Lake Charzykowskie (Figure 24) is administratively located within the Pomeranian Voivodeship in the municipality of Chojnice (Figure 25). In terms of physical-geography it is situated in a macroregion of the South Pomeranian Lakeland on the Charzykowska Plains, within the Polish largest forest complex of the Bory Tucholskie.
The lake fills the southern part of the largest glacial gutter in the Polish lowlands. Its length is 17 km, width reaches up to 2.5 km and its depth is up to 50 m. In addition to the Lake Charzykowskie also Dlugie, Karsinskie and Witoczno lakes fill the form.
The gutter is the result of complex processes that took place since the Krajenska sub-phase of the last glaciation (16,8 thousand BP). According to Pasierbski (1973), during the oscillation of the ice sheet, the glacier lobe entered the older subglacial gutter and significantly increased the narrow form (glacial erosion). During deglaciation, the gutter was preserved by gutter blocks of dead-ice and was buried under the ground moraine deposits and fluvioglacial sediments. According to Nowaczyk (1994), the melting of the dead-ice blocks and the beginning of the biogenic accumulation is dated 1540 BP. The original shape of gutter was transformed due to accumulation of river and Aeolian/biogenic processes. This resulted in several lake terraces, river scars, coastal embankments and constantly developing river deltas. (Dysarz, 2003).
The lake is located in Brda River catchment, which takes the beginning in Lake Smolowe and disgorge into the Vistula River. The Brda River is the lake’s main tributary; it is 238 km long and the reception basin is 4627.2 km2. The remaining tributaries are Red Stream (Czerwona Struga), Jarcewska Stream (Jarcewska Struga), Seven Lakes Stream (Struga Siedmiu Jezior ) and several smaller in the rich in water south. Outflow of water from the reservoir is through Brda to the Karsinskie Lake (Dlugie Lake).
Lake Charzykowskie is a large tank with an area of 1357 ha which can store a water volume of 135 Mln m3 of water. It consists of three pools separated by underwater thresholds limiting the free exchange of water (Figure 25). The southern pool, with a depth of 25 m, is supplied by small tributaries draining the numerous springs flowing out from moraine formations. On the border with central pool flows Jarcewska Stream that discharges wastewater from a large city of Chojnice. To the deepest (29 m) central pool flows Red Stream. Through the most shallow (11 m) northern pool flows Brda river. There also flows Seven Lakes Stream.
Lake Charzykowskie catchment is located in the South-Baltic Lakeland, where the climate is influenced by the impact of the Atlantic Ocean. It is overlapped by the impact of the Baltic Sea, allowing the air mass impact from the Ocean, far transmissivity deep inland. The mean annual temperature in the lake region is 7°C, ranging from -3°C in January and 17°C in July. Annual precipitation is ca 600 mm, with the highest amounts concentrated in summer.
Due to this climatic characteristics, most of the area is covered by forest, i.e. 59%of the total catchment area of Lake Charzykowskie is wooded (Figure 26), mostly national forests with an implemented policy for harvesting timber. However, the north-eastern part of the catchment, the newly established Bory Tucholskie National Park (1996) is under the legal protection. The lake was also incorporated into a indigenous part of a Man and Biosphere Reserve of the Bory Tucholskie in 2010. In addition, the lake is located within the protected area of the Zaborski Landscape Park and it lies in the buffer zone of the Bory Tucholskie National Park created in 1996. Further, the lake lies within the Special Area Protection for Birds “Wielki Sandr Brdy” (PLB220001; Natura 2000) and parts of the lake catchment are situated in the Special Areas of Conservation.
Over 35% of the catchment is occupied by agricultural corps (Figure 26). Urban development constitutes minimal percentage but has an impact on the lake. The lake is orientated around 4 towns: Charzykowy, Funka, Bachorze and Male Swornegacie. These towns perform a holiday resort function during summer.
The annual nutrient loads from the main lake inlet (River Brda) have been estimated as very low (ca. 1 Kg P m-2 year-1 and 20 Kg N m-2 year-1 in the hydrological year 2007) and among the lowest within the lake district of the Zaborski Landscape Park (Bogdanowicz & Cysewski, 2008). However, the major nutrients loads are reaching the lake from tributaries entering the lake from its eastern side, in particular from River Jarcewska Struga, which is a recipient of industrial and domestic sewage treated in waste water plant in Chojnice (Figure 27). It was estimated that the Jarcewska Struga load alone accounts for about 20% of the total phosphorus entering the lake (Goszczynski & Jutrowska, 1997)
Based on a research carried out in 2009 year by the Regional Inspectorate for Environmental Protection in Gdansk, Slupsk delegature, average annual chlorophyll-a resulted to be 16.7 µg L-1, dissolved oxygen was 1.6 mg L-1, TN was 0.98 mg N L-1 and TP was 0.103 mg P L-1. Accordingly to Carlson’s Trophic State Index (TSI) the lake is currently classified as mesotrophic (Table 3).
However, significant symptoms of eutrophication have been recently observed in several lakes situated in the Zaborski Landscape Park (ZPK), where intensive algae blooms are occurring in summer months (Wisniewski & Nowicka, 2003). On the contrary, previous research did not indicate any important pollution sources within the Park borders, with the exception of some fish farms, small settlements and few holiday centres situated on the lakes banks (Wisniewski & Nowicka, 2003).