Small stream Characterisation su

Small stream Characterisation survey Calculation

This synopsis is provided to help the reader understand the calculations and how the SSCS final Characterisation determination is arrived at.

The SSCS endeavours to determine the pollution level of the sample area by measuring specific categories of invertebrates, giving them weighted scores and thereby arriving at a score that determines the characterisation of the survey.

There are five categories of invertebrates to consider in the survey. Some of these categories are pollution sensitive whilst some are insensitive to pollution, these different categories have different weightings in the calculation.

The categories are;

1) Mayfly

2) Stonefly

3) Caddis

4) Gold (snails, worms, small fly, Daddy Long legs, Leeches, Flatworms)

5) Asellus

Categories 1, 2 are pollution sensitive and are treated similar for weighted scores.

Category 3 is less pollution sensitive and has a lower weighting than cat. 1 &2.

Category 4 has weighted scores that vary high and low dependent upon the types and quantities, each river system requires some of these invertebrates, too few and too many are not good and the weighted scores reflect this.

Category 5 relates to invertebrates that live on decaying matter, absent from the sample has a significant impact, whereas a few or more of these can have a significant impact on the overall score.

When the survey is taken the assessor identifies the specific larvae type in each category and counts the actual number in the sample. For each type the number are;

A	1-5
B	6-20
C	20-50
D	50-100
E	100+

For each assigned letter a value is allocated in the relevant abundance category eg, A=1, B=2, C=3 etc.

When all the types in the category have been counted with a letter entered and a relative abundance assigned then, the total relative abundance is added and the number of different types are added.

The sub score for the categories are calculated from the following tables;

Category 1 Mayflies

Number of types	1		2+
Relative Abundance	1-2	3+	2	3+
Sub Score	4	6	4	8

Category 2 Stoneflies

Number of types	1		2+
Relative Abundance	1 - 2	3+	2	3+
Sub Score	4	6	6	8

Category 3 Caddis (Cased & Caseless)

Number of types	1 - 2		3+
Relative Abundance	1 - 2	3+	3+
Sub Score	2	4	6

Category 4 Gold (Snail, Worm, Fly Larvae, Flatworms, Leeches)

Number of types	1 - 2		3+
Relative Abundance	1 - 6	7+	1 - 6	7+
Sub Score	2	0	4	0

Category 5 (Water Louse)

Number of Individuals	Absent	Few (1 - 20)	Common (21+)
Sub Score	6	2	0

When all the categories are calculated, the sub scores are added together to obtain the final characterisation score.

17 or less is an "At Risk" determination

18 is considered "Indeterminate" determination - further sampling suggested

19 or greater is "Not at Risk" determination

The maximum score attainable is 32 and it can be seen that the max Mayfly and Stonefly sub scores together provide 50% of the max with the absence of Water Louse providing 20% of the max.

These three categories are a strong indicator of the polluted state of the watercourse.