A9. Unit-area invertebrate survey

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Figure 1: Unit area invertebrate survey with Surber net

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1 Objectives

To obtain replicated samples and allow robust statistical assessments, a quantitative approach to sampling is recommended. Sampling a standardized area allows density data to be collected. Due to the need to identify and count all invertebrates obtained within a sample this approach is time consuming. It is only applied by professionals or academics in specific studies. When appraising a river restoration scheme, it is important to follow the – BACI principles (Before, After, Control, Impact) (Underwood, 1994).

2 Method summary

Figure 2: Sorting and species identification in laboratory © Judy England - Environment Agency

A variety of quantitative sampling devices are available (See the ISO 10870:2012 standard), all of which enable samples to be taken from a given area and allow results to be calculated for a spatial area. A Surber sampler consists of a quadrat on a hinged frame that can be pushed into the riverbed substrate and a net with side wings to help reduce the loss of invertebrates. The quadrat frame used within this survey encloses a specific area (0.1 - 0.04 m2). Its use is demonstrated in Figure 1. The sediment within the quadrat is disturbed to dislodge the invertebrates present, larger stones are washed individually to remove any invertebrates attached to the surface. Cylinder samplers operate in a similar way by enclosing a given area which is disturbed so that invertebrates are washed into a net attached to its side.

Samples may be collected randomly from a site or following a stratified approach - e.g. replicated sampling randomly within specific habitats. The number of samples needed to characterize an invertebrate community and be statistically representative will depend on the river.

Once the location of each sample has been determined such as by the generation of randomly selected co-ordinates, samples should be collected from the downstream limit of the site first then moving progressively upstream to avoid disrupting the sampling area.

Invertebrates are stocked in alcohol. Sorting and species identification in laboratory Figure 2.

3 Advantages

  • Surber samplers are best suited for sampling shallow gravel substrate in flowing water conditions
  • The results can be analyzed statistically to provide richness, densities, abundance, and equitability

4 Disadvantages

  • Need a lot of replicates
  • More intensive and time consuming than the kick sampling method
  • Various formulae for computing the number of samples required see (Sokal and Rohlf, 1981; Resh and McElravy,1993; Casey and Kendall, 1997; Crawley, 2013)

5 Recommendations for methods application

  • Avoid low flow and flood periods, and sample at the same time period over years (usually spring (March-May) and autumn (September-November)
  • Not applicable on every type of stream, in channel or marshy streams
  • When applied to a river restoration scheme it is important to follow the BACI (Before, After, Control, Impact) (Underwood, 1994)
  • Check and clean your material after each sampling site to avoid bias data with animals from previous samples

6 Cost

Material cost < £200. Labour intensive method for data collection and particularly for species identification and counting.

7 Protocol and data analysis

UK invertebrate sampling and analysis procedure for the STAR project

8 References

  • Casey, R. J. and Kendall, S. A. (1997) ‘Sample number and colonization patterns of benthic macroinvertebrates and organic material on artificial and natural substrata’, Journal of Freshwater Ecology, 12(4), pp. 577–584. doi: 10.1080/02705060.1997.9663572.
  • Crawley, M. J. (2013) The R book, Wiley. Edited by John Wiley & Sons Ltd. Chichester, West Sussex. doi: 10.1017/CBO9781107415324.004.
  • ISO 10870:2012, Water quality — Guidelines for the selection of sampling methods and devices for benthic macroinvertebrates in fresh waters
  • Resh, V. H. and McElravy E.P. (1993), ‘Contemporary quantitative approaches to biomonitoring using benthic macro-invertebrates.’ Freshwater biomonitoring and benthic macro-invertebrates. D. M. a. R. Rosenberg, V.H. New York, Chapman and Hall: 195-233.
  • Sokal, R. R. and Rohlf F. J. (1969), ‘Biometry, The principles and practice of statistics in biological research.’ New York, W.H. Freeman and Company.
  • Underwood, A.J. (1994) ‘On beyond Baci: Sampling designs that might reliably detect environmental disturbances.’ Ecological Applications 4:3-15.