What is sampling and its types

Sampling and processing

Sampling of the PoD takes place as so-called "Multiple Habitat Sampling" (MHS) and aims at the most complete possible recording of the algae coverings occurring in the running water and the estimation of their abundances. The examination follows the work steps

Preliminary work

  • Determination of the procedural variant
  • Determination of the sampling time
  • Selection of the trial site
  • Determination of the sampling section
  • Determination of the biocenotically relevant PoD type

Sampling in the field

  • Mapping and sampling
  • Labeling of the samples
  • Preparation of the field protocol and estimation of the abundances
  • Transport and fixation of the samples

and Preparation in the laboratory, i.a. Determination. The basis for the implementation is the "Procedural Instructions for the Ecological Assessment of Rivers for Implementation of the EC Water Framework Directive: Macrophytes and Phytobenthos (PHYLIB)" from 2012.

Preliminary work

Determination of the procedural variant

There are two variants of the procedure for examining the PoD. The so-called "full procedures“All indicator taxa detected at the location, including the rare forms, are included in the assessment.

The so-called. "reduced procedures“On the other hand, it is limited to the taxa that are macroscopically visible at the site or appear in large numbers in the microscopic analysis.

The probability of achieving a reliable assessment is significantly lower when using the reduced procedure. The sampling in both methods does not differ.

Determination of the sampling time

Basically, the recommendation applies to take the sample at a time when the water level is as low as possible and after a stable discharge phase. Although some species of PoD are generally available all year round, a one-off sampling in summer (mid-June to early September), which can be carried out together with macrophytes and diatoms, is planned.

Selection of the trial site

The trial site should be a representative and as undisturbed section of the stretch of flowing water as possible.

Determination of the sampling section

Sections of at least 20 m in length are sampled for streams and 50 m in length for rivers. In practice, many samplers orientate themselves to the section of 100 m provided for the macrophytes.

Determination of the biocenotically relevant PoD type

The test site is assigned to a PoD type based on the ecoregion, the geochemical characteristics and the bed substrate. The PoD typology therefore often corresponds to the LAWA type. However, there are also deviations.

The LAWA types of the siliceous river types 5, 5.1 and 9 of the low mountain range correspond to a PoD type PB 3. The LAWA types 14 and 16 are differentiated on the basis of their geochemical characteristics. An auxiliary criterion for the assignment is a total hardness or acid capacity of <1.6 mmol / l for silicate imprinting or> 1.6 mmol / l for carbonate imprinting. Analogous to this, water bodies of LAWA types 11 and 12 in the north German lowlands are divided into types with low-base and high-base characteristics. Due to the different bed substrate, LAWA type 9.1 differentiates between the loess, Keuper and chalk regions from the Muschelkalk, Jura, Malm, Lias, Dogger and other limestone regions.

Sampling in the field


  • large bucket for transportation
  • Waders or rubber boots
  • Viewing box / viewing tube / Aquascope
  • Hand magnifier
  • Spoon, tweezers, spatula, scalpel or knife (rustproof)
  • Glass or plastic jars (15-20 ml)
  • Freezer bags of various sizes
  • Lugol's solution or neutralized formaldehyde
  • prefabricated waterproof labels or fabric tape and waterproof felt-tip pens for labeling the samples
  • Log book or field log sheet and pen
  • Cool boxes with ice packs or fans
  • topographic maps on a scale of 1:25,000 or 1:50,000 or a GPS device
  • Photo camera
  • if necessary life jacket and safety rope
  • if necessary, rake or pliers with a long handle
  • if necessary pipettes, Petri dishes (plastic)
  • if necessary, white plastic tray (2 to 3 l) for sorting the material
  • If necessary, quick tests to determine the water hardness or acid capacity if insufficient information is available about the characteristics of the PoD type

Carrying out the sampling

For the recognition and differentiation of the algae in the water, the knowledge of the different growth and storage forms, their different colors and their consistency is crucial. In addition, information on the habitat (substrate, spatial occurrence) is often required for a later determination. A detailed description of the deposits and sampling can be found in the field guide "Benthic algae without diatoms" (Gutowski & Foerster, 2009).

