Excess seawater salinity in the Puck Lagoon, a shallow part of Puck Bay (southern Baltic Sea)

Seawater salinity is an essential variable describing the state of seawater. Salinity strongly influences seawater density and, therefore, affects dynamical processes and the formation of water masses in the sea and ocean (e.g. Dera, 1992; Massel, 2010). The functioning of different marine ecosystems depends on the ability of fauna and flora to adapt to given salinity conditions (e.g. Smyth & Elliott, 2016). In inland seas and coastal water bodies (gulfs, bays, and lagoons), salinity is closely related to local water balance components such as river runoff, inflows from adjacent seas, precipitation, and evaporation. In the Baltic Sea, which is primarily subjected to river runoff (e.g. Leppäranta & Myrberg, 2009), the mean salinity level is low, but salinity is characterised by large variability at various space and timescales (Lehmann et al., 2022). Changes in water balance components result in changes in the salinity regime.

Recent investigation on hydrological conditions in the Puck Lagoon, a small water body on the southern coast of the Baltic Sea, carried out in the summer of 2020–2022 has revealed higher than expected seawater salinity. The results are presented in this study, and the possible reasons responsible for such a situation are considered.

Due to the small depths in the Puck Lagoon basin, the water column was well mixed, and salinity mostly did not change from the sea surface to the bottom. Only near the Reda River was there a two-layer stratification, with water of distinctly lower salinity in the upper layer. Water with salinity ≥7.5 g∙kg⁻¹ usually occupied most of the Puck Lagoon area, that is, its northern, central, eastern, and even southeastern parts in June and August 2020 and August 2022 (Fig. 3). In June 2021, it covered a smaller area of the eastern and middle parts of the lagoon.

Figure 3

Spatial distributions of surface seawater salinity in the Puck Lagoon in the summer of 2020–2022.

Some statistics of surface salinity values is given in Table 1.

The highest salinity values were in the range of ~7.6–7.7 g∙kg⁻¹, except in June 2020, when they ranged within ~7.55–7.6 g∙kg⁻¹ (Table 1). Generally, they were observed in the zone extending from Kuźnica Hollow, the deepest natural bottom depression in the Puck Lagoon (maximum depth c.a. 10 m), through the shallows of Virgin and Western Sands (c.a. 1 m of minimum depth), to the northern part of Puck Lagoon (Figs 1 and 3). Some salinity values were even higher than those observed in the surface waters of the southern Baltic Sea (Bornholm Basin), for example, ~7.6 g∙kg⁻¹ in August 2022 (original value of practical salinity used by permission of Cormano Sp. z o. o.). The southern part of the Puck Lagoon is directly influenced by the Reda River. There, the lower surface salinity values were noted. However, the lowest ones, in the range of ~6.6–7.0 g∙kg⁻¹, occurred usually near the Rewa Cape (Z23) and once (30.06.2021) at sampling site Z22 located closer to the river mouth (Table 1).

At the reference sampling sites located in the Outer Puck Bay (Fig. 2), seawater surface salinity rarely exceeded c.a. 7.4 g∙kg⁻¹ (Fig. 4A). On 30 June 2021, it was elevated to c.a. 7.5 g∙kg⁻¹ at sampling sites K6 and K13 (Fig. 2) located near the Kuźnica Passage (Fig. 4A). High salinity values (~7.55–7.7 g∙kg⁻¹), as observed in the Puck Lagoon, were noted below the homogenous surface layer even at the depth of 25–30 m (Fig. 4A). The increase of salinity down in the water column resulted from the advection of saline and cold water into Puck Bay from the Gulf of Gdańsk. The most distinct difference in surface seawater salinity between the two parts of Puck Bay was registered in August 2022 (Fig. 4B). In Outer Puck Bay, salinity ranged from c.a. 7.1 g∙kg⁻¹ to 7.4 g∙kg⁻¹, while surface salinity in the Puck Lagoon was dominated by values in the range of 7.5–7.7 g∙kg⁻¹. The salinity decrease in the surface layer of the Outer Puck Bay was caused by the migration of water from the area adjacent to the Vistula River mouth induced by the wind from the eastern sector before the fieldwork started.

Figure 4

Vertical salinity variability at reference sampling sites in the Outer Puck Bay in the summer of 2020–22 (A). Surface seawater salinity [g∙kg⁻¹] distribution in the Puck Bay on 17–19 August 2022 (B). Surface seawater temperature [°C] distribution in the Puck Bay on 17–19 August 2022 (C).

Previous studies on the Puck Bay hydrology did not report such a noticeable difference in salinity between both parts of the bay during summer. In 1969–1972, Klekot (1980) noted that in June and August, over-bottom water salinity in the Puck Lagoon could be as high as c.a. 8 g∙kg⁻¹, which is even higher than the values reported in this work. It was suggested that under northern and western winds, surface water in the bay is pushed to the south and east, respectively, and compensating sea current from the Gulf of Gdańsk originating at greater depths and carrying more saline water gets into the Puck Lagoon. However, there were no data shown from the Outer Puck Bay or the Gulf of Gdańsk to support this idea. There are also no signs of such circulation in the vertical salinity profiles from the sampling sites south of Seagull Sandbar (Fig. 4A). A large number of data on salinity in Puck Bay were collected at the Institute of Oceanography of the University of Gdańsk during monitoring of the Gulf of Gdańsk between 1972 and 1990 (Nowacki, 1993c). Based on this material, it was found that in the summer, the mean surface salinity spatial distribution in Puck Bay revealed a plume of water characterised by salinity increase up to 7.5 g∙kg⁻¹ in the northern part of the Puck Lagoon (Nowacki, 1993c). This picture indicates the occurrence of water with higher salinity in the lagoon, but it is strongly different from the salinity surface distributions presented in Fig. 3.

