Despite a long history of Baltic Sea studies, still today, an important objective for investigations is the dynamics of water exchange between the North Sea and the Baltic Sea. A critical study object is the transition area between the two seas: the Danish Straits.

Furthermore, pronounced changes in the salinity in the Gotland Deep, situated in the central Baltic, are strongly dependent on major saltwater inflows, MBIs. These events, which are a subset of LVCs, represent a specific type of barotropically driven inflows. Their formation depends on specific favorable meteorological circulation patterns (e.g., Matthäus et al., 2008; Leppäranta and Myrberg, 2009; Lehmann and Post, 2015) and spatiotemporal grouping of deep cyclones (Lehmann et al., 2017).

Recently, a number of investigations have been carried out on the role of meteorological forcing and its impact on the hydrographic conditions in the Danish Straits, as well as the total freshwater supply to the Baltic Sea before the occurrence of highly saline barotropically driven inflows. Höflich and Lehmann (2018) proposed a mechanistic explanation including the salinity in the Danish Straits as well as the time variability of meteorological forcing in connection with an inflow taking place. Freshwater supply played only a modulating role; i.e., it does not lead to a change in frequency or intensity of the events. However, it had an adjusting role in observed transports of saline water to the Baltic Sea.

The third-largest MBI ever observed took place in December 2014, generating interest in studying the saline inflows further (e.g., Mohrholz et al., 2015; Gräwe et al., 2015; Rak, 2016; Neumann et al., 2017; Liblik et al., 2017). Mohrholz (2018) reanalyzed the time series of major inflows. He used long-term data on sea levels, river runoff, and observed salinity in the Belt Sea and the Sound. As a result, an ongoing time series (revised list of MBIs) of barotropically driven inflows was formed from about 1890 until today. The new time series by Mohrholz (2018) were compared with those composed by Fischer and Matthäus (1996), which could be considered a traditional list of MBIs. There were apparent differences between the two series from the 1980s onwards. There were many reasons behind the observed deviations. Not many accurate data during the 1976–1991 period were available. After that time, the methods to observe the inflows were changed, which caused a bias in the statistics. Moreover, the locations where the measurements took place were changed (Mohrholz, 2018).

There is a clear difference in the revised time series of the MBIs compared with the earlier, traditional ones. Namely, according to Mohrholz (2018), there is no clear trend in the frequency and intensity of the MBIs on the decadal timescale. In the traditional assumption, climate change would have impacted MBIs as a decreasing trend in their frequency. On the other hand, variability was found in the frequency of the inflows with a timescale of about 30 years (Mohrholz, 2018; Radtke et al., 2020).

This decadal variability was also found in surface and bottom salinities, river runoff, and salt transport across the Darss Sill (Radtke et al., 2020). It also turned out that LVCs and MBIs are not the only events which transport salt into the Baltic Sea. There are also smaller inflows of barotropic origin. These occur during all seasons, having a low variability between the years. Such inflows bring about 30 % of the entire salt transport to the Baltic Sea. Normally, the salinity of the inflowing water is not high enough to substitute the bottom water of the deep basins. Thus, this water will be stratified in the corresponding density layer within or below the halocline (e.g., Reissmann et al., 2009; Naumann et al., 2018). This variability of the saline water inflows in time does not explain the worsening of oxygen conditions in the Gotland Basin (anoxic bottoms) and the observed prolonged periods of stagnation (Mohrholz, 2018). Large barotropic inflows and the associated dense bottom currents form one branch of the Baltic Sea overturning circulation and deepwater ventilation. Holtermann et al. (2017) investigated the dynamics of the deep waters and vertical mixing in the central part of the Baltic Sea while major Baltic inflows took place, thus providing new information on dense bottom gravity currents on their way to the deep Gotland Basin and associated turbulent mixing. They showed that the large-scale inflow event could be separated into three phases extending over 5 months. During Phase I, near-bottom intrusions of only moderate oxic waters could be detected, most likely by displaced saline deep water from the Bornholm Basin. The main inflow pulse arrived in the deep Gotland Basin in Phase II, leading to an instantaneous expansion of the oxic deepwater pool. This was followed by Phase III, a longer period of oxic intrusions accumulating in almost the same amount of oxic waters as in Phase II. Furthermore, mixing was mostly happening in a thin bottom gravity current propagating down the slope, but at the same time the interior region remained nonturbulent.

