Benthic macroinvertebrate sampling in Polish rivers follows methods aligned with CEN standard EN 27828 and the national guidance developed under WFD implementation. The choice of sampling device and effort depends on substrate type, wading depth and the purpose of the assessment.
Field Conditions and Site Selection
Before selecting a sampling method, field conditions at a Polish river reach need to be characterised. Key variables include mean wetted width, dominant substrate (gravel, sand, clay, silt), depth of thalweg and presence of macrophyte beds. In Polish lowland rivers — such as those draining the Mazovian Plain or the Noteć valley — soft substrates and slow current dominate, which favours different techniques compared to gravel-bed Carpathian tributaries.
Site selection for WFD biological monitoring should target representative stretches with a minimum length of 100 m, avoiding immediate downstream influence of point-source discharges, bridges or major tributaries for at least 500 m.
Kick-Net Sampling
The kick-net (or pond net) is the most widely applied method in Polish routine biomonitoring. A rectangular frame net with 500 µm mesh is held downstream while the operator disturbs the substrate by kicking for a fixed duration — typically three to five minutes per sampling unit in EN 27828.
Procedure in Gravel-Bed Streams
In Carpathian foothill streams with riffle–pool morphology, the operator works upstream through riffles, placing the net frame against the substrate and disturbing an area of approximately 0.09 m² per kick. Cobbles and larger clasts are turned by hand and washed into the net opening. Organic debris, moss and algal material in the sample are particularly important to retain, as many Plecoptera and Trichoptera larvae shelter within them.
Procedure in Lowland Sandy and Silty Rivers
Lowland reaches with sand and silt substrates require a modified approach. The net frame is pressed into the substrate to 5–10 cm depth before kicking, to disturb oligochaetes, Chironomidae and Ephemeroptera burrowers. Sampling effort is typically distributed across multiple microhabitats — open sand, macrophyte beds, detrital margins — proportionally to their representation within the reach, following the multi-habitat (MHS) protocol.
The multi-habitat sampling (MHS) protocol allocates sub-sampling effort proportionally to the percentage cover of each habitat type within the reach. In Polish monitoring programmes, a standard of 20 sub-samples per site (each covering 0.09 m²) is commonly applied, with sub-sample allocation reflecting the relative area of each habitat.
Surber Sampler
The Surber sampler encloses a fixed area (typically 0.09 m²) with a frame and downstream net. It is suited to hard substrates in flowing water where the enclosed area can be worked systematically. In Polish monitoring, Surber samplers are mainly used in upland streams and smaller Sudeten or Carpathian tributaries with stable gravel substrates.
All substrate within the frame is disturbed to a depth of approximately 5 cm, and stones are individually cleaned into the net. The enclosed design reduces loss of fast-swimming organisms compared to open kick-net methods.
Hand-Sorting in the Field
Polish national guidance recommends initial coarse sorting in the field using a white enamel tray and forceps. Large organisms (body length > 10 mm) are picked by hand; the remaining material is preserved in 70–80% ethanol for laboratory sorting. This approach reduces preservation volume and limits deterioration of fragile taxa such as small Plecoptera nymphs.
Habitat-Specific Considerations
| Habitat Type | Recommended Method | Key Taxa |
|---|---|---|
| Gravel/cobble riffle | Kick-net, Surber sampler | Plecoptera, Ephemeroptera, Trichoptera |
| Sand/silt lowland | Kick-net (MHS protocol) | Chironomidae, Oligochaeta, Ephemeroptera burrowers |
| Macrophyte beds | Sweep net in vegetation | Gammarus, Asellus, Hydracarina |
| Organic detritus accumulations | Hand-net, tray sorting | Simuliidae, Lepidostoma, Sericostoma |
| Woody debris | Hand-picking, brush sampling | Elmis, Limnius, Esolus (Riffle beetles) |
Laboratory Processing
After preservation, samples are sorted under a stereomicroscope (typically 10–40× magnification). Identification follows standard Central European keys, including Freshwater Invertebrates of Central Europe (Nilsson 1996), Identification Keys to Polish Invertebrates (PWN series) and family-level guides for Diptera larvae. Identification to genus or species level is required for taxa used in MMI (Polish multimetric index) calculations.
Seasonal Timing
For WFD compliance surveys in Poland, spring sampling (April–May) and autumn sampling (September–October) are the standard windows. Spring surveys capture late-instar EPT larvae before adult emergence; autumn surveys document the composition after summer baseflow conditions. Single-season surveys for preliminary assessments are typically conducted in spring, when EPT diversity is highest in Polish streams.
References
- CEN (1994). EN 27828: Water quality — Methods of biological sampling — Guidance on handnet sampling of aquatic benthic macro-invertebrates.
- Chief Inspectorate for Environmental Protection [GIOŚ] (2021). Methodology of biological monitoring of rivers — Macroinvertebrates. Warsaw.
- Buffagni, A. et al. (2006). A simple procedure to harmonize class boundaries of assessment methods across European rivers. Hydrobiologia 566: 1–16.
- European Environment Agency — Water quality data and assessments.