Sediment sample collecting and sample processing

Five localities alongside stream profile were chosen for sampling sediment and interstitial water samples based on previous investigations (Figure 2, Table 1). Hyporheic sediments were collected with a freeze-core using N2 as a coolant (Bretschko & Klemens 1986) throughout summer period 2009-2011. At each locality, three cores were taken for subsequent analyses. After sampling, surface 0-25 cm sediment layer and layer of 25-50 cm in depth were immediately separated and were stored at a low temperature whilst being transported to the laboratory. Just after thawing, wet sediment of each layer was sieved and only particles < 1 mm were considered for the following microbial measurements and for all microbial activity measurements since most of the biofilm is associated with this fraction (Leichtfried 1988).

image176

Figure 2. Graphic depiction of the thalweg of the Sitka stream with sampling localities. The main source of pollution is an effluent from Sternberk city sewage water plant, located just in the middle between stretch II and III.

Variable/ Locality

I.

II.

III.

IV.

V.

elevation above sea-level [m]

535

330

240

225

215

distance from the spring [km]

6,9

18,2

25,6

30,9

34,9

channel width [cm]

523

793

672

444

523

average flow velocity [m. s-1]

0,18

0,21

0,46

0,42

0,18

stretch longitude [km]

12,6

9,3

6,3

4,7

2,3

stretch surface area [km2]

0,043

0,06

0,043

0,024

0,012

stretch surface area (%)

24

32

24

13

7

dominant substrate composition

gravel

gravel

gravel

silt-

clay

gravel-

sand

grain median size [mm]

12,4

12,9

13,2

0,2

5,4

surface water PO43- [mg L-1]

0,15

0,24

7,0

2,6

1,8

surface water N — NO3- [mg L-1]

0,01

0,21

1,2

0,5

0,18

surface water N — NH4+ [mg L-1]

0,39

0,26

0,66

0,72

0,61

surface dissolved oxygen saturation [%]

101,7

110,0

105,8

108,5

103,5

surface water conductivity [pS. cm-1]

107,5

127,5

404,8

394,0

397,7

hyporheic water conductivity [pS. cm-1]

115,3

138,3

414,5

506,5

416,2

surface water temperature [°C]

8,1

9,7

10,7

11,1

8,9

surface water DOC [mg L-1]

2,47

0,81

2,62

2,69

3,74

hyporheic water DOC [mg L-1]

2,05

1,31

2,71

5,76

2,62

Table 1. Longitudinal physicochemical patterns of the Sitka stream (annual means). Hyporheic water means mix of interstitial water taken from the depth 10 up to 50 cm of the sediment depth

A few randomly selected subsamples (1 mL) were used for extraction of bacterial cells and, consequently, for estimations of bacterial numbers; other sub-samples were used for measurement of microbial activity and respiration, organic matter content determination, etc. Sediment organic matter content was determined by oven-drying at 105 oC to constant weight and subsequent combustion at 550 oC for 5 hours to obtain ash-free dry weight (AFDW). Organic matter values were then converted to carbon equivalents assuming 45 % carbon content of organic matter (Meyer et al. 1981). Sediment from another freeze-core was oven-dried at 105 °C and subjected to granulometric analysis. Grain size distribution and descriptive sediment parameters were computed using the database SeDi (Schonbauer & Lewandowski 1999).