Liblik et al. (2018) studied the impact of the December 2014 MBI downstream from the eastern Gotland Basin to the Gulf of Finland. Although the deep Gotland Basin was well ventilated, oxygen conditions in the area north of the Gotland Deep did not improve but rather worsened in the northern Baltic Proper and Gulf of Finland. A further study by Stramska and Aniskiewicz (2019) showed that remote sensing altimetry could be a complementary source of information about barotropic inflow events.

A Baltic-specific physical feature is the so-called cold intermediate layer (hereafter denoted as CIL), which is formed annually. The CIL appears as a temperature minimum between the thermocline and the permanent halocline from spring to autumn. Vertical convection due to cooling of the atmosphere and wind mixing erode the seasonal thermocline during autumn and winter (Leppäranta and Myrberg, 2009; Stepanova, 2017). During this process, the water masses of the fresher upper layer and the sub-thermocline saltier layer are mixed. As a result, the seasonal salinity maximum in the surface layer occurs in winter (Reissmann et al., 2009). This mixing process extends down to the upper boundary of the permanent halocline (Figs. 2 and 3). In areas where the permanent halocline does not exist, such as the Gulf of Riga and the Gulf of Bothnia (Fig. 1), mixing leads to a complete turnover of the entire water column almost every winter (Raateoja, 2013; Raudsepp, 2001). With the formation of the seasonal thermocline, the CIL is formed as a separated layer between the thermocline and the permanent halocline. Its thickness has been estimated to be 20–50 m (Liblik and Lips, 2017; Stepanova, 2017). Despite the rapid warming of the thermal mixed layer, the temperature of the CIL only slowly increases during summer and autumn (Hinrichsen et al., 2007a; Liblik and Lips, 2011). However, the CIL can be traced in the water column until the next winter (Liblik et al., 2013; Stepanova, 2017), when a new CIL is formed. The water temperature in the CIL correlates with the severity of the previous winter (Hinrichsen et al., 2007a; Liblik and Lips, 2011). After the formation of the thermocline in spring, CIL temperature is often lower than the temperature of maximum density (Tmd), most probably due to lateral advection of slightly more highly saline, dense water. This buoyancy flux is stronger than the destabilizing effect caused by the warming of the water, when T<Tmd (Chubarenko et al., 2017a; Eilola and Stigebrandt, 1998).

According to Chubarenko and Stepanova (2018), colder and slightly saltier water, which has its origin from the upper layer of the Bornholm Basin, advects to the east and forms the core of the CIL in the Baltic Proper. Wind-driven pycnocline variations, including coastal upwelling and downwelling events, considerably alter the depth and thickness of the CIL (Liblik and Lips, 2017). No remarkable changes occurred in temperature and salinity in the CIL from 1982 to 2016 (Liblik and Lips, 2019).

The eastern Gotland Basin as part of the Baltic Proper (Fig. 1) is the most prominent region to investigate the impact of barotropic inflows and long-term salinity changes in the Baltic Sea. The salinity dynamics of the eastern Gotland Basin are also affecting the different sub-basins and lagoons surrounding it. The salinity dynamics there represent the development of salinity and stratification with sufficient accuracy in the entire Baltic Proper. Changes in the mean salinity calculated from Gotland Deep's position are only about 2 % different from the calculation based on all sub-basins (Winsor et al., 2001, 2003; Elken et al., 2015). However, only about 12 % of the total area of the Baltic Sea has a depth of more than 100 m and 2.7 % more than 150 m (Leppäranta and Myrberg, 2009). Observed surface salinity of the eastern Gotland Basin (Figs. 2 and 4) reveals a low-salinity period starting in the 1980s (Elken et al., 2015; Vuorinen et al., 2015; Liblik and Lips, 2019) and lasting until 2002. After the MBI in 2003, the surface salinity slightly increased and fluctuated until 2018, but it remains relatively low (<6.5 g kg-1; Fig. 4). The deepwater salinity decreased from the late 1970s until 1993 and then increased until 2018 (Fig. 2). Major saltwater inflows after 1994 can be traced by the abrupt salinity increase in the layers below the halocline. There are also smaller barotropic inflows (Mohrholz, 2018), keeping the salinity below the halocline on a high level (Fig. 2). A negative salinity trend of about 0.1–0.2 g kg-1 per decade can be detected at the surface. The surface temperature increases by about 0.4–0.6 ∘C per decade, whereas surface oxygen decreases by 0.1–0.2 mL L-1 per decade (Fig. 3). Generally, the temperature trend at the surface of the Baltic Sea follows the trend in air temperature. Increasing temperatures reduce the solubility of oxygen and at the same time enhance oxygen depletion rates. Maximum negative trends in oxygen up to 1 mL L-1 per decade can be found in the area of the halocline (Bornholm and Gotland Basin, Fig. 3). The surface salinity trend is decreasing, whereas salinity below the halocline is increasing (0.2–0.25 g kg-1 per decade), leading to enhanced stratification between the surface and deep layer. However, the frequency of barotropic and major Baltic inflows did not increase. The decreasing trend in surface salinity might be due to increased runoff and net precipitation (Liblik and Lips, 2019). The volume-averaged salinity also shows a drop until 1992, and with the MBI in 1993, a gentle increase occurred (Figs. 4 and 5).

