In particular branches of the Earth sciences the term estuary is used with various meanings. The term estuary is applied to objects that often differ from the original geographic meaning of the word. The phenomena that accompany them and processes that cause them are called also estuary phenomena and processes. The aim of the study is to describe, in a geographic context, the range in meaning of both the term and names of phenomena and processes derived from it. In the years 2002 - 2005, a survey of the distribution of chloride concentrations was conducted on five lakes characteristic of the southern Baltic coast: Jamno, Bukowo, Wicko, Kopań and Łebsko. It was observed that seawater intrusions into the coastal lakes only resembles those that occur in classic estuaries, though the results are similar to seawater intrusion processes. In the Polish section of the southern Baltic coast, coastal lakes are not the only places where these phenomena are observed. We suggest that all inland water bodies that are under the influence of marine intrusions should be called coastal waters, instead of the misleading term estuary.

The paper discusses hourly changes in specific conductivity in two lakes and compares them to changes over longer time intervals. The short time intervals between measurements are designed to help assess the course of seawater intrusions. Two lakes on the Polish coast were selected for research purposes – Lakes Gardno and Łebsko. Specific conductivity was measured using an automatic YSI Sontek 6920V2 probe. It was shown that Lake Łebsko has a permanently elevated specific conductivity, whereas Lake Gardno experiences episodes of fluvial influence. The specific conductivity was shown to change constantly in both lakes, as evidenced by multi-day, daily and hourly data

The aim of the study was to locate and describe groundwater outflows in a selected lake basin and measure the spring output of water and the physical characteristics of the studied waters (temperature and conductivity). The final aim was to gauge the effects of the spring output on the lake hydrology. The time scope of the work included the period from January 2011 to September 2012. The spatial scope of the work included the area of Lake Raduńskie Górne and its direct catchment located in the Kashubian Lake District in northern Poland. Four groundwater outflows were located in the course of the study. Their total output was 4.6 L s⁻¹ and ranged between 0.5 and 2.5 L s⁻¹. This produces an annual yield of about 145 000 m³ year⁻¹. These results confirm that one of the forms of supply to the lakes are the under-lake springs. For lakes located in the area of young glacial accumulation this can be decisive.

The purpose of the paper is to provide some calculations on the current water balance for the Vistula Lagoon, which is exceptionally valuable in terms of biology and hydrology. It is located along the southern coast of the Baltic Sea in both Poland and Russia. In the era of marked climate change, but first and foremost the plans of the Polish side of the cross-cutting of the Vistula Spit, there has been a need to update the balance data of the Vistula Lagoon. In the near future, they may be an excellent comparative material to changes in the proportions of individual water circulation components in the Vistula Lagoon and changes in its entire ecosystem, caused by the implementation of the project. In the literature on the subject, balance sheet data from 50 years ago are used (these data were compiled in the initial part of the study), hence the concept of attempting to update them has appeared. Due to the lack of data from the Russian part of the catchment area, the main emphasis in the work was put on the completion and modernization of the components of the balance sheet of the Polish catchment. A novelty at work is the refinement of the potamical inflow to the Vistula Lagoon with small but numerous streams flowing from the Elbląg Upland, as well as the inflow forced by polder discharges. Climate water balance data are shown based on data available from IMGW in Warsaw and include monthly precipitation totals for the period 1996–2010 obtained at six gauging sites located near the Vistula Lagoon. Evaporation was calculated using Tichomirov’s formula. River water influx was determined based on data available in Kruk (2011), data from IMGW (Pasłęka River) and data from papers Bogdanowicz (2007, 2009) and documents associated with the Program for biological passages in rivers in Warmińsko-Mazurskie Province (2007). Raw data were also obtained from the Office of Water Management in the city of Elbląg for the period 2006–2011. These data covered water flow moving from the polders. Groundwater influx was estimated using values provided by Silicz (1975). The volume of seawater influx was estimated using the data provided by Silicz (1975) and Chubarenko and Chubarenko (2002). The study confirms that water exchange in coastal lagoons is quite complex due to the presence of several different sources of recharge and several different places where water is lost. The complexity of water cycle is enhanced by a complex hydrographic system of lagoon catchments and a complicated system of water exchange with the sea. The total water volume involved in circulation in the Vistula Lagoon is estimated to be 24,225 mln m3 per year. The largest part of the water received by a coastal lagoon comes from the sea. In this case, it is about 77% (18,130 mln m3 per year). In addition, an array of catchment sources yields about 21% of the studied lagoon’s water. Its catchment provides an additional 4,974 mln m3 of water. The Pregolya River produces 55% of the catchment influx. The volume of water forcibly produced by area polders is so small that it does not produce an observable effect on the water balance in the Lagoon (about 0.04% of total influx). The precipitation constitutes 2.3% of the power supply source of the Vistula Lagoon. On the other hand, the Strait of Baltiysk constitutes the main outflow pathway for water exiting the Vistula Lagoon – close to 98% of all outflow (23,694 mln m3). The remaining 2% is lost due to evaporation.