Sampling takes place in several steps. First, the structural diversity of the sample site is observed in order to identify the different habitats and substrates of the benthic algae at the sample site. In waters that can be waded, the stretch of water is then zigzagged as far as possible against the current and searched for macroscopically conspicuous algae coverings and growth forms. A viewing box for viewing the bottom of the water is usually helpful for this. During the inspection, samples are taken of every conspicuous growth form. Samples from deeper areas can be picked up with a rake or pliers. Thin coverings or crusts as well as thick, soft coatings and small tufts of short threads on hard substrate as well as epiphytic algae on substrate can be removed directly with the substrate. If this does not succeed, parts of it are scraped off. Other growth forms such as long threads, reticulated braids, broad threads or flat thalli as well as gelatinous forms can be taken directly. All samples (sub-samples) are kept separately. The material is placed in freezer bags without added water or in vessels with water added. If macrophytes or mosses are found at a sample site, a squeeze sample is also created by putting material in a freezer bag together with water and squeezing it. The suspension is transferred to a vessel. In this way epiphytic and metaphytic species can be obtained.

The number of sub-samples fluctuates depending on the variety of macroscopically recognizable algae growth. It is recommended that 4 to 8 sub-samples be taken. With a smaller number of sub-samples, the evaluations according to PHYLIB often remain uncertain. If there are coverings that look similar in a larger abundance at the test site, it is often useful to take parallel samples at different points, since such coverings are often composed of different types with different abundances.

It is advisable to photographically document the investigation section upstream and downstream as well as conspicuous occurrences of benthic algae.

In the case of larger bodies of water that cannot be waded, a longer investigation section of the shallower bank areas is sampled.

Labeling of the samples

Each sub-sample must be carefully labeled with at least the sample location number, the sub-finding number and the date of sampling. It is important to ensure that the lettering adheres to the jar or the freezer bag and remains legible despite later fixing or storage. If possible, the client and the name of the body of water and the measuring point should also be noted.

Preparation of the field protocol and estimation of the abundances

The sub-samples (sub-findings) are listed individually in a field protocol (Fig. 1). For this purpose, numbers are assigned to the samples. In the description of the samples, information about the growth or storage form, color, consistency and possibly location in the water is noted. In addition, the substrate should be specified. The indication of the degree of coverage of the vegetation is indispensable for a later assessment.

Fig. 1: Field protocol.

For the evaluations, three macroscopically recognizable abundance classes (3 to 5) of the field protocol are considered later (Tab. 1). However, it does not make sense to limit the entries to these three classes in the field protocol. More suitable are details in percentage steps ( 1%, 5%, then in 5% steps up to 40% or 10% steps over 40% coverage). By specifying the percentages, the total abundance of each taxon can later be better determined for similar coverings. It should be noted that in the procedure for the coverage levels of 5% and 33%, the abundance classes are changed.

Tab. 1: Definition of the macroscopically recognizable abundance classes.

Abundance class



massive, covering more than 1/3 of the river bed (> 33%)


often, but covering less than 1/3 of the river bed (<33%)


macroscopically rare, barely recognizable (single findings or <5%) or microscopically en masse

Transport and fixation of the samples

Fresh samples are brought to the laboratory in cool boxes and processed there as quickly as possible. If rapid processing is not possible, the samples must be fixed. This can be done in a number of ways. Soft substrates should be fixed with Formol (37%) or Lugol’s solution as soon as possible after sampling, at the latest on the evening of sampling. Over-fixation should be avoided (in the case of Formol because of its toxicity and in the case of Lugol because of the too intense, too dark coloration, which makes microscopy difficult). Freezing (cryofixing) is recommended for stones.