Volume 10 (2010): Issue 3-4 (December 2010)

The paper presents a method of assessment of the ecological state of lakes. It is based on methodology proposed by the Polish Limnological Society. According to this methodology, a set of indicators may be determined and may serve as a basis to make recommendations on what actions should be taken to maintain or restore a good ecological state of a lake. The set of indicators used for remedy recommendations includes: environmental pressure indicators that determine the rate of eutrophication in bodies of water, indicators of the state of the environment describing water quality and the balance of the water resources of a lake, and reaction indicators providing information on the degree of eutrophication of a lake.

The study was undertaken to illustrate a wide range of application of a gradient thermal probe in thermal characterisation of water reservoirs on the example of Lake Morskie Oko whose water temperature has been measured since 2006. For analysis, the data collected in 2007 were chosen because of the least missing data; in this year only the data from 13 days were not recorded. The data permitted analysis of changes in daily mean temperatures of water at particular depths, with the daily means calculated as arithmetic means from 144 measurements made at every 10 minutes. The daily amplitudes of temperatures at particular depths and mean daily amplitudes for particular months were determined, which permitted an assessment of the scale of the temperature amplitude damping with increasing depth. High frequency of temperature reading permitted detection of short-duration anomalies, taking place e.g. over a time span of 10 minutes, or their vertical extent. It has been established that measurement at 2 a.m. is the optimum for determination of the annual mean temperature. The time moments at which the minimum and maximum temperatures are recorded within a day at the surface were identified. Different variants of temporary jump changes in the surface water temperature within the day were analysed. The dynamics of temperature changes from the surface to the bottom of the lake was discussed.

Lake Syczyńskie is a water body that represents a group of the smallest Łęczna-Włodawa Lakes. The lake is supplied with waters from the four streams, while the outlet from the lake is directed into the Świnka River. The residence time of water is very short in Lake Syczyńskie. A role and variation of water supply from the catchment area were calculated using the mean seasonal loads transported from lakes sub-basins. A range of ionic migration into the basin of the lake was presented by means of daily and five-day radiuses. Among all inflows, C2 and C4 streams that drained Chełm Hills structure, were the most stable ones. The C3 stream that drained lake vicinity, was less important in shaping the quality of the lake water, while stream C1 was the one with a wide range of seasonal loads inconstancy. The highest fluviodynamic and matter transport occurred during the cold season. However, during the warm season in-lake processes were significant in terms of shaping the quality of lake water.

Lake disappearing is a natural process which contemporarily escalates in consequence of human activity. It is estimated that within the area of Northern Poland from the last glaciation period (ca. 17 000 years ago) a half of lakes totally have disappeared. Areas exposed after water basins desiccation have become native rocks for new soils. Reduced water level results in changes of morphology and properties of the soils situated in direct vicinity of former water basins. The aim of this study was to estimate impact of the catchment groundwater level fall on morphology and properties of direct lake catchment soils, exemplified by the lake Sumowko (Northern Poland) as well as description of new soils formed of lake sediments. The analysis covered 11 soil profiles emerging within former lake basin (newly formed soils) and soils from direct vicinity of former lake (modified through ground water level fall). Obtained results prove that newly formed soils (Limnic Histosol Drainic and Haplic Gleysol) in majority are utilized as grasslands. Soils of the former lake surroundings prove relic features of gleying while they are also subject to mucking process because of dehydration.

Rusałka Lake is a shallow, man-made, strongly eutrophicated reservoir, being a place of recreational activities for Poznan citizens. Restoration measures with the use of iron sulphate were conducted in years 2006-2007. Phosphorus and organic matter concentrations in lake sediments were analyzed in years 2005-2007 at two research stations (profundal and littoral) to determine its changes under the influence of restoration. Greater phosphorus concentrations were noted in profundal (max 1.55 mg P g^-1 dry weight (DW)) than in littoral (max 0.98 mg P g^-1 DW). An increase of P amount in sediment was observed after iron treatment. Among phosphorus fractions Res-P dominated, i.e. phosphorus biologically unavailable for organisms. Organic matter content reached 23.1% in profundal, while 14.8% in littoral. Its amount decreased in following years.

