Acta Facultatis Ecologiae - Technická univerzita vo Zvolene

115Typha latifolia, Carex sp., Scirpus sp., etc. dominate,among two fill slopes (www.zitava.sk).Taking of water samples was realized from 6sampling sites in the NR. Water samples were takenregularly during the whole year, 15 th day ofeach month. The sampling sites were proposed toobtain the best possible data for the evaluation ofthe changes in concentrations of dissolved oxygenand organic substances in water in dependence onspace and time. We have established the followingsix sampling sites:1. Inflow of the river Žitava into the Aluvium.Phragmites australis and Salix sp. grow alongthe river Žitava and they are also typical for thewhole area of the NR. The average depth is 0.32m.2. and 3. The sampling sites are typical wetlandecosystems. There is a very dense stand ofPhragmites australis and Salix sp. in this part ofthe NR. Water level is covered by Lemna minor.Water in these sites flows very slowly and itshigh changes in dependence on weather duringthe year. The average depth is about 0,30 m.4. It is situated near the bridge on which there isa road to the village Martovce. It is also the narrowestpart of Aluvium; therefore water flowreaches the highest speed in the river Žitava.There are typical stands of Phragmites australis,Salix sp., Alnus sp. on the banks of the river. Theaverage depth is 0.40 m.5. and 6. They are characterized as typical wetlandecosystems. The river Žitava flows out of itswatershed here while rapid snow melting duringspring months and intensive precipitationamount in summer months. In comparison withthe second and the third sampling site the riverfloods the whole area between two slopes. Thewater level decreases about few meters duringdry weather in summer months. This part ofAluvium is mostly represented by an open waterarea. Typha latifolia, Phragmites australis, Alnussp. and Salix sp. grows along the river. Thewater level in the sixth sampling site is coveredwith Lemna minor which forms a typical greencover. This sampling site is situated about 120 mfrom the place where the river Žitava joins theriver Nitra. The average depth in the 5 th samplingsite is 0.26 m and 0.39 m in the 6 th samplingsite.In taken water samples the concentrationsof dissolved oxygen were determined by inoLabMulti Level 3 with galvanic oxygen sonde StirrOxG. The concentrations of organic substances weredetermined by an indirect method based on theiroxidation with dichromate potassium by means ofsolutions of Merck spectrophotometrically. Themethod is analogical with ISO 6060, EPA 410A.The results of dissolved oxygen and organic substancesconcentrations are given in mg O 2.l –1 .The water in the sampling sites was ranked inthe categories of surface water quality. It was performedby comparison of the calculated characteristicvalues of indicator and the correspondingsystem of limit values which are stated by the standardSTN 75 7221 (Classification of the surfacewater quality). According to the standard STN, thecharacteristic value of indicator is the value withthe probability of non-overfullfilment of 90 % inCOD Crand with the probability of overfullfilment90 % in dissolved oxygen.RESULTS AND DISCUSSIONFrom the results of average concentrationsof dissolved oxygen in water of the NR AluviumŽitavy in dependence on sampling time shows thatthe highest values were during winter and earlyspring months. The maximum value was meausredin March (9.0 mg O 2. l –1 ). Since April there wasa tendency of successive decrease of average concentrationsof dissolved oxygen. The minimumvalue was measured in July (1.6 mg O 2.l –1 ) (Fig. 1).This fall is probably related to successive increaseof water temperature. Thus good conditions werecreated for decomposition of organic matter by microorganismsin water in which they use dissolvedoxygen. We supposed that very low average concentrationof oxygen in July is connected with thehighest temperature of water (22 °C) measured inthis month in the NR (Fig. 2). It is known from literature(Losos et al., 1984, Pitter, 1999) that the dissolubilityof oxygen in water subsides with risingwater temperature. Beňačková, Noskovič (2005)found out similar dependence between dissolvedoxygen concentration and water temperature in theNR Žitavský luh.