This study describes summer phytoplankton communities in 27 lakes in the Suwałki Landscape Park (SLP) using in situ fluorescence methods. Low chlorophyll-a concentrations were noted in most of the studied lakes, particularly in the deepest lakes with highest surface area. Green algae, diatoms and cryptophyta were dominant components of lake phytoplankton. Higher chlorophyll-a concentrations in the shallow or more eutrophicated lakes were connected with an increase of cyanobacteria and cryptophyta concentrations as well as with a decrease in the share of diatoms inphytoplankton structure. Vertical distribution of phytoplankton in stratified lakes revealed the presence of deep chlorophyll layers just below the thermocline where the maximum concentrations of phytoplankton were up to 15 times higher than in the epilimnion zone. The deepest maximum concentration of phytoplankton was noted at a depth of 16.5 metres in Lake Jeglówek. In some lakes two or three significant increases of phytoplankton concentration in the vertical profile were observed, caused by intensive development of different algae groups.
The spatial differentiation of the physico-chemical parameters (KdPAR, pH, DOC, nutrient concentrations) and plankton communities (phytoplankton, Crustacea) was considered in the system: the water column without macrophytes, pleustophytes - lemnids (duckweeds) and hornwort Ceratophyllum demersum. It was demonstrated that at the study sites different habitat conditions occurred, which were related to the macrophyte assemblages. The highest values of the phytoplankton biomass and the greatest algal biodiversity were found in the site without vegetation, in both species-specific and functional terms. Zooplankton, dominated by Copepoda, reached the greatest density and species diversity at the Ceratophyllum site. Despite the substantial differentiation of habitats, the plankton diversity was quite low.
Summer field research and chemical analyses in 2015 provided the basis for the presentation of the trophic status of waters of 28 lakes of the Suwałki Landscape Park in NE Poland, located in the catchment of the River Neman, in the Natura 2000 area – Ostoja Jeleniewska. Lakes Hańcza and Szurpiły are currently mesotrophic ecosystems. The remaining lakes are moderately eutrophic, while four lakes in the group, in addition to anaerobic hypolimnion, have infertile epilimnion similar to that of mesotrophic lakes. Lake Hańcza shows considerable multiannual dynamics of water fertility, periodically reaching a status approximate to eutrophy. In Lake Szurpiły and the majority of smaller lakes of the Suwałki Landscape Park, reduced precipitation in 2015 resulted in an inconsiderable decrease in trophic status. The lakes are under the strong influence of their catchment area, as confirmed by statistically significant correlations between TSI and the Ohle and Schindler ratios. Lakes Kluczysko and Kopane currently remain the most fertile lakes in the Park. Unless rapidly developing cattle breeding leads to excessive fertilisation of waters in the catchment, the forecasted variability of weather conditions during global climate transformations will lead to fluctuations in the fertility of the majority of the analysed lakes at the threshold of meso- and eutrophy. A hypothesis is also stated regarding the considerable role of groundwater alimentation and short retention time in the maintenance of higher than average resources of silica in limnic waters.
Studies of summer rotifer fauna of 11 shallow and 6 deep lakes of the Suwałki Landscape Park were conducted in an aim to assess the role of lake depth and biotic and abiotic factors in determining the diversity of rotifer communities in lakes undergoing a natural rate of eutrophication not accelerated by anthropopression. Some differences were observed when the group of shallow lakes was compared with that of deep lakes. The former were more differentiated as regards a list of rotifer species, partly due to the high proportion of species with low frequency. Consequently, their faunal originality was markedly higher and values of the percentage similarity of rotifer communities were much lower than in deep lakes. Species richness of rotifer fauna was also dependent on the trophic state of the studied lakes, the character of a nutrient limiting primary production, as well as biomass of diatoms and green algae.
Rotifer abundance and species composition in lakes of the Suwalki Landscape Park were studied in the years 1983–1985, 2009, 2012 and 2015. Rotifer trophic state indices (TSIROT) were used to assess changes in the trophic state of the studied lakes. In most lakes of the Suwalki Landscape Park, there were no changes in rotifer communities over the 25–32 year period to indicate a decline in trophy of the lakes. However, in lakes with the strongest decline in TSIROT values, the values of indicative parameters widely varied in 2015, which may indicate that the status of rotifer communities in the lakes may be dependent on non-trophic factors.