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Table 1 Statistical key parameters of mean annual values of all stressor variables. Stressor’s relevance is expressed as percent samples above the risk threshold (RT)

From: The hierarchy of multiple stressors’ effects on benthic invertebrates: a case study from the rivers Erft and Niers, Germany

Stressor

Stressor code

Description

Mean

Median

Min

Max

RT

%Samples at risk

Physico-chemistry

T

Maximum water temperature [°C]

20.48

20.70

12.50

24.90

–

–

O2

Minimum oxygen concentration [mg/L]

8.03

8.10

5.70

10.20

7

22%a

TP

Mean total phosphate concentration [mg/L]

0.13

0.14

0.02

0.24

0.1

67%

TN

Mean total nitrogen concentration [mg/L]

4.38

4.65

0.50

7.91

–

–

Cl

Mean chloride concentration [mg/L]

72.67

76.60

17.38

237.63

200

2%

SO4

Mean sulphate concentration [mg/L]

83.83

91.39

24.39

252

200

2%

Fe

Mean total iron concentration [mg/L]

0.91

0.92

0.15

2.53

0.7

63%

NO2-N

Mean total nitrite nitrogen concentration [mg/L]

0.04

0.04

0.01

0.13

0.05

43%

NH4-N

Mean total ammonium nitrogen concentration [mg/L]

0.12

0.11

0.03

0.37

0.1

55%

Mixture toxicity

RQmix,acute

Risk quotient of acute mixture toxicity

0.23

0.18

0.06

1.29

1

2%

RQmix,chr

Risk quotient of chronic mixture toxicity

0.77

0.73

0.20

1.98

1

18%

Hydrological alteration

fh5

High flow frequencyb

19.05

20.50

4.00

41.00

–

–

dl16

Low flow pulse durationc

8.89

5.18

3.15

36.72

–

–

ra5

Number of day risesd

0.38

0.38

0.27

0.49

–

–

MQMNQ

Quotient of the long-term mean and mean low-flow discharge

3.20

2.47

1.54

13.67

–

–

Morphological degradation

HP1

Channel development

6.24

6.00

4.00

7.00

3

100%

HP2

Longitudinal profile

5.38

5.00

3.00

6.50

3

96%

HP4

Cross profile

5.32

5.00

3.00

7.00

3

94%

HP5

Bank structure

5.68

6.00

3.00

7.00

3

96%

  1. RT of physico-chemical variables were chosen in accordance with the German surface water directive (Oberflächengewässerverordnung, 2016). If different stream type-specific thresholds were available, the strictest value was used. RTs were set to class 3 for morphological quality (1 = natural, 7 = fully degraded). For mixture toxicity of micropollutants a quotient of 1 indicated measured concentrations exceeding the respective ecotoxicological effect concentration. Statistical parameters of each catchment are shown in Additional file 1: Supplements S3
  2. aValues are strongly influenced by the timing of sampling during the day and only represent a rough estimation
  3. bAverage number of events above the median flow of the flow record
  4. cMedian of the yearly average durations of flow events below the 25th percentile
  5. dPercentage of days in the flow record in which the flow is greater than at the previous day