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An analysis of the species composition of zooplankton of Lake Gardno was based on the materials collected in the years 1998–2002, from July to August. It was found that the numbers of zooplankton species, their abundance and biomass were greatly variable, and depended on abiotic factors and lake trophy. An analysis of the structural characteristics of zooplankton as bioindicators of lake eutrophication revealed the presence of some species of rotifers and crustaceans in the zooplankton community, considered good indicators of lake trophy. The zooplankton-related trophic state indices enabled to classify Lake Gardno as eutrophic and meso-eutrophic.
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Estuaries - types, role and impact on human life

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Estuaries are transition zones between the land and the sea which form aquatic ecosystems, often referred to as marginal filters. Numerous classifications of estuaries are based on such criteria as: geomorphological structure, the character of water circulation and stratification, the ratio between freshwater and marine water and the type of energy flow. However, not all estuaries fall into one of the above categories. Some of them represent an intermediate type, while the type of others may change depending on the season and related changes in water inflow rate. Fisheries, including bi-valve mollusks, depend on the high productivity of estuarine areas, which can be also used as ports or sites of recreation and industrial activities. Estuaries play a fundamental role in maintaining an ecological balance in the biosphere.
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A zooplankton-based study of coastal lakes

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Numerous brackish estuarine water bodies are situated in the Polish Baltic Coastal Zone, including smaller and bigger lakes like Sarbsko, Łebsko, Gardno, Wicko, Kopań, Bukowo, Jamno, Resko Przymorskie, Liwia Łuża, Koprowo and Wicko Wielkie. Despite a similar genesis and geomorphological type, these lakes are characterized by different hydrological and hydrochemical conditions, determined by the predominance of terrestrial or marine factors. The origin of Polish coastal lakes and transboudary water bodies is closely related to the history of the Baltic Sea and the last glaciation. Coastal brackish waters have been inhabited by both marine and freshwater organisms.
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Estuaries of the Polish Baltic coastal zone

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Estuaries are dynamic ecosystems characterized by great and constant variability of physicochemical gradients and biological parameters. Considerable fluctuations in salinity levels, temperature, pH, concentrations of oxygen, nutrients and organic matter are quite natural in such water bodies. There are many estuarine forms at the Polish Baltic Coast, including open-sea gulfs (the Gulf of Gdansk and the Pomeranian Gulf), water bodies connected to the Baltic Sea (Lake Łebsko) and periodically closed water bodies connected to the Baltic Sea by channels or through river-mouths (Lake Jamno and Lake Gardno). The mouths of large rivers, like Odra and Vistula, also show features of estuaries. Estuaries perform a very important function of specific natural filters for a variety of chemical compounds contained in waters flowing through this area. In this way they form an effective buffer zone that prevents the penetration of toxic biological substances into seawater.
The analysis of zooplankton community structure in Lake Długie was based on the materials collected during the years 2000-2004 in a monthly cycle. Zooplankton was studied following lake restoration by various methods as well as in the year when no restoration was carried out. It was found that the number of zooplankton species, their abundance and biomass were greatly variable and depended on lake trophy and abiotic environmental factors. The restoration of Lake Długie resulted in gradual changes in the zooplankton community structure, including a wider species diversity and a decrease in the densities of indicator species of high trophy levels.
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Cercopagis pengoi as an invasive species

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The fishhook waterflea (Cercopagis pengoi) native to the Ponto-Caspian region, is an excellent example of an invasive species. Within one decade from its first appearance, Cercopagis pengoi has colonized nearly the entire Baltic Sea and most of the Great Lakes of North America. The species spread rapidly owing to its ecological and biological profile. The fishhook waterflea is an euryhaline organism marked by a predominance of parthenogenetic reproduction over sexual reproduction. This taxon quickly colonized inhabited ecosystems to create stable populations. The ecological and economic consequences of the species’ appearance are noticeable, but they have not been fully investigated and require further analysis.
The composition of food consumed by larval and juvenile smelt (Osmerus eparlanus) in the Vistula Lagoon was analyzed taking into account changing environmental conditions. The fish and zooplankton were collected at two stages of research, at the end of May and at the beginning of June 2000. The meteorological conditions and physicochemical parameters of water were determined within a limited research area, where a dense network of 15 sampling sites was established. The digestive tracts of 2552 smelts with a standard body length (SL) of 10 to 34 mm were subjected to preparation. A qualitative and quantitative analysis of diet composition was performed within 5-mm fish body length classes, taking into consideration time- and space-variation, as well as changing environmental conditions, primarily wind strength and the depth of sampling sites. Particular attention was paid to the occurrence of a non-specific component in the diet of the pelagic smelt – copepods of the order Harpacticoida, found in bottom deposits. These organisms were present both in the zooplankton and in the food of smelt, and the intensity of their occurrence varied in time and space. The analysis revealed that the presence of the Harpacticoida in deep waters and in the diet of smelt was directly proportional to wind strength and inversely proportional to the depth of sampling sites.
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