There is also a simulation approach to the hydrology of Puck Bay developed within the WaterPUCK project (https://waterpuck.pl). The numerical model of seawater salt transport in the Puck Bay yields a reasonable approximation of the salinity field in the water body of the Outer Puck Bay, and discrepancies were noted in shallow waters, for example, near the Głębinka Passage (Dybowski et al., 2019). However, the model results were not tested in the Puck Lagoon area. Visual inspection of the surface salinity distributions in the lagoon, presented at the WaterPUCK website for the days when the measurements were performed, revealed that modelled values strongly underestimate the real ones.

Precipitation and evaporation from the sea surface are basic components of the water budget. Climatological mean world ocean surface salinity and evaporation minus precipitation flux are highly correlated, and salinity changes over time, in significant part, can result from the alteration to evaporation and precipitation (e.g. Durack, 2015). Such a dependency is also observed during a year at some coastal seas and reservoirs (e.g. Liblik et al., 2024; Ratnawati et al., 2018). Precipitation over the sea results in the seawater salinity decrease. Precipitation over the land also affects seawater salinity because excess rainwater is discharged into the sea by rivers. The monthly sums on precipitation in the region of the Puck Bay were taken from meteorological coastal stations located at Rozewie Cape, in the town of Gdynia, and Hel (https://danepubliczne.imgw.pl/pl/datastore) (Fig. 1 and Table 2).

Generally, there was a distinct precipitation deficit in the summer of 2020–2022. Only in a few months was precipitation approximately similar to or even higher at certain stations than the long-run averages, as, for example, in June 2020 or in August 2021 (Table 2). Reduced precipitation was accompanied by a decrease in Reda River discharge. The monthly means of the river water flow at Wejherowo, ~10 km to the west from the river mouth (https://danepubliczne.imgw.pl/pl/datastore), were on average about 15% smaller compared to multiyear monthly means (Cieśliński et al., 2022). The occurrence of dry months (August 2020, June 2021, and August 2022) and higher salinity level, up to c.a. 7.7 g∙kg⁻¹, coincided, whereas, in June 2020, salinity was lower, up to c.a. 7.6 g∙kg⁻¹.

While precipitation and river discharge cause the dilution of seawater, evaporation acts in the opposite direction. Evaporation is more intensive when water is warmer (e.g. Eaglson, 1970). Due to shallow depth, the water in Puck Lagoon warms quickly. Heat transfer from the air to the water in this area can be efficient since there is usually no strong water dynamics (e.g. Dybowski et al., 2019). Therefore, in summer, the water temperature in the Puck Lagoon is higher than in the surface layer of the Outer Puck Bay (Nowacki, 1993c, Fig. 4C). Additionally, evaporation could also be enhanced in connection with global warming. It is considered that this process is responsible for the long-term growth of seawater temperature reported for the eastern part of Polish coastal waters, for example, at Hel (Świątek & Girjatowicz, 2019). Another factor intensifying evaporation in a shallow basin is increased roughness of its surface in comparison to deep water under the same wind conditions (e.g. Panin et al., 2006). According to the multiyear approach, evaporation exceeds precipitation in Puck Bay on average by ~10 mm∙m⁻² in summer months (Chlost & Cieśliński, 2022). In the light of precipitation data presented in Table 2, the advantage of evaporation had to be significantly larger than 10 mm∙m⁻². Therefore, evaporation seems to be the factor that should be considered responsible for the increase in seawater salinity. It could be expected that high salinity levels in Puck Lagoon are related to the occurrence of dry years in the Puck Bay region. According to Chlost and Cieśliński (2022), in the 1981–2015 period, dry years occurred irregularly, sometimes one after another. The seawater salinity increase in the Puck Lagoon was similar to that observed in Väike Strait, a shallow strait in the eastern Baltic Sea, where in June and August, evaporation caused the salinity to be in the range of c.a. 7.6–8 g∙kg⁻¹ (Liblik et al., 2024).

In summer, when the sea level is low and there are no severe windstorms, water exchange between the two parts of Puck Bay is hampered because of the presence of Seagull Sandbar (Nowacki, 1993b). In the absence of strong seawater flows through the passages, Puck Lagoon and Outer Puck Bay can be well isolated from each other and then the difference in seawater salinity becomes more pronounced. Such situations likely happened on 30 August 2020 and 17 August 2022, when salinity spatial distributions in the lagoon displayed a pattern with dominance of more saline water (Fig. 3). However, even when sea level is low, water exchange through the passages can be intensive, leading to distinct changes in salinity distribution in the lagoon. Probably, such an effect was observed at the end of July 2021, when the most saline water occurred in the eastern part of the lagoon at the entrance of the Kuźnica Passage, and increased salinity was also noted on the other side of the passage in the Outer Puck (Figs. 3 and 4A). It seems that there was water outflow from the lagoon through the Kuźnica Passage. At the same time, there was an inflow of less saline surface water from Outer Puck Bay to the lagoon through the Głębinka Passage located at the other end of Seagull Sandbar. This inflow, together with freshened water near the Reda River mouth, was directed to the north, causing salinity decrease in the lagoon (Fig. 3). In this case, despite very low precipitation (Table 2), the impact of evaporation influence on seawater salinity was disturbed in a large part of the lagoon area by enhanced water circulation.

To sum up, it is suggested that the occurrence of seawater with increased salinity in the summer of 2020–2022 in Puck Lagoon mainly resulted from evaporation. The coastal shallow parts of the sea isolated to some extent from the open waters are particularly sensitive to changes in water balance components. Confirmation of this hypothesis requires further research combining salinity records and spatially expanded meteorological observations in the region of the Puck Lagoon.