The Gulf of Riga is a seasonally stratified, semi-enclosed basin in the eastern Baltic Sea (Fig. 1), where the water column is fully mixed every autumn–winter. The gulf has two shallow connections with the Baltic Proper: the Irbe Strait (sill depth 25 m) and the Väinameri Sea area (sill depth 5 m). The water budget in the gulf is determined by the water mass transport through these two openings (Laanearu et al., 2000; Lilover et al., 1998; Otsmann et al., 2001) and river discharge, which is concentrated in the southern part of the gulf. Due to the shallow straits, the sub-halocline salty water does not intrude from the Baltic Proper to the gulf, and no permanent halocline exists there. Stratification in early spring is dominated by haline stratification (Stipa et al., 1999), especially close to the freshwater sources, but later in spring and summer, thermal stratification becomes more important in stabilizing the water column (Berzinsh, 1995; Liblik et al., 2017). Thus, the water column is stratified from spring to late autumn (Berzinsh, 1995), but the mean salinity difference between the upper and deep layers is only 0.7–1.0 g kg-1 (Raudsepp, 2001; Skudra and Lips, 2017). There is quite a high correlation between river runoff in spring and mean salinity in the upper mixed layer in August (Skudra and Lips, 2017). Bottom layer salinity in the gulf is well correlated with the near-bottom salinity in the Irbe Strait (Skudra and Lips, 2017). Long-term changes in the average salinity are characterized by an increase from the 1960s to the late 1970s and a consecutive decrease in the 1980s–1990s (Berzinsh, 1995). The latter trend of decreasing salinity in the gulf coincided with the corresponding changes in the Baltic Proper in the layer above the halocline during the stagnation period until the mid-1990s (Raudsepp, 2001; Figs. 2 and 4).

Wind-driven processes modify the transport of saltier water from the Irbe Strait and the advection of riverine water (Liblik et al., 2017; Lips et al., 2016b, c; Soosaar et al., 2014, 2016). Most of the fresh water from the Daugava River is transported to the north along the eastern shore during the cold season (Lips et al., 2016b). An anticyclonic gyre in the southern part of the gulf (Soosaar et al., 2014) or the entire gulf (Lips et al., 2016b) could form in spring–summer under specific wind forcing. Modeling experiments have also indicated that cyclonic eddies could develop and transport the saltier water from the Irbe Strait towards the central gulf (Lips et al., 2016c). High-resolution measurements have shown an entering of the subsurface warmer, saltier, and oxygen-rich buoyant patches from the Irbe Strait into the gulf intermediate layer in summer. The exact shape, fate, and impact of these sub-mesoscale features are unknown, but they showed up as strong subsurface salinity maxima in the time series (Liblik et al., 2017).

