Water conductivity in 23 lakes of the Suwałki Landscape Park (SLP) was tested in the years 2012–2014. Conductivity profiles were made at the deepest place every two months between spring and autumn water mixing. The collected measurement data, supplemented with historical data, were used to identify factors that shape the spatial variability of water conductivity and to reconstruct its multiannual changes. The range of variability of the mean conductivity of surface water of the SLP lakes ranged from 178 to 522 µS cm−1. The strong negative relationship between conductivity and lake elevation (R = 0.816, p < 0.000) suggests that in the territorially compact complex of the SLP lakes, conductivity is a consequence of the location of the lake in the catchment, which, in turn, affects the structure of its water supply. However, the physical and environmental parameters of the catchment and morphometric parameters of lake basins are of secondary importance. In dimictic lakes and in those showing signs of “spring meromixis”, the magnitude of conductivity differences in the water column is determined by the lakes’ susceptibility to wind mixing expressed by the exposure index value. The developed climate models have also shown that conductivity, an indicator of water quality, is very sensitive to climate change. The analysis of the reconstructed chronological conductivity sequences shows that the deterioration of the quality of the waters of the SLP lakes first occurred at the beginning of the 21st century and a clear increasing trend has been maintained over the last decade.

Maximum depth of colonization (zC) and total area covered by a population of Lobelia dortmanna, as well as underwater light regime were studied in 25 soft water lobelia lakes in north-western Poland. Variations in underwater light conditions among the lakes were described by Secchi disc depths (zSD), and by attenuation coefficients of irradiance within photosynthetically active radiation range (Kd,PAR), and euphotic zone depths (zEU) derived from photometric measurements conducted twice a year (in midspring and midsummer) during the period 2014–2015. Maximum depth of colonization of water lobelia ranged from 0.1 to 2.2 m (median zC = 0.8 m; mean zC = 1.0 m). Nine lakes showed the relative coverage of the littoral zone (RCLZ) by L. dortmanna to be greater than the mean value, which was 4.8%. Studies showed that light requirements of water lobelia increase when the maximum depth of colonization also increases. This pattern could be partially related to the greater energy needs of deeper growing individuals due to enlarged seed production and their incubation, and for the creation of much heavier inflorescences. Assessment of the light requirements of L. dortmanna along the depth gradient indicates that relative irradiance (percentage of subsurface irradiance of PAR) should be at the level of: (i) 47–50% (annual total of quantum irradiance 3083–3280 mol m−2 yr−2) for plants growing within a depth range of 2.0–2.5 m; (ii) 44–47% (2886–3083 mol m−2yr−1) for plants growing within a depth range of 1.5–2.0 m; (iii) 41–44% (2690–2886 mol m−2yr−2) for plants growing within a depth range of 1.0–1.5 m; and (iv) 34–41% (2230–2690 mol m−1 yr−1) for those growing in the littoral zone at a depth of between 0.5 and 1.0 m. In average conditions in the Pomeranian lakes, the maximum depth of colonization by L. dortmanna accounts for approximately a third of the Secchi disc depth and a fifth of the depth of the euphotic zone with irradiance of PAR at zC equal to about 43% of subsurface irradiance. It has also been demonstrated that the light factor is a crucial one that limits the absolute maximum depth of lobelia population occurrence in Pomeranian lakes. The cleanest and most transparent lakes of this region have light attenuation coefficients (Kd,PAR) within the range of 0.35–0.42 m−1, which corresponds to the maximum colonization depths of 1.8–2.2 m.

Secchi disc transparency and optical properties of water, based on measurements for downward irradiance using an underwater radiometer PER-700, were studied in 23 lakes of the Suwałki Landscape Park (SLP) in 2013. Measurements of optically significant constituents of water were also performed (YSI EXO-2) and analysed. Data on water transparency from published studies were used to assess long-term changes in the optical conditions in the deepest water body in Poland -Lake Hańcza. In terms of the optical characteristics of the water, the analysed SLP lakes can essentially be divided into two distinct groups representing different optical lake types. These are optical categories of lakes most common in Poland, namely types II and IIIb. Regional equations describing the relationships between the attenuation coefficient for downward irradiance as well as optically significant constituents of water were developed. The optical properties of the SLP lakes are mainly affected by suspended solids, mostly phytoplankton. Interannual fluctuations in Sechi disc transparency, as well as the attenuation coefficient for downward irradiance and other apparent optical properties of mid-summer water calculated from it, are determined by the spring thermal conditions and intensity of phytoplankton blooms during this period. Application of the attenuation coefficient for downward irradiance as a criterion for the assessment of the trophic status of the SLP lakes indicates that these are generally eutrophic lakes. The largest and deepest lakes of the park, however, are mesotrophic. Lake Hańcza is currently in the stage of α-mesotrophy.