Perla marginata stonefly nymph — a sensitive indicator of clean stream water
Perla marginata nymph. Stonefly nymphs require well-oxygenated, cold water and are absent from degraded reaches. Source: Zoologische Staatssammlung Muenchen / BOLD Systems, CC BY-SA 3.0.

What Are Macroinvertebrates?

Macroinvertebrates are animals visible to the naked eye that lack a backbone and live in or on stream substrates — gravel, cobble, leaf packs, woody debris, and aquatic vegetation. The group includes aquatic insect larvae (the largest and most studied component), as well as crustaceans, worms, snails, and mussels.

In bioassessment practice, the focus is almost always on insects because they are abundant, diverse, relatively easy to identify, and well-documented in ecological literature. Their life cycles are closely tied to local water conditions, meaning that community composition at any given site reflects conditions over weeks to months rather than just at the moment of sampling.

The EPT Index

The term EPT stands for the three insect orders most commonly used as water quality indicators:

  • Ephemeroptera (mayflies)
  • Plecoptera (stoneflies)
  • Trichoptera (caddisflies)

EPT richness — the total number of different EPT taxa found at a site — is one of the most widely cited metrics in rapid bioassessment protocols. The underlying principle is straightforward: EPT taxa, particularly Plecoptera, are intolerant of low dissolved oxygen, elevated temperatures, and many organic and inorganic pollutants. Their disappearance from a site is a meaningful ecological signal.

Note on tolerance: Not all EPT taxa are equally sensitive. Within Trichoptera, some families (e.g., Hydropsychidae) show moderate tolerance and may persist in mildly degraded streams. Plecoptera, especially Perlidae, are considered the most sensitive group overall.

Stoneflies (Plecoptera)

Stonefly nymphs are flattened insects with two tails and prominent leg claws that anchor them to stones in fast-flowing water. They are almost exclusively found in lotic (flowing) environments and require cold, well-oxygenated water — conditions typical of upland or forested headwater streams.

In Poland, stoneflies are found throughout the Carpathian, Sudeten, and Świętokrzyskie mountain streams. The species Perla marginata and members of the family Leuctridae are among the commonly recorded taxa in clean Polish rivers.

Stonefly nymph found in Houghton Creek, NY — typical of clean, fast-flowing streams
Stonefly nymph from Houghton Creek (NY, USA). The body form is characteristic across most Plecoptera families. Photo: Dave Huth, CC BY 2.0.

Mayflies (Ephemeroptera)

Mayfly nymphs are recognizable by their three tails (two cerci and a central terminal filament, though some families lack the central tail), feathery gills along the abdomen, and a single tarsal claw. They inhabit a range of microhabitats depending on the family: burrowers (Ephemeridae) dig into fine sediment, crawlers (Ephemerellidae) move across substrates, and clingers (Heptageniidae) flatten themselves on exposed rock surfaces in swift currents.

Mayfly diversity is a strong predictor of overall stream invertebrate richness. Many species complete a single generation per year, with nymphs developing over several months before emerging synchronously — a familiar spectacle on Polish rivers like the Dunajec and San.

Spiny crawler mayfly Ephemerella subvaria nymph — a sensitive stream invertebrate
Spiny crawler mayfly, Ephemerella subvaria, with a Perlodid stonefly nymph in the upper left. Photo: Bob Henricks, CC BY-SA 2.0.

Caddisflies (Trichoptera)

Caddisfly larvae are soft-bodied insects that many species protect by constructing portable cases from sand grains, small stones, bark fragments, or leaf pieces, bound together with silk. The case material and construction method are often species-specific, making case morphology a useful identification aid even without examining the larva directly.

Free-living caddisfly families (e.g., Rhyacophilidae) are predators that move openly across substrate. Net-spinning families (Hydropsychidae) construct fixed silk nets to filter fine organic particles from the current. Both free-living and case-building Trichoptera are important components of stream food webs, converting leaf litter and algae into animal biomass consumed by fish and other predators.

Caddisfly larvae from a stream in the Wayne National Forest, Ohio
Caddisfly larvae from Wildcat Hollow, Athens Ranger District, Wayne National Forest. Photo: Wayne National Forest / L. Sells, CC BY 2.0.

Biotic Index Methods

Several scoring systems translate invertebrate community data into a single water quality value. The most commonly applied in European streams include:

Method Approach Common Use
EPT Richness Count of distinct EPT taxa Rapid screening
BMWP Tolerance scores summed per family UK, central Europe
ASPT BMWP score / number of scoring families Adjusts for sample size
MMI (Multimetric) Composite of multiple biological metrics EU Water Framework Directive

Sampling in Polish Streams

The standard sampling method in Poland for macroinvertebrate surveys follows the EU Water Framework Directive protocols, which are based on a combination of kick-net sampling (using a 500 µm mesh net held downstream while the substrate is disturbed) and hand-picking from selected microhabitats. Surveys are typically conducted in spring (March–May) or autumn (September–October) when populations are at predictable stages.

GIOŚ (Główny Inspektorat Ochrony Środowiska) maintains the national biological element monitoring network, with published data available through the RBDH (River Basin District Hydrological) system and the European Environment Agency's waterbase portal.

Further Reading