Copepoda of lowland springs: diversity patterns and integrative taxonomy of <i>Cyclops</i>

Groundwater copepods in lowland Europe remain insufficiently studied compared to those from mountainous regions. Here, we present the results of a faunistic and ecological survey of lowland springs, conducted in the context of earlier research on groundwater copepods from over 100 wells in the same region. Springs, as natural interfaces between aquifers and surface waters, provide more diverse habitats and favorable conditions for groundwater-affiliated fauna. We examined copepod assemblages in 36 lowland springs in northeastern Poland, representing rheocrene, limnocrene, and helocrene types, which were sampled in summer and autumn. We identified a total of 23 species of Copepoda, comprising 13 Cyclopoida and 10 Harpacticoida. Most of these species were associated with hyporheic or groundwater habitats. Cyclopoida dominated in terms of abundance, especially in limnocrene springs, and showed little seasonal variation, while Harpacticoida exhibited markedly higher diversity and abundance in summer. The Cyclopoida assemblage was dominated by Eucyclops serrulatus, Diacyclops bicuspidatus, and Cyclops strenuus, whereas Harpacticoida were represented mainly by Attheyella crassa, Canthocamptus staphylinus, and by five Bryocamptus species. Many of the common copepod species were also found in groundwater wells in the same region. Still, overall richness, particularly of Harpacticoida, was higher in springs due to the presence of heterogeneous benthic microhabitats. We identified three Cyclops species in the springs that were not recorded in the groundwater (wells) of this region. Conversely, we did not detect C. furcifer in the studied springs, despite its presence in nearby wells and temporary puddles. To present taxonomic relationships within the genus Cyclops, we applied an integrative taxonomic approach that combined morphological traits with 12S rRNA and ITS-1 markers. This confirmed the presence of three distinct species: C. strenuus, C. insignis, and C. borealis (considered a senior synonym of C. heberti), for which we provide descriptions of key morphological traits together with molecular data. These findings highlight the function of lowland springs as ecotonal habitats fostering distinct copepod assemblages, including taxa characteristic of both groundwater and benthic microhabitats.