Changes in the quantitative relations of the phytoplankton in heated lakes
Long-term changes in phytoplankton biomass and structure were studied in three heated lakes that were included into a power plant cooling system in 1958 and 1970. Since the mid 1990s, the share of Vallisneria spiralis L., which is a thermophilic hydrophyte species that is alien to Poland, has been increasing. The phytoplankton biomass fluctuated in these basins in the 1992-2003 period from 0.2 to 49.3 mg dm-3. Two phytoplankton assemblages were noted: one was typical of the cold season and the other of the warm season. The phytoplankton in the summer comprised cyanophytes, chlorophytes, dinoflagellates, and cryptophytes. Diatoms developed on a massive scale during cold periods. The results obtained indicate that the contemporary phytoplankton assemblages are shaped by the varied impact of water heating, increased flow rates, the highly productive waters, and the developing submerged hydrophyte phytocenosis along with the abundant growth of the epiphyte assemblage.
A total of 313 lakes with charophyte vegetation were identified based on the data presented in 1111 manuscripts under the shared title of “Assumptions for the fisheries management project in Lake (lake name)”, based on the results of an environmental inventory carried out by the Inland Fisheries Institute in 1953-1968. The lakes’ morphological characteristics were described, hydrophyte species that most frequently accompanied charophytes were identified, differences in summer water transparency were analyzed in 281 lakes with charophyte vegetation and the results were compared with the observations made in 657 other lakes, and the potential trophic state of lakes with charophyte vegetation was determined using Carlson’s trophic state index (1996). More than half (54%) of 171 dimictic lakes with charophyte vegetation were classified as mesotrophic and 31% as oligotrophic, whereas 50% of 110 polymictic lakes were classified as eutrophic, and 40% as mesotrophic. The frequency of taxa that contribute to the eutrophication (degradation) of water bodies, including Ceratophyllum spp. Myriophyllum spp. and Elodea canadensis, increased with a decrease in the percentage of charophytes in communities of submerged hydrophytes. Regardless of the proportion of charophytes in submerged hydrophyte communities, water in the lakes colonized by charophytes was more transparent than in the 659 lakes without charophytes. Water in many polymictic and dimictic lakes with charophyte vegetation, including lakes with a small contribution of charophytes, was more transparent than in lakes without charophytes, which suggests that charophytes are sensitive indicators of water quality and are components of ecological memory in aquatic ecosystems.
The possibility of doing a back assessment of the ecological status of a lake based on archival bathymetric maps indicating areas overgrown with rushes and aquatic vegetation was verified. This assessment was assumed to be in accordance with that performed with the official Polish macrophyte-based method for lake assessment (Ecological State Macrophyte Index, ESMI). The study was conducted on Lake Dobrąg located in the Warmian-Masurian Voivodeship (surface area - 108 ha, maximum depth - 27.9 m, mean depth - 11.6 m). It included the hydroacoustic distribution of submerged macrophytes along 85 evenly distributed belt transects (perpendicularl to the shore line), creating a bathymetric chart and maps of vegetation occurrence and identifying areas occupied by hydrophytes (Cmax) and the maximum depth of lake colonization (Z). Analogous data were read from archival bathymetric chart dating from 1964-1968. The values obtained were compared with the means (and their confidence intervals) of 83 stratified lakes in Poland in different ecological status classes. Analysis of changes indicated that the ecological status of the lake had deteriorated. In the mid-1960s, the status of the lake was less than “very good” while the current status borders between “good” and “moderate.” The results indicate that the proposed method could be useful when attempting to assess changes in ecological status using archival bathymetric charts showing areas overgrown with vegetation and the distribution of it in lakes.
The aim of the study was to perform an ecological status assessment based on the phytoplankton in the vendace-type Lake Dejguny in accordance with the requirements of the Water Framework Directive. The phytoplankton analyses were conducted during three growth seasons in 2006, 2007, and 2008. The phytoplankton multimetric Phytoplankton Metric for Polish Lakes (PMPL) was used to assess the lake’s ecological status. The average total biomass of phytoplankton ranged from 1.6 to 3.9 mg dm-3, while the average biomass of Cyanoprokaryota ranged from 0.4 to 1.4 mg dm-3, with the minimum noted in 2008. The phytoplankton assemblages were dominated mainly by filamentous Cyanoprokaryota and pennate Bacillariophyceae in 2006 and 2008, or exclusively by pennate Bacillariophyceae in 2007. The relatively low total biomass and Cyanoprokaryota biomass, as well as seasonal phytoplankton dynamics with the dominant taxa of Tabellaria flocculosa, Dinobryon sociale, and D. divergens confirmed the lake’s mesotrophic state and some of the features of reference conditions in this lake, whereas the predominance of filamentous species Planktothrix agardhii and Planktolyngbya limnetica was characteristic of eutrophic conditions. However, a clear tendency towards progressive eutrophication stemming from the significant domination of filamentous Cyanoprokaryota was observed, and according to the PMPL the assessment indicated that Lake Dejguny had a good ecological status in 2006 and 2007 and even a high one in 2008.