Multihabitat sampling: BMWP index

1. Select a river stretch that has not been flooded recently.

Samples should be taken in the central areas or at the edges at depths greater than 0.2 m.

2. Divide the river stretch into different areas according to the different substrates present:

With high current speed and hard substrate (1)
Lentic with hard substrates (2)
Among the submerged portion of the emergent vegetation of the river edges (3)
Among submerged macrophytes or macroalgae (4)
With sand, gravel or mud (5)

It is very important to select a river stretch that has the maximum number of different substrates in order to obtain all the macroinvertebrate biodiversity.

3. Sample once at each habitat following the appropriate methodology.

In habitats (1) and (2)

Remove the large stones from an area of 2 m².
If the stones are less than 10 cm in diameter in diameter, kick an equivalent area and collect all the material disturbed, holding the net against the flow of the river.

In habitats (3) and (4)

Use the net to collect organisms present among the vegetation, submerged roots and macrophytes.

In habitat (5)

Kick the bed and collect the suspended material with the net.

In all cases sampling should continue until no new families appear in the successive samples for each habitat.

As the sampling is qualitative, a representative sample should be obtained. An exhaustive search for macroinvertebrates is not necessary.

Remember that the net should have a mesh of 250 µm and the opening should have a diameter of at least 30 cm.

The net should be carefully cleaned between two consecutive sampling stations in order to prevent the presence of animals from the previous sampling.

4. Sorting, identification and counting

This stage is performed mostly in the field, with the following steps:

Place the sample in a white tray with some water
Identify the animals to family level, directly with the naked eye or using a magnifying glass.
Identification should proceed until no new families are detected.
If the macroinvertebrates cannot be properly identified in the field, the animals should be kept in vials and taken to the laboratory for correct identification under a stereoscope.
The following abundance code should be used:

1 2 or fewer individuals

2 3 - 10 individuals

3 11 - 100 individuals

4 more than 100

In the event of the observer having little experience in taxonomy, the whole sample (all the habitats together or separately) should be preserved and sorted later at the laboratory following the steps used in the FBILL methodology.

Caution should be taken if the tray is full of litter. In this case the sample should be divided into several parts and each of them carefully examined for macroinvertebrates.

Developed by the Department of Ecology, University of Barcelona, with the collaboration of the Department of Environment of the Barcelona Council