 1 km. [ABSTRACT FROM AUTHOR]

After the dramatic eutrophication-induced decline of intertidal seagrasses in the 1970s, the Wadden Sea has shown diverging developments. In the northern Wadden Sea, seagrass beds have expanded and become denser, while in the southern Wadden Sea, only small beds with low shoot densities are found. A lack of documentation of historical distributions hampers conservation management. Yet, the recovery in the northern Wadden Sea provides opportunity to construct robust habitat suitability models to support management., We tuned habitat distribution models based on 17 years of seagrass surveys in the northern Wadden Sea and high-resolution hydrodynamics and geomorphology for the entire Wadden Sea using five machine learning approaches. To obtain geographically transferable models, hyperparameters were tuned on the basis of prediction accuracy assessed by non-random, spatial cross-validation. The spatial cross-validation methodology was combined with a consensus modelling approach., The predicted suitability scores correlated amongst each other and with the hold-out observations in the training area indicating that the models converged and were transferable across space. Prediction accuracy was improved by averaging the predictions of the best models., We graphically examined the relationship between the consensus suitability score and independent presence-only data from outside the training area using the area-adjusted seagrass frequency per suitability class (continuous Boyce index). The Boyce index was positively correlated with the suitability score indicating the adequacy of the prediction methodology., We used the plot of the continuous Boyce index against habitat suitability score to demarcate three habitat classes - unsuitable, marginal and suitable - for the entire international Wadden Sea. This information is valuable for habitat conservation and restoration management., Divergence between predicted suitability and actual distributions from the recent past indicates that unaccounted factors limit seagrass development in the southern Wadden Sea., Synthesis and applications. Our methodology and data enabled us to produce a robust and validated consensus habitat suitability model. We identified highly suitable areas where intertidal seagrass meadows may establish and persist. Our work provides scientific underpinning for effective conservation planning in a dynamic landscape and sets monitoring priorities. [ABSTRACT FROM AUTHOR]

This study provides an inventory of the recent benthic macrofaunal communities in the entire Baltic Sea. The analyses of soft-bottom benthic invertebrate community data based on over 7000 locations in the Baltic Sea suggested the existence of 10 major communities based on species abundances and 17 communities based on species biomasses, respectively. The low-saline northern Baltic, characterized by silty sediments, is dominated by Monoporeia affinis, Marenzelleria spp., and Macoma balthica. Hydrobiidae, Pygospio elegans, and Cerastoderma glaucum dominate the community in sandy habitats off the Estonian west coast and in the southeastern and southern Baltic Sea. Deep parts of the Gulf of Finland and central Baltic Sea often experience hypoxia, and when oxygen levels in these regions recover, Bylgides sarsi was the first species to colonize. The southwestern Baltic Sea, with high salinity, has higher macrofaunal diversity compared with the northern parts. To spatially interpolate the distribution of the major communities, we used the Random Forest method. Substrate data, bathymetric maps, and modelled hydrographical fields were used as predictors. Model predictions were in good agreement with observations, quantified by Cohen's K of 0.90 for the abundance and 0.89 in the wet weight-based model. Misclassifications were mainly associated with uncommon classes in regions with high spatial variability. Our analysis provides a detailed baseline map of the distribution of benthic communities in the Baltic Sea to be used both in science and management. [ABSTRACT FROM AUTHOR]

Abstract: The seasonal dynamics of molybdenum (Mo) were studied in the water column of two tidal basins of the German Wadden Sea (Sylt-Rømø and Spiekeroog) between 2007 and 2011. In contrast to its conservative behaviour in the open ocean, both, losses of more than 50% of the usual concentration level of Mo in seawater and enrichments up to 20% were observed repeatedly in the water column of the study areas. During early summer, Mo removal by adsorption on algae-derived organic matter (e.g. after Phaeocystis blooms) is postulated to be a possible mechanism. Mo bound to organic aggregates is likely transferred to the surface sediment where microbial decomposition enriches Mo in the pore water. First δ98/95Mo data of the study area disclose residual Mo in the open water column being isotopically heavier than MOMo (Mean Ocean Molybdenum) during a negative Mo concentration anomaly, whereas suspended particulate matter shows distinctly lighter values. Based on field observations a Mo isotope enrichment factor of ε =−0.3‰ has been determined which was used to argue against sorption on metal oxide surfaces. It is suggested here that isotope fractionation is caused by biological activity and association to organic matter. Pelagic Mo concentration anomalies exceeding the theoretical salinity-based concentration level, on the other hand, cannot be explained by replenishment via North Sea waters alone and require a supply of excess Mo. Laboratory experiments with natural anoxic tidal flat sediments and modelled sediment displacement during storm events suggest fast and effective Mo release during the resuspension of anoxic sediments in oxic seawater as an important process for a recycling of sedimentary sulphide bound Mo into the water column. [Copyright &y& Elsevier]