The Gulf of Finland is an elongated sub-basin of the Baltic Sea located in the northeastern extremity of the sea (Fig. 1). The length of the gulf is about 400 km, and its width varies between 48 and 135 km (Myrberg, 1998). The mean depth of the gulf is 37 m, with the maximum depth being 123 m (in the Baltic Sea 459 m). The drainage area of 420 990 km2 is 20 % of the total drainage area of the entire Baltic Sea. The water budget in the gulf is mainly determined by unrestricted and continuous water exchange with the northern Baltic Proper in the west and river discharge, which is mainly concentrated in the eastern part, where the largest river of the entire Baltic Sea is located, namely the river Neva. The water column can be divided into three layers – the upper mixed layer, the cold intermediate layer, and the sub-halocline near-bottom layer separated by the seasonal thermocline and the quasi-permanent halocline (Alenius et al., 1998, 2003; Soomere et al., 2008). The seasonal thermocline vanishes on a yearly basis during autumn–winter. Strong wind events, reversing the estuarine circulation (Elken et al., 2003), can occasionally destroy the halocline for more than a month in large areas of the gulf in winter (Liblik et al., 2013). Surface salinity increases from about 1 g kg-1 in the easternmost part to 6 g kg-1 in the western part. On average, surface salinity is higher near the southern coast than the northern coast (Kikas and Lips, 2016; Liblik and Lips, 2017) due to the general cyclonic circulation scheme (Andrejev et al., 2004a, b). The westward-flowing current along the northern coast changes its location across the gulf mainly due to wind forcing (Kikas and Lips, 2016; Liblik and Lips, 2017; Lips et al., 2016a).

Wind-driven processes, such as the along-gulf advection, coupled upwelling and downwelling events, and vertical mixing, play an important role in the salinity dynamics. Westerly winds bring the saltier upper layer water to the gulf from the Baltic Proper (Lilover et al., 2016; Suhhova et al., 2018), weaken the stratification, and deepen the upper mixed layer (Liblik and Lips, 2017). Easterly winds have an opposite effect; these intensify the transport of fresher waters to the west (Elken et al., 2003; Liblik and Lips, 2012), make the mixed layer shallower, and strengthen the haline stratification (Liblik and Lips, 2017). The latter process can lead to the formation of the shallow haline stratification in winter (Liblik et al., 2013). Shallowing or deepening of the upper mixed layer due to the prevailing winds can be an important factor influencing the primary production and species dominance during the summer cyanobacteria blooms (Kanoshina et al., 2003).

Coupled upwelling and downwelling events bring denser water from the cold intermediate layer to the surface layer, where it mixes with the ambient upper layer water (Myrberg and Andrejev, 2003; Lips et al., 2009). Upwelling–downwelling events at the southern and northern coast have several distinctive characteristics. Less wind forcing is needed to generate upwelling along the southern coast than the northern coast (Kikas and Lips, 2016; Liblik and Lips, 2017). Stronger lateral salinity, temperature, and density gradients occur in the upper layer in the case of upwelling along the southern coast (Kikas and Lips, 2016; Liblik and Lips, 2017). Eastward advection in the surface layer and downwelling along the southern coast generated by westerly winds can form a thick upper mixed layer (>45 m) in summer (Liblik and Lips, 2017). A positive trend in the upwelling occurrence along the northern coast was detected in 1990–2009 (Lehmann et al., 2012). However, no long-term trends were seen in the upwelling-favorable winds in 1982–2013 (Liblik and Lips, 2017).

In situ measurements and modeling experiments have been conducted to characterize sub-mesoscale processes and their impact on the development of stratification and vertical mixing in the gulf (Lips et al., 2016a; Väli et al., 2013; Vankevich et al., 2016).

The gulf is impacted by estuarine circulation reversals caused by westerly wind impulses (Elken et al., 2003), which considerably weaken the halocline and lower salinity in deep layers (Elken et al., 2003; Lilover et al., 2016; Stoicescu et al., 2019). In the case of long-lasting, strong westerly winds, circulation reversals can lead to the vanishing of the stratification in large areas of the gulf in winter (Liblik et al., 2013; Lips et al., 2017). Stratification collapse events have become more frequent since the 1990s (Elken et al., 2014). More frequent and stronger westerly winds during winters (Keevallik and Soomere, 2014) generate more reversals and likely cause a salinity minimum in the annual cycle of the deep layer (Lehtoranta et al., 2017; Maljutenko and Raudsepp, 2019). The reversals, together with upward salt flux created by convective and wind mixing, cause a maximum in the annual cycle of the upper layer salinity in the winter period. A salinity maximum or minimum usually occurs in the deep or surface layer in summer, when the seasonal thermocline restricts vertical mixing and westerly winds are not that dominant. Additionally, the seasonal maximum in river runoff during spring will lead to a surface salinity reduction (e.g., Eilola and Stigebrandt, 1998). Another minimum in the sea surface salinity might occur due to a lack of vertical mixing in the ice-covered areas in late winter (Merkouriadi and Leppäranta, 2014).

