BS/EN 17136-2019 pdf download.Water quality – Guidance on field and laboratory procedures for quantitative analysis and identification of macroinvertebrates from inland surface waters.
After decanting the remaining sand and/or gravel should be screened for rapidly sedimenting organisms like medium-sized molluscs and case-bearing caddisilies.
7.23 Sub-sampling
7.2.5.1 General
If one or more groups are represented by a large number of individuals, the sample can be divided in sub-samples in one or more steps (e.g. 1/4. 1/16, 1/64) to limit the time of analysis. Sub-sampling can be done with a 2- or 4-compartment Folsom sample splitter or by spreading across a sieve or tray. The justification is to have at least a minimum number of individuals of the dominant group(s) (Table 1) in the sub-samples selected for identification and counting. The minimum number of identified individuals is dependent on the species richness and evenness of macroinvertebrates in combination with the applied assessment method. When designing assessment methods that include or are sensitive for the presence/absence of rare species, it is important that a specific hit rate of detection should be defined for such species to prevent false negatives (not finding a rare species when It Is actually present). Typical threshold values are 100 organisms for species-rich taxon groups and 50 organisms for species-poor taxon groups 12).
When sub-sampling, one should always analyse at least three sub-samples selected at random from the sample to be able to judge the accuracy of the sub-sampling (see also LZI). Large variation In number of organisms and/or taxa (groups) between sub-samples might be a reason for counting more subsamples or even the whole sample.
By sub-sampling. taxa present in the sample with a low abundance might he missed. The probability of missing a rare taxon Is dependent on both the distribution of the numbers per taxon and the total sub-sample size. This probability could be estimated by a hypergeometric probability distribution. For instance in a sample with 1 000 organisms at least 258 organisms have to be identified to be reasonably (95 %) sure that you have not missed a taxon (group) with an abundance of> 1 %. However if the distribution has a long tail with many rare taxa still several can be missed.
NOTE I The probability to find rare species depends strongly on the sampling effort, the number of samples and the sample size. It is not uncommon to miss up to 40 % of the present species just by the sampling strategy Itself.
NOTE 2 For specific samples It can be more efficient to sort all the groups and split the abundant group(s) afterwards (using a splitter) to select in a random way the organisms to be identified.
7.2.5.2 Sample splitter
Clean samples with minor amount of debris can best be subdivided with a Folsom sample splitter (see AnnexA). To load the sample to the splitter, transpose the rinsed sample into a 1 500 ml beaker. Fill the beaker to 50 % with tap water to facilitate the splitting of the sample. Mix the content of the beaker in such a way that the material is evenly suspended. Pour the sample directly into the splitter which should be placed on a horizontal surface. Move the wheel of the splitter back and forth until the sample is evenly distributed over the compartments. When the sample is evenly distributed turn the wheel In such a way that the Content flows into the reservoirs under the wheel, Check if the Content of the separated reservoir should be divided in sub-samples again (e.g. 1/4, 1/16. 1/64). In this case repeat the procedure. Put the content of each reservoir separately on a 500 pin sieve (to remove the water) and transpose to a plastic jar and continue with sorting (2.32 or L3) or add ethanol with a final concentration of 70 % for storage. Write sub-sample size and a unique identification on the jar with indelible ink or pencil.
7.2.5.3 Sub-sampling by evenly spreading on a sieve
If this sub-sampling method has been chosen the pre-treatment should start by putting the empty sieve on a balance and tare. After draining (7.2.2) and rinsing (72.3). the sample should be evenly dispersed over the sieve. After excessive water has leaked from the sieve weigh the complete sample and carefully take at least three equally weighted “slices of pie” from the side to the centre of the sieve. A sub-sample should consist of a complete slice. Put each part in separate plastic jars and continue with sorting (LIZ or 7,j3) or add ethanol with a final concentration of 70 % for storage. Write sub-sample size and a unique identification on the jar with indelible ink or pencil.
Instead of weighing slices of pie they can also be taken on a volume c.q. surface area basis.
7.2.5.4 Sub-samplIng by evenly spreading on a pan (Aqem-Star method)
For this method a sorting tray consisting of two parts, a rectangular plastic or Plexiglas pan (36 cm 30 cm) with a rectangular sieve insert [31 is used (see Annex..A). The sample Is placed on the sieve, in the pan and dispersed evenly. When a random grid(s) is selected, the sieve is lifted to temporarily drain the water. A square “cookie-cutter like metal frame 6 cm x 6 cm is used to clearly define the selected grid; debris overhanging the grid may be cut with scissors. A 6 cm flat scoop is used to remove all debris and organisms from the grid. Put each part in separate plastic jars and continue with sorting (2.3.2 or Z33) or add ethanol with a final concentration of 70 % for storage. Write subsample size and a unique identification on the jar with Indelible ink or pencil.
7.3 Quantitative sorting
7.3.1 General
Quantitative sorting means that the macroinvertebrates are collected from the sample or sub-samples and identified in a quantitative and reproducible way. This means that accuracy can be specified and that there exist no unknown bias by selective sorting of taxa and individuals.
Sorting is done in water. Use a (soft) pair of tweezers to pick out the organisms. For small molluscs, water mites, etc. a pipette can be handy.BS/EN 17136-2019 pdf download.
BS/EN 17136-2019 pdf download
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