This study examined the overgrowth in two eutrophic lakes during the period 1961-2008. In addition, for comparative studies, analyses of plant community structures in these lakes were performed to assess the effects of human pressure, such as water level reduction and increase of nutrient availability on changes in the range of the littoral zone surface. To achieve these aims aerial photos, ortophotmaps, GPS measurements were used to evaluate changes in plant cover in two lakes: Niepruszewskie and Tomickie. Considerable differences between the studied lakes located in an agricultural catchment were observed. Lake Niepruszewskie was characterised by a threefold higher rate of overgrowing, measured per hectare, in comparison with Lake Tomickie. Moreover, in comparison with lake areas, rates of overgrowing were substantialy higher in Lake Tomickie than Lake Niepruszewskie, which is almost seven times bigger. The littoral zones of both lakes are dominated by emergent vegetation. Compared to the data from 1961, the zone of Lake Niepruszewskie increased almost threefold. The biggest changes in overgrowing were observed after reduction of water level (>2002) which was 1.30 ha yr^-1. During almost 50 years, emergent vegetation in Lake Tomickie increased more than fivefold. The most intensive growth was observed in years from 1961 to 1986, which amounted to 0.73 ha yr^-1. Probably, water level reductions in Lake Niepruszewskie and high eutrophication in Lake Tomickie were recognised as major factors causing intensive overgrowing in the studied lakes.

The method of measuring transparency of water environments using the Secchi disc is still applied, despite a certain level of subjectivity of obtained results. At present, it should be used as a supplement to more reliable methods of measuring the thickness of the euphotic zone. A serious drawback is the low informative value of isolated measurements of the transparency of water environments, representing different stages of cyclic annual variability. This is also confirmed by the summer visibility measurements in the waters of lakes near Ełk of 2003 and 2005. Reservoirs investigated twice were sometimes characterised by a significant diversity of the observed absolute values of the Secchi disc visibility in particular years. However, treating them as benchmark reservoirs makes it possible to determine relative relationships of optical properties among water environments of all the 21 lakes analysed then. They can be expressed in the form of an ordered sequence of reservoirs with increasingly better optical properties of water environments. The ordered sequence was verified on the basis of the research results of 2006. The observed stability of the position of the benchmark reservoirs with respect to each other makes it possible to abandon treating the observed isolated values of the Secchi disc visibility in other reservoirs as, out of necessity, representative ones of the investigated lakes. The knowledge of the values of scopes of the disc visibility in the waters of the benchmark lakes makes it possible to precisely estimate, against them, this value in less frequently investigated lakes.

The research in Lake Głębokie was conducted in the years 2008-2010. Sampling from three selected sites was designed in such a way that vertical profiles of the characteristics of interest were obtained. Measurements were performed to determine the content of dissolved oxygen, oxygen saturation expressed as a percentage, water temperature, total phosphorus, chlorophyll a, dry seston mass, and Secchi disk depth. During the course of a year, water temperature varied from 0.58°C to 24.72°C. However, a permanent thermal stratification was not observed in the summer season due to complete water mixing. Dissolved oxygen concentrations in the water varied within a range that started at hard-to-measure insignificant values and reached up to 9.63 mg O₂ l^-1, while oxygen saturation reached a maximum of 116% O₂. At the same time, it was observed that oxygen conditions gradually worsened as the depth became greater. The largest oxygen deficits occurred in the winter season of 2009-2010. The oxygen conditions were typical for an eutrophic lake. Its high trophic state was confirmed by an analysis performed using the method proposed by Carlson (1977) as well as by an analysis performed according to the directives of the OECD (1982). The trophic state of this lake is borderline eutrophic and hypertrophic. The oxygen conditions found in this lake ought to be seen as not beneficial to fish.

The aim of this study was to estimate ecotoxicity of the sediments from the shallow, eutrophic dam reservoir - Sulejowski. Research was carried out using Phytotoxkit microbiotests, which measure inhibition of seeds germination and roots growth retardation of the three species of plants (dicotyl Lepidium sativum, Sinapis alba, monocotyl Sorgum saccharatum). Tested plants gave different response to sediment samples, with decreasing sensitivity gradient: L. sativum, S. saccharatum, S. alba. The relationship of phytotoxicity of sediments with its water content was recorded (r = 0.63, p < 0.05). The results obtained showed spatial diversification along the reservoir, with the highest values in its opposite ends of the upper and lower parts (up to 87%), which are separated by a central area with the lowest toxicity (up to 45%). Possible reasons and implications of bottom sediments phytotoxicity are discussed.