Multiyear changes in salinity in the deep layer depend on the occurrence of MBIs (Laine et al., 2007; Liblik et al., 2018; Liblik and Lips, 2011). If the water exchange with the North Sea was artificially limited in a numerical experiment, salinity would decrease in the deep layer of the gulf (Lessin et al., 2014). After the recent MBIs (Naumann et al., 2018), salinity peaked at 10.77 g kg-1 in the near-bottom layer of the central gulf (Liblik et al., 2018), which is the highest value since 1974 (Alenius et al., 1998). Former deep layer water from the northern Baltic Proper was pushed to the gulf 9 months after (Liblik et al., 2018) the MBI occurred in December 2014 (Mohrholz et al., 2015). The MBI water, which originates from the depths of 110–120 m in the eastern Gotland Basin (compare with Fig. 2), arrived in the gulf 14–15 months after the inflow (Liblik et al., 2018).

Decadal trends of salinity show vertically distinct changes. The surface salinity decrease since the early 1980s has been estimated to be in the range from 0.005 g kg-1 (Liblik and Lips, 2019) to 0.02 g kg-1 yr-1 (Almén et al., 2017). Long-term records close to the island of Utö revealed a sea surface salinity decrease from the early 1980s to mid-1990s (Laakso et al., 2018; compare with Figs. 2 and 4). However, the surface salinity increased by 0.5 g kg-1 during 1927–2012 in the northwestern part of the gulf (Merkouriadi and Leppäranta, 2014). The salinity trend in the deep layer of the central gulf has been estimated to be 0.04 g kg-1 yr-1 from 1982–2016 (Liblik and Lips, 2019).

The mean depth of the Gulf of Bothnia is 55 m, and its surface area is 30 % of the entire Baltic Sea. The Gulf of Bothnia is connected to the northern Baltic Proper by sills and archipelagos. Its hydrography is quite different from other parts of the Baltic Sea (Fig. 1). The sill between the Åland Sea and Baltic Proper prevents the northward propagation of deepwater flow. It is assumed that the water masses in the Bothnian Sea are renewed mainly by inflowing surface water from the Baltic Proper (Marmefelt and Omstedt, 1993; Meier, 2007). The net water exchange through the Archipelago Sea is estimated to be low compared to the Åland Sea (Omstedt et al., 2004; Myrberg and Andrejev, 2006; Tuomi et al., 2018). In the Gulf of Bothnia, the salinity stratification is weak. The surface salinity in the Åland Sea is about 5.25–6.25 g kg-1,, whereas at a depth of 200 m, salinity varies between 7 and 7.75 g kg-1. Deep salinity in the Åland Sea and the Bothnian Sea stems from the upper homohaline layer of the northern Baltic Proper. Additionally, a small fraction of more saline deep water flows in over sills. The inflow of saline water through the Åland Sea over the sills can cause a corresponding flow of fresher water out into the Gotland Basin. This strengthens the stratification in the Bothnian Sea (Leppäranta and Myrberg, 2009).

In the Bothnian Sea (Fig. 1), the surface salinity varies between 4.8 and 6.0 g kg-1, and in the lower layer at 150 m depth, the salinity is 6.4–7.2 g kg-1. In the Bay of Bothnia (Fig. 1), the salinity is between 2 and 3.8 g kg-1, and at 100 m depth near the bottom it varies between 4 and 4.5 g kg-1. The Gulf of Bothnia has many rivers, and near the river mouths, the salinity is close to zero. Even in the Bothnian Sea, the salinity stratification is relatively weak, and overall oxygen conditions have remained fairly good, not to mention some specific coastal areas. However, oxygen conditions in the deepest layer have somewhat deteriorated over the recent 2 decades, although there is no real evidence that hypoxic conditions will occur in the future (Raateoja, 2013).

