Predatory diet and potential effects of Orconectes limosus on river macroinvertebrate assemblages of the southeastern Baltic Sea basin: implications for ecological assessment

Invasive crayfish can affect macroinvertebrate assemblages and thus alter conventional macroinvertebrate-based ecological assessment. We aimed to reveal potential impacts of the North American crayfish Orconectes limosus on river assessment in the Neman River basin (southeastern Baltic Sea). A laboratory experiment using identical macroinvertebrate assemblages was conducted to compare feeding selectivity and effects between O. limosus and the European Astacus leptodactylus. Field experiments were conducted to evaluate potential impacts of O. limosus on disturbed and undisturbed crayfish-free macroinvertebrate assemblages: one dominated by Oligochaeta vs. one co-dominated by Ephemeroptera-Plecoptera-Trichoptera and Mollusca (EPT-codominated). In the laboratory experiment, both crayfish species preferred feeding on Diptera (mostly chironomids), but O. limosus also selected Trichoptera and Ephemeroptera. Family richness did not change, but both species inflated the Shannon Diversity index by reducing Diptera domination. Astacus leptodactylus treatments had higher Shannon Diversity and percentage of EPT abundance compared to O. limosus treatments. Field experiments indicated (1) negative, (2) assemblage-specific, or (3) no effects of O. limosus on macroinvertebrate metrics. A negative effect, especially in the undisturbed assemblage, was observed on simple additive metrics based on taxa presence data, such as total or EPT family richness, or BMWP (Biological Monitoring Working Party) score. Assemblage-specific effects were indicated for some metrics based on relative abundances. In the Oligochaeta-dominated assemblage, Shannon Diversity was inflated and the percentage of Oligochaeta abundance was reduced. In the EPT-codominated assemblage Shannon Diversity was deflated while the relative abundance of Oligochaeta was not affected. No effects were observed when using the ASPT (Average BMWP Score Per Taxon) or percentage of EPT abundance. We conclude that O. limosus may have a more diverse predatory diet than A. leptodactylus, and thus can have a stronger effect on macroinvertebrate taxa sensitive to disturbances. Therefore, the invasion of O. limosus can alter macroinvertebrate assemblages and compromise conventional ecological assessment, even when it displaces resident Astacus species.