Abstract: We apply the three-dimensional COupled Hydrodynamical Ecological model for REgioNal Shelf seas (COHERENS) to compute water renewal time scales for Hervey Bay, a large coastal embayment situated off the central eastern coast of Australia. Water renewal time scales are not directly observable but are derived indirectly from computational studies. Improved knowledge of these time scales assists in evaluating the water quality of coastal environments and can be utilised in sustainable marine resource management. Results from simulations with climatological September forcing are presented and compared to cruise data reported by Ribbe (2006. A study into the export of saline water from Hervey Bay, Australia. Estuarine Coastal and Shelf Science 66, 550–558). A series of simulations using idealised forcing provides detailed insight into water renewal pathways and regional differences in renewal timescales. We find that more than 85% of the coastal embayment''s water is fully renewed within about 50–80 days. The eastern and western shallow coastal regions are ventilated more rapidly than the central, deeper part of the domain. The climatological simulation yields temperature and salinity patterns that are consistent with the observed situation and water renewal time scales in the range of those derived from idealised model studies. While the reported simulations involve many simplifications, the global assessment of the renewal time scale is in the range of a previous estimate derived for this coastal embayment from a simpler model and observational data. [Copyright &y& Elsevier]

Neobiotic species can have profound impacts on food webs and entire ecosystems. The American razor clam Ensis leei was introduced into the Wadden Sea by vessels in the late 1970s and has since spread widely. It has been suggested that Ensis does not interact strongly with other benthic species. The abundance and biomass of E. leei were recorded in 2393 samples in the north-eastern Wadden Sea and 800 samples in the south-eastern Wadden Sea over a total period of 9 years. Using an interdisciplinary approach, we developed a habitat prediction model using sedimentological and hydrodynamic predictors to help understand the shape of the ecological niche occupied by Ensis in the Wadden Sea. Our model showed that Ensis preferred areas with moderately high bed shear stress and prolonged or constant water coverage. Ensis preferred coarse sediments in the northern sub-area but coarse and muddy sediments in the southern sub-area and was negatively affected by the sand mason worm Lanice conchilega in the northern sub-area. Predictions of the spatial distribution of Ensis using the northern and southern datasets revealed major differences in predicted hot-spots throughout the entire study site. This study thus highlights the need to collect a sufficiently large dataset from different sub-areas of the Wadden Sea to allow valid conclusions to be drawn regarding the spatial distribution of Ensis. The negative effects of L. conchilega on Ensis abundance and biomass as well as the occurrence of Ensis in muddy sediments in the south suggest that the ecological niche of this neobiotic species is likely to overlap partly with the native fauna of the Wadden Sea. • Long-term data were used to determine ecological niche and predict habitat potential of Ensis leei. • Abundance and biomass of Ensis were related with sedimentologic and hydrodanamic predictors. • Ensis occurred both, in coarse sediments as well as in muddy environments. • Ensis abundance and biomass declined in areas with dense Lanice conchilega reefs. • The invasive Ensis is thought to overlap with the pristine benthic fauna particularly in muddy sediments. [ABSTRACT FROM AUTHOR]

Here we present the first digital, publicly available, ecotope map of the trilateral Wadden Sea covering the Netherlands, Germany and Denmark. This ecotope map, representative for the time period 2008–2012, was developed on the basis of bathymetry, salinity, flow velocity, exposure time, sediment composition, hard substrates and salt marshes. Ecotopes are discrete classes of the physical environment based on the distributions of communities in an ecosystem. An ecotope map can give a first–order estimate of the potential spatial distribution of species and communities. The use of a single, consistent and well-defined ecotope system made it possible to compare the proportions of the different ecotopes and compare properties of the tidal basins over the entire trilateral Wadden Sea. The 39 tidal basins within the Wadden Sea were clustered in four distinct types, using an unsupervised clustering algorithm. These four types included: 1) basins that are characterised by a large proportion of low-dynamic low-littoral ecotopes, 2) basins with a high proportion of high-dynamic sublittoral ecotopes, 3) shallow basins with over 50% low-dynamic mid-littoral ecotopes and 4) basins with an equal distribution of low-dynamic low-littoral and low-dynamic mid-littoral ecotopes. The clustering in four major tidal basin types gives clear guidance for comparative ecological and morphological studies between tidal basins. The ecotope map can be used for environmental research, policy and conservation purposes of the trilateral Wadden Sea in an integrated manner. • We produce the first digital, publicly available trilateral Wadden Sea ecotope map. • Tidal basins of the Wadden Sea were clustered in four main types. • The power law relation between channel and basin area was confirmed for all 39 basins. [ABSTRACT FROM AUTHOR]

