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In this study we present new insights into the overturning circulation in non‐tidal, fjord‐type estuarine systems associated with diahaline mixing. As a realistic example we analyze 2 years of numerical model results for the Baltic Sea, a brackish semi‐enclosed marginal sea, characterized by strong freshwater surplus. An isohaline water mass transformation framework is applied to quantify and decompose the diahaline exchange flow. Time‐averaged effective vertical diahaline velocity is directly calculated from the divergence of the transports below the respective isohaline surface. It is furthermore indirectly estimated from the gradient of local mixing per salinity class with respect to salinity. Under the assumption of negligible horizontal diffusive salt transports both estimates should be identical. Our analysis shows a high correlation between the spatial patterns of the two estimates for the diahaline exchange flow. Two dominant types of diahaline exchange flow are analyzed. First of all there is a large scale overturning circulation with inflow at places where the isohaline surface is close to the bottom and with outflow at places where the isohaline is surfacing. Secondly, there is the well‐known small‐scale overturning circulation localized inside the bottom boundary layer over sloping bathymetry, associated with boundary mixing. Both types of circulation are visualized across selected vertical transects in physical and in salinity space. One major result is that about 50% of the diahaline exchange flow patterns are generated by numerical mixing caused by the truncation error of the advection scheme, despite the fact that an anti‐diffusive advection scheme and vertically adaptive coordinates are used. Plain Language Summary: We study the major circulation in estuaries controlled by mixing of water masses. A theoretical framework is applied that replaces depth with salinity, since the latter is descriptive of the circulating water masses. The circulation is described from a point of view where the flow crosses areas of constant salinity. This is the diahaline circulation. The framework breaks down the circulation into smaller chunks, and allows us to deduce how the circulating flow depends on mixing. An application of this theory in a 2‐year long computer simulation of the large and non‐tidal estuary of the Baltic Sea is demonstrated. Two dominant types of diahaline circulation are analyzed. The first is a large‐scale circulation where water masses flow into the estuary where the areas of constant salinity touch the bottom, and out where they encounter the surface. The second is a small‐scale circulation close to the bottom where the ocean bathymetry is sloping, and is controlled by boundary mixing. Both types of diahaline circulation are visualized in this study. One major result is that around 50% of the circulation is generated by unwanted spurious mixing, which is created by the computer simulation and does not exist in reality. Key Points: The local diahaline mixing per salinity class is associated with the overturning circulationThe spurious numerical contribution to the total diahaline exchange flow is around 50% over a large range of salinity classesHot spots for diahaline exchange flow are located at sloping topography and where isohalines encounter surface (outflow) or bottom (inflow) [ABSTRACT FROM AUTHOR]

Observations from the global ocean have long confirmed the ubiquity of thermohaline inversions in the upper ocean, often accompanied by a clear signal in biogeochemical properties. Their emergence has been linked to different processes such as double diffusion, mesoscale stirring, frontal subduction, and the recently discussed submesoscale features. This study uses the central Baltic Sea as a natural laboratory to explore the formation of salinity inversions in the thermocline region during summer. We use realistic high‐resolution simulations complemented by field observations to identify the dominant generation mechanism and potential hotspots of their emergence. We propose that the strongly stratified thermocline can host distinct salinity minima during summer conditions resulting primarily from the interaction between lateral surface salinity gradients and wind‐induced differential advection. Since this is a generic mechanism, such salinity inversions can likely constitute a typical feature of the upper ocean in regions with distinct thermoclines and shallow mixed layers. Plain Language Summary: The upper ocean is characterized by a well‐mixed surface layer, below which temperature decreases rapidly with depth, forming the so‐called thermocline region. A corresponding salinity increase with depth is typically anticipated for stable density stratification to occur. Temperature and salinity inversions can, however, emerge in the upper ocean. Such thermohaline inversions have been observed in different regions of the world's oceans, and various mechanisms have been proposed to explain their generation. Here, the central basin of the Baltic Sea is used as a natural laboratory to explore the formation of distinct salinity minima in the thermocline region during summer conditions. Using high‐resolution numerical simulations and measurements from a field campaign, we show that inversions are abundant and can emerge throughout the entire basin. They increase with increasing wind speeds and concentrate mainly in regions with strong lateral salinity differences. We propose that thermocline salinity minima can occur during summer when the wind transports saltier water over less saline surface waters. This is a generic mechanism that can therefore be responsible for the formation of the salinity inversions observed worldwide in areas with distinct thermoclines and shallow mixed layers. Key Points: Observations collected in the central Baltic Sea during summer indicate patches of distinct salinity minima in the thermocline regionRealistic high‐resolution simulations are used to explore the origin of the salinity minima and to identify the hotspots of their genesisLateral surface salinity gradients interacting with wind‐induced differential advection are shown to generate most of the inversions [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]

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]

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]

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]