Species composition and density of Gastrotricha in bottom sediments were studied, on an annual basis, in the littoral of the mesotrophic Lake Piaseczno (Łęczyńsko-Włodawskie Lakeland). The number of species from spring to autumn remained on the same level (from 24 in spring to 22 in autumn), but in winter in littoral of the lake there were recorded only 8 species. Species diversity expressed by the Shannon index, in spring, summer and autumn was high and amounted 2.86; 2.69 and 2.77 respectively. In winter the value of the index was significantly lower and amounted 1.80. In individual seasons the mean density of Gastrotricha valued from 50 103 indiv. m-2 during winter to 1238 103 indiv. m-2 in spring. The density of Gastrotricha in spring was more than 24 times higher than that winter. There were no significant differences in species diversity Gastrotricha from spring to autumn, although the peak abundance was recorded in the spring. It is necessary to add, that the density of Gastrotricha during the spring peak was one order of magnitude higher from that in summer and autumn. The similarity of the fauna found in winter to the fauna occurring in other seasons, is significantly low, ranging from 24 to 27%. Seasonal changes in amount and diversity of the gastrotrich fauna are probably the result of temperature changes and food availability.
Seasonal changes in the Antarctic krill (Euphausia superba Dana) autoproteolytic activity were followed throughout the year. Using the kinetic formula for the first order reaction, the initial reaction rate (y0), the rate after 5 minutes (y5) and the average reaction rate (yx) after 0, 5, 10, 15 and 20 min of incubation of mixed homogenate at 40° + 0.2°C were determined in each sample. Changes in the krill autoproteolytic activity over the year were found to follow a sinusoid with a maximum during the austral summer (January) and a minimum during the austral winter (July-August). The maximum initial reaction rate was about ten times the minimum initial rate, which is an evidence of a considerable seasonal variation in the krill autoproteolytic activity associated presumably with the krill feeding intensity.
In the present paper we show results of our research on the contents of selected heavy metals in the surface and subsurface water layers of the estuarine lake Gardno. Obtained results show that there are substantial differences between concentrations of lead, copper and zinc in surface microlayer and subsurface waters. We observed that the surface microlayer is capable of accumulating much higher amounts of these metals than the subsurface waters. Analysis of seasonal changes in heavy metal concentrations in surface microlayers and subsurface waters shows some periodicity of these changes.