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The main purpose of the paper was to identify day-and-night variations of the selected physico-chemical water characteristics (discharge, electrical conductivity, temperature, pH and dissolved oxygen) of a small urban river that had been heavily transformed by human activity (Sokołówka river catchment with the area of 7.71 km²). The data came from measurements conducted in the period between 4 October 2011 and 3 October 2012, using the YSI 6920 V2 (multi-parameter probe) and ISCO 2050 (automatic current meter), in 15- and 10-minute intervals, respectively. The research has proven the existence of rhythms of diurnal and seasonal variability to the selected water characteristics. The variability of the hourly values of the tested characteristics in particular days depends to a large extent on the value of the analysed feature itself. The variability coefficients of discharge and conductivity increase with the increase in the values thereof. For dissolved oxygen, pH, and water temperature, the variability coefficients decrease with the increase in the values thereof. In the case of water pH, its conductivity, and temperature, the correlations between the given feature and its variability observed at different times of day are not linear, but they run along loops of varying curvature. The average day-and-night variability of the Sokołówka river’s SEC and pH was significantly changed as a result of human impact. The maximum readings of the conductivity and the pH shifted to the morning hours, with no change to the minimum readings.
Zooplankton was investigated at fixed site in 24 hours in Kongsfjorden, a glacial fjord situated on the west coast of Spitsbergen (Svalbard) (79°N, 12°E), in order to unveil the level of diurnal variability in community composition and abundance. Parallel to zooplankton study water temperature and salinity were measured while information on local tides and winds was obtained from external sources. Observed changes did not exceed the range of variability regarded intrinsic, resulting from the nature of plankton. Because of this low variability we are of the opinion that the data presented can be regarded a valid measure of the natural heterogeneity of zooplankton communities in hydrologically dynamic Arctic coastal waters in summer. The observed changes in zooplankton were primarily induced by the complex dynamics of the fjord's water masses. In spite of importance of tidal forcing, the variability in zooplankton did not demonstrate similar temporal fluctuations due to modification of the water movement by other irregular forces (local wind). Also, we have not found any indication of diel vertical migration in coastal water in the Arctic under the condition of midnight sun.
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