Volume 14 (2014): Edizione 3 (September 2014)

Since 2005, great cormorants have been observed on the Lake Góreckie (Wielkopolski National Park) shoreline. The population of these birds occurring within the lake has gradually increased. In autumn 2008, more than 100 individuals were observed. In the period 2009-2012 the number of birds occupying the island periodically exceeded 250 individuals. So far, there is no breeding colony of great cormorants, but the birds have established a roosting colony on the island. In the period 2009-2012 we conducted research on the impact of the colony of great cormorants on the accumulation of nitrogen, phosphorus and other elements in soils beneath the colony and transfer of chemical elements from the colony to a nearby freshwater ecosystem. Our results show that a relatively small and recent colony of great cormorants can significantly affect the chemistry of soil. Compared to a control, the soil beneath the colony was characterized by statistically higher concentrations of nitrogen and phosphorus. A significant accumulation of nutrients was observed in the topsoil zone (to a depth of about 20 cm). Enrichment of soil in chemical elements has resulted in their further transport to a nearby lake. Compared to the control, the groundwater and surface runoff from the colony area revealed several-fold higher concentrations of nitrogen and phosphorus. The maximum abundance of cormorants in the roosting colony was reflected in the elevated concentrations of nitrogen and phosphorus in the littoral water near the colony. Our study demonstrates that the roosting colony of great cormorants can play a significant role in accelerating the eutrophication of surface waters.

This paper presents lake surface area changes that have taken place in the Gwda River basin. The studies were conducted on the basis of the cartographic materials released since the beginning of the twentieth century until the present times. The starting point was the area of all lakes greater than 1 ha which are present on the MPHP map from 2010. The assessment of the changes in the surface area of lakes in the Gwda River basin during approximately the last 100 years was possible thanks to the use of German topographic maps, so called Messtischblatt, at a scale of 1: 25 000 released between 1919 and 1944. The area of all the studied lakes has decreased by 465.09 ha (from 12783.62 ha at the beginning of the twentieth century to 12318.53 ha at the present time). Despite the general trend of lake atrophy, in particular cases one may observe an increase in the water surface area. This is the result of hydrotechnical works leading to river and lake damming, which in turn hampers the pace of atrophy.

The spatial variation of water transparency of the largest Polish lakes was presented on the basis of documentary material collected by the Voivodeship Inspectorates for Environment Protection as well as university geography departments in the period of 1991-2013. Seasonal results of the Secchi disc measurements (spring, summer) for 706 lakes were gathered. This is more than 68% of the total number of Polish lakes with a surface area larger than 50 ha. The mean water transparency of these lakes was 1.87 m, while the median was 1.58 m. Distinct inter-regional differences of the Secchi disc transparency were found, and it was also possible to distinguish three relatively homogeneous groups of macroregions where lakes have different optical characteristics. They can be divided into areas with lakes with high (3.05 m), medium (1.99-2.11 m) and low (1.22-1.52 m) water transparency. The Secchi disc transparency of these lakes was also analysed in relation to concentration values of the main forms of the optically significant constituents of water (coloured dissolved organic matter, suspended matter). The mean transparency of turbid, stained and clear lakes was, respectively, 1.16, 2.02, and 2.92 m. Moreover, regional empirical models to predict Secchi disc transparency from chlorophyll a concentration were also developed. It was assumed that inter-regional differences between models could be due to the probably different time of the ice sheet recession. The influence of the lake morphometry was most fully reflected in the impact of the depth conditions.

The research on aluminium content was conducted in water and on aquatic flora of Polish lakes in the central part of the coast. The study included the lakes Sarbsko, Choczewskie, Bia.e, K.odno, D.brze and Salino investigated in the summer of 2013. The examined lakes belong mainly to the direct basin of the Baltic Sea. Samples of aquatic plants and lake waters were collected. In the water samples pH and electrolytic conductivity were measured. The aluminium content was determined both in water and aquatic plants. Submerged hydromacrophyte studies included Myriophyllum alterniflorum L., Potamogeton perfoliatus L. and Ceratophyllum demersum L. Emergent hydromacrophyte studies included Phragmites australis (Cav.) Trin. ex Steud., Juncus bulbosus L., Iris pseudacorus L., Eleocharis palustris (L.) Roem. % Schult., Phalaris arundinacea L., Carex riparia Curt., Mentha aquatic L., Stratiotes aloides L., Alisma plantago-aquatica L., Glyceria maxima (Hartman) Holmb., Sagittaria sagittifolia L., Scirpus lacustris L. and Typha angustifolia L. The purpose of this investigation was the determination of the aluminium content in submerged and emergent hydromacrophytes and also the definition of their bioaccumulative abilities. The average concentration of aluminium in water was 2.68 fęg Al dm.3. The average content of aluminium in plants was 2.8015 mg Al kg.1. The bioaccumulation factor ranged from BCF=19.74 to BCF=16619. On the basis of the analysis of the aluminium content in water and aquatic plants results show that both water and plants were characterized by a moderate level of aluminium. The recorded concentrations indicate a mid-range value and are much lower than those which are quoted for a variety of surface waters in various parts of the world.

The aim of ongoing studies was to expand the non-destructive monitoring of Isoëtes lacustris (Vöge 2004) resulting in a description of the vitality and structure of the population. The influence of the environment on the development of the plant was the subject of extended studies carried out between 1995 and 2014 in more than 100 lakes throughout Europe. The environment was characterized through conductivity, pH and Secchi depth of the lake water and the accompanying submersed plant species, as well as the plant performance through the mean number of leaves of 20 sporogenous plants. The counting of the leaves was performed whilst diving. The vitality level was defined based on Vöge (2004) and related to the environmental conditions; these are referred to in the context. The inventory of the accompanying plant species may reflect a changing environment. The method is demonstrated through case examples. A highly significant correlation between the mean and the smallest number of leaves of sporogenous plants enables separating juvenile from adult plants. Finally, recent results show a relationship between the vitality level and the structure of the population.