Overgrowing of two polymictic lakes in Central-Western Poland
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.
Variability of nutrient concentrations in sediments and wetland plants during the vegetation season under different sediment moisture conditions
Changes of nutrient concentrations in sediments and plant biomass as well as above-ground biomass production of four emergent macrophytes during the vegetation season were studied. Particularly, variations over time in N:P and N:K ratios in sediments between sites with and without vegetation as well as seasonally flooded and permanently flooded sites were investigated. In plant covered sites, biomass production and nutrient concentrations in the four wetland plant species were measured monthly from March to September 2008. The study was conducted in the littoral zone of the polymictic Lake Niepruszewskie. The performed analyses of N, P and K concentrations in plants and sediments indicated that measurements of nutrients in plant biomass better reflected nutrient availability for plant growth than analyses of nutrient contents in sediments. The vegetation biomass and nutrient concentrations in plant biomass in the shallow littoral zone were more sensitive to changes of water depth in comparison with the permanently flooded part of the littoral zone. The present study showed that the N:P biomass ratio was a better indicator of moisture changes than the N:K ratio or individual nutrients. Preferences of the studied macrophytes for nutrient concentrations in sediments depended more on the N:P ratio in the sediment than N or P or K concentrations individually. Glyceria maxima preferred sites with high N and K concentrations (N:K=0.9) while Typha angustifolia occurred in sites with the lowest nutrient concentrations among the studied species (N:P and N:K = 0.1).
Changes in water resources in selected lakes in the middle and lower catchment of the River Warta
Lakes play a very important function in water retention, particularly in areas with strong water deficiency. Water resources in lakes determine the possibility of using them for different purposes, e.g. economic development (irrigation or drinking water, water supply for industry). With ongoing processes causing a reduction in lake capacity (shrinking and shallowing), it is necessary to investigate the scale and rate of decreasing lake resources, in terms of the economic and natural consequences of this process. The aim of the study was to test changes in water resources in lakes situated in the middle and lower River Warta catchment. Special attention was paid to lakes characterised by reduced water resources in order to identify factors contributing to this phenomenon. Our current studies of 127 lakes showed a 10% decrease in water resources over 50 years (1910-1960) in the analysed part of the catchment. Our results indicated that the predominant process causing the disappearance of lakes is shallowing. This factor should be monitored carefully to avoid further lake loss.
Agnieszka Ławniczak, Janina Zbierska, Sylwia Machula and Adam Choiński
Fluvial lakes affect on phosphorus and potassium concentrations in the Samica Stęszewska River
The aim of the study was to evaluate the effect of shallow lakes on reactive, total phosphorus and potassium concentrations in the river water and analyse the effect of changes in water retention in lake on nutrient concentrations in river waters. The study was carried out in the Samica Steszewska River. This is lowland river, which flows through two polymictic lakes. The study site is located in the Wielkopolska Lowland, Central-west Poland. Water samples were collected at depths of 0.5 m below the water surface. Reactive, total phosphorus and potassium were analysed monthly by standard methods. The study was carried out from January to December 1999-2002 (period with high water retention), 2005-2008 (period with low water retention), at four control points. Sites were located at the inflow and outflow of the Samica Stęszewska River into and out of Niepruszewskie and Tomickie Lakes. From 1974 to 2002, Lake Niepruszewskie was regulated at its outlet by a weir. In 2002, the water level was reduced. Changes of water retention in Niepruszewskie Lake influenced water discharge of the Samica Stęszewska River. The results indicate that changes in water retention have significantly influenced water quality in the river, particularly total and reactive phosphorus concentrations; however, this influence was not observed in respect to potassium concentrations. Nutrient concentrations in river strongly depend on water quality of the lake ecosystem and their buffering capacity. Additionally, by improving water quality, increased oxygen concentrations, and decreasing dissolved nutrient concentrations, as well as increased amplitude of water level fluctuations in lakes may increase P-fixation rates in outlet streams.
One of the most important elements in the protection of water quality is buffer zones. In order to protect water quality, appropriate management of these areas is necessary. In the paper, changes in the land use in the buffer zone of 200 m width around lakes in the Mała Wełna catchment over 20 years were analysed. For the study eight lakes larger than 50 ha were chosen. Changes in the land use within the buffer zone were studied from 1980 to 2000, based on topographic maps in the scale 1:10 000. Results show both positive and negative aspects of land changes. An increase in forested areas and grasslands through tilled land were positive aspects of these changes. On the other hand, the enhancement of suburban development in these zones caused increased probability of water pollution from these areas. Calculations of potential nutrient loading from different types of land use in these zones indicated an enhancement of nutrient sources in the lakes within the 20-year study period. However, these changes are not significant in comparison to the nutrient loads contributed by the tributaries. Our study indicated that in the case of flow lakes, more important is an improvement in water quality in the tributaries and a reduction in nutrient sources in the catchment with the purpose of reducing the input of nutrients into the lake. Management of the buffer zone is one of the issues which have to be taken into consideration in lake protection after elimination of the major water pollutant sources.