CNR-IRSA activities related to INHABIT

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Artificial substrates

Sampling technique through artificial substrates is generally used when direct sampling procedures (e.g. through handnet or Surber net) are not suitable or unreliable, or in those water bodies where a ‘standard’ sampling would be difficult or dangerous due to problematic site access or to the limited accessibility of certain microhabitats. These circumstances are often found in potamal reaches showing steep and/or eroding banks, extremely wide channels and deep waters. Among the advantages offered by the use of artificial substrates in such water bodies, the following can be mentioned:
The possibility of obtaining quantitative data entirely comparable among different sites, although limited to a single microhabitat. The sampling technique is based on a given, and constant, sampling surface, likewise the Surber net used in wadable rivers. The sampling subjectivity is strongly reduced and a sample of benthic fauna is  obtained from the same habitat in each sampling station, even when visibility is poor and substrates difficult to reach;

  • the possibility of obtaining a sample representative of all the period of substrate permanence (about 30 days), giving a whole framework of the state of the water quality influencing the community composition, with limited influence from physical habitat conditions (excluding current velocity and particulate transportation);
  • the possibility of limiting the amount of silt and organic matter in the collected samples, allowing an easy sorting of the sample;
  • the easiness of assembling the substrates with a low cost.

The artificial substrates sampling technique shows some obvious drawbacks, such as the following:

  • It has been clearly demonstrated how artificial substrates favour the colonization of some taxa (e.g. Amphipoda, Chironomidae, Baetis genus of Ephemeroptera) comparing to others, showing preferences for natural, or other, substrates (e.g. Tubificidae, Lumbricidae, Smuliidae, Gasteropoda, Anisoptera Odonata); other groups seem instead less dependent from substrate variations. Considering this, it is apparent how a sample collected through artificial substrates cannot be indicative of the real abundance ratios of the different taxa, due to the selectiveness of the technique. This does not hamper the comparison among sites and the correct definition of water quality on the biological basis; nevertheless, dedicated calibration and tuning of the biological methods is requested both for the investigated stream types and the sampling techniques;
  • artificial substrates can get lost following flooding or acts of vandalism, or can be covered by flowing vegetation or by silt preventing the colonization of the substrate by rheophilic taxa; also, substrate can emerge from water surface during drought periods;
  • a relatively long immersion period (about 30 days) is needed in order to complete the colonization by the macrobenthic groups. This period follows also a phase of ‘stabilization’ and ‘maturation’ of the SAs for the action of algal and bacterial community.

The sampling technique consists of: the use of artificial substrates (SA) in Masonite plates; the positioning in the river water of groups of SA attached to floating structures; the recovery of the SAs after about 1 month and the invertebrates sorting; the treatment of the collected taxa.
Suggested substrates are of a modified Hester-Dendy type. Each single artificial substrate is formed by 10 squared, 100cm2 each side, Masonite plates, 2-3 mm thick.

plates


The plates are fixed together through a grub screw (or two), 10-12 cm long and 4-6 mm of diameter, carrying a ring on one side and a butterfly nut on the other.

Assemblage of the plates


The assemblage of the plates, even if a wide space is left, allows the colonization of one side only. The surface to be considered is thus related to only one of the two sides of the plate (i.e. 100 cm2 X 10). Also, it is important to consider that the top and bottom faces of the outer plates are generally scarcely colonized. The total surface, useful to the invertebrates colonization, associated to a single 10plates SA is considered equals to 0.1 m2. The sampling unit is formed by a group of 5 SA, in order to consider a surface similar to the one used in most of the Italian wadable rivers (0.5 m2).

Plates