The long-term changes in the salinity of the Baltic Sea depend to a large extent on net precipitation, river discharge, and wind forcing (Winsor et al., 2001, 2003; Meier and Kauker, 2003; Gustafsson and Omstedt, 2009); higher salinity appears during dry periods and lower salinity during wet periods. Furthermore, on shorter timescales, the mean salinity of the Baltic Sea is governed by the water exchange between the North Sea and the Baltic Sea, which is governed by the prevailing atmospheric conditions. Generally, westerly winds force inflow of saline water and easterly winds force outflow of brackish Baltic Sea water. Multi-decadal oscillations control the long-term variations of the surface salinity and its meridional gradient with a period of about 30 years (Radtke et al., 2020). A statistically significant positive trend of centennial changes in the north–south gradient of the surface salinity has been found (Kniebusch et al., 2019b). Increased river runoff from the most northern catchment could explain this trend. Observations reveal (Fig. 5, see Sect. 5.1) that after the minimum around 2003/04, the volume-averaged salinity increased again until 2018. Fluctuation in the accumulated anomalies of river runoff coincides with the variability in mean salinity, confirming the role of river runoff in controlling the mean salinity of the Baltic Sea (Kniebusch et al., 2019b; Radtke et al., 2020).

The long-term salinity dynamics within the Baltic Sea are controlled by the large-scale internal water cycle (e.g., Elken and Matthäus, 2008). There is a surface layer circulation and a deepwater circulation decoupled in the Baltic Proper by the permanent halocline. The wind and freshwater surplus drive the upper layer circulation. It is mainly Ekman dynamics in combination with complex coastlines and upwelling and downwelling. In the lower layer, the flow, which is the dense bottom current, is driven by internal pressure gradients steered by the complex bottom topography consisting of deep basins and channels and restricted by sills (Leppäranta and Myrberg, 2009). The vertical branch of this circulation system, termed the Baltic Sea haline conveyer belt (Döös et al., 2004), is restricted by the strong permanent saline stratification. Convection, mechanical mixing, entrainment, and vertical advection determine the vertical salt flux across the halocline. It is not only the mean salinity of the Baltic Sea which varies over the years; there are also considerable changes in the strength of the permanent salinity stratification. Liblik and Lips (2019) found a strengthening of the permanent halocline in the deep basins of the Baltic Sea over the period 1982–2016. They argued that the accumulated river runoff probably caused the decrease in surface salinity. However, they found no correspondence between increased runoff and decreased surface salinity in the second half of the period (compare Figs. 4 and 5). They argued that changes in the vertical salt transport might be the reason for this, which might be related to changes in meteorological forcing. However, the volume-averaged salinity of the eastern Gotland Basin is highly correlated with the accumulated river runoff (Fig. 5). After the major inflow in January 1993, salinity increased in the lower layer of the eastern Gotland Basin (Fig. 2). This increasing trend of deep layer salinity (Fig. 3) due to stronger lateral salt transport from the Kattegat could not be explained by a growing number of barotropic and major Baltic inflows (Mohrholz, 2018). So, the reason for the change in the haline stratification in the deep basins of the Baltic Sea over the recent 3 decades remains unclear.

Generally, the Baltic Sea's oxygen distribution results from the input through the atmosphere–ocean interface, physical transport, and the consumption of oxygen due to respiration and biogeochemical processes. The layer above the halocline is well oxygenated due to vertical convection during winter. In and below the halocline, the ventilation happens only by horizontal advection of water originating from the Danish Straits and the Kattegat. In the deep basins below the halocline, there is often no sufficient oxygen supply so that continuous oxygen consumption leads to hypoxic or even anoxic conditions. In the Bothnian Sea and Bay of Bothnia (Fig. 1), the salinity stratification is weak so that vertical deep convection during winter has prevented these basins from experiencing oxygen deficiency so far.

Climate warming impacts not only the deepwater oxygen distribution. The solubility of oxygen at the surface depends on the water temperature. In warmer water, the solubility is reduced and oxygen consumption is increased by enhanced decomposition of organic matter. Thus, even after major saltwater inflows (MBIs), which can reach the deep basins of the Baltic Sea, improved oxygen conditions will turn back to hypoxic conditions faster than in former times (Naumann et al., 2018). The trend in oxygen depletion is about 0.1 mL L-1 per decade in the surface layer and up to 1 mL L-1 per decade in the halocline (Fig. 3).

Roughly 100 fish species occur in the Baltic Sea. Their spatial distribution is primarily governed directly by salinity and oxygen. Marine species (∼70 species) dominate in the central Baltic Sea, while freshwater species (30–40 species) mainly occur in coastal areas and the northeastern parts of the Baltic Sea (HELCOM, 2002). Cod, herring, and sprat comprise most of the fish community in biomass and numbers. The marine species sprat, herring, cod, various flatfish, salmon, and the freshwater species pike and perch are commercially important before severe stock declines (Nilsson et al., 2004).