In this contribution we present a comprehensive analysis of sea level data around the Baltic Sea from 1777 to the present with a focus on the southwestern part, concentrating on quantifying and understanding long-term trends and the influence of inter-annual and decadal variability. Our analysis focuses on a new monthly mean sea level (MSL) dataset, generated from a first time available compilation of high-resolution sea level measurements combined with already existing monthly MSL time series. Our final MSL dataset consists of records at 139 sites in the Baltic Sea with a temporal resolution of at least 19 years. Additionally, we analyze outputs of barotropic and baroclinic reanalysis runs from a 3D hydrodynamic numerical ocean model to quantify different meteorological and climatological processes contributing to the local sea level budget. Throughout the 20th century (1900 to 2015), an average linear trend for the whole Baltic Sea of 1.66 ± 0.14 mm/yr could be determined (corrected for the in the region dominant vertical land motion signal from Glacial Isostatic Adjustment). This is in principal consistent with current GMSL trend estimates (1.3 to 2 mm/yr) for the same period (Church and White 2006, Hay et al., 2015; Dangendorf et al., 2017) within their specific error bars. However, within the basin large differences in trends from different stations are found, which reach from minimum values in the southwestern part (0.94 ± 0.15 mm/yr in Kolobrzeg) to maximum values in the eastern Gulf of Finland (2.21 ± 0.32 mm/yr in Kronstadt). These differences can primarily be explained by an intensification of westerly winds and a decrease of atmospheric pressure over the 20th century pushing lowering MSL rates in the west and increasing them in the east. With our study we outline major existing knowledge gaps in understanding sea level change and variability in the southwestern Baltic Sea. A better understanding of the past and present development of the MSL provides the basis for deriving more reliable future projections of sea-level rise. Church and White (2006): A 20th century acceleration in global sea-level rise. In: Geophysical Research Letters, Jg. 33, 1, n/a. doi: 10.1029/2005GL024826.Dangendorf et al. (2017): Reassessment of 20th century global mean sea level rise. In: Proceedings of the National Academy of Sciences of the United States of America, Jg. 114, 23, 5946-5951. doi: 10.1073/pnas.1616007114.Hay et al. (2015): Probabilistic reanalysis of twentieth-century sea-level rise. In: Nature, Jg. 517, 7535, 481-484. doi: 10.1038/nature14093. [ABSTRACT FROM AUTHOR]

Insight into processes contributing to the onset and breakdown of stratificationenhances our understanding of the ecosystem dynamics of shallow seas. In theGerman Bight, a region of freshwater influence (ROFI), stratification is mainlyinfluenced by tidal mixing, fresh water river inflow and wind forcing. The physicalregime of the German Bight in the ROFI region switches between stably stratified tosemi-diurnal stratification. In order to investigate the processes contributing to thisvariability, a three dimensional high resolution numerical model is setup basedon the General Estuarine Transport Model (GETM). The model has a horizontalresolution of 300 m in the inner German Bight and incorporates 30 vertically adaptivelayers. The model results are first compared against observations, showing that the model is ableto reproduce the observed physical regime. The model results are then used to calculate theterms in a dynamic equation for the potential energy anomaly (φ). These termsinclude the horizontal advection, depth-mean straining, non-mean straining, verticaladvection and vertical mixing. The contribution of these processes to stratification isassessed under different conditions in the ROFI region. The results show that inthe semi-diurnal stratification condition, the depth-mean straining and horizontaladvection terms are the most important processes. Both of these processes are in phaseleading to stratified conditions during low water and mixed conditions during highwater. On the other hand, in the stably stratified condition, vertical advection is asimportant as depth-mean straining and horizontal advection. In this condition, thedepth-mean straining and horizontal advection are out of phase and therefore oppose eachother, while the depth-mean straining and vertical advection are in phase. Thisresults in minimum stratification (minimum φ) occurring at maximum ebb andmaximum stratification (maximum φ) occurring at slack after ebb (low water). [ABSTRACT FROM AUTHOR]