Cod eggs in the Baltic Sea have a vertical distribution concentrated in deep water and near or below the permanent halocline (Wieland and Jarre-Teichmann, 1997). Thus, cod eggs are frequently distributed in water layers with very low oxygen concentration (Nissling et al., 1994; Wieland et al., 1994). The eastern Baltic cod has a prolonged spawning period from March to September (Köster et al., 2017). The reproductive layer thickness for eastern Baltic cod is limited to minimum environmental threshold values (salinity 11 g kg-1, temperature 1.5 ∘C, and oxygen 2 mL L-1; Wieland and Jarre-Teichmann, 1997; Nissling and Westin, 1991; Nissling et al., 1994; Wieland et al., 1994). The reproductive layer thickness for western Baltic cod in February–March (the peak spawning time of this stock) is limited to minimum environmental threshold values (salinity 16.5 g kg-1, temperature 2 ∘C, and oxygen 2 mL L-1; Nissling and Westin, 1997) to keep western eggs in the Arkona Basin floating to allow successful development and survival. In a modeling study, Hinrichsen et al. (2016a) showed that the spatial extent of the habitat suitable for successful fertilization and development of eggs of the eastern cod stock is primarily determined by oxygen and salinity conditions at spawning. The highest survival of eastern Baltic cod eggs occurred in the Bornholm Basin, with a pronounced temporal decrease in survival in the Gdansk Deep and the Gotland Basin. Relatively low survival in these areas was attributable to oxygen-dependent mortality. Compared to eggs spawned in these eastern spawning grounds, eggs spawned in the Arkona Basin were affected mainly by sedimentation, i.e., the lack of sufficiently saline water at the bottom to ensure successful egg fertilization and development. However, since the mid-2000s, a substantial increase in sedimentation-related mortality has also been observed for the Gdansk Deep (Hinrichsen et al., 2016a).

For the Bornholm Basin, Hinrichsen et al. (2016b) estimated the geographic extent of the area with hydrographic conditions suitable for egg development in the early (April–May), middle (June–July), and late (August–September) spawning season. Egg survival depends on their buoyancy, which is related to female age and/or size. Large eggs, for example, are spawned by large, old females and float at low water density. The seasonal timing of spawning does not matter for these large eggs, while small eggs spawned by young females sink towards the bottom and suffer higher mortality due to exposure to hypoxic conditions. The geographic area suitable for their survival is concurrently lower than for larger eggs, with the most favorable conditions occurring late in the spawning season owing to the summer inflows.

The Arkona Basin, a relatively shallow area (max depth ∼ 40 m) mainly occupied by the western Baltic cod stock, has recently also been utilized as spawning ground by the eastern cod stock (Bleil et al., 2009; Hüssy, 2011). Generally, the reproductive conditions appear to be more favorable for the eastern stock, with several occasions of relatively large reproductive layer thicknesses since 1999, which was extremely seldom for the western stock. Vertical resolution data on the reproductive layer thickness for both stocks showed improved reproductive conditions for eastern cod of about 10 m overall layer thicknesses in June to August since 2000. A certain fraction of the egg production of the eastern cod stock can be expected to sink to the bottom and die. However, in a combined stock identification and drift model study, Hüssy et al. (2015) showed that eastern Baltic cod have progressively immigrated into the Arkona Basin during recent years. This resulted in stock mixing with the western stock, showing a marked increase in proportion between 2005 and 2008 with a fairly stable proportion of approximately 70 %–80 % since then. Even though this stock mixing is purely a physical admixture without interbreeding (Hemmer-Hansen et al., 2019), the immigrating eastern cod may have contributed to the recruitment of the eastern cod stock in this management area. However, this was only possible for relatively late-spawning eastern cod in years characterized by specific conditions, i.e., after summer inflows of saline water. The decreasing surface salinities in the 1980s (see Sect. 5.1) led to a southward shift of the salinity range of 5–7 g kg-1, which is the threshold for both freshwater and marine species distribution and diversity (horohalinicum). Vuorinen et al. (2015) discussed potential ecological consequences of varying surface salinity due to increased rainfall and river runoff. Species distribution and abundance will be affected favorably or negatively according to their origin. Decreasing surface salinity will lead to further expansion of freshwater species at the expense of marine species.