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Biological invasions can have major impacts on freshwater and marine ecosystems. Therefore, it is vital that non-indigenous species are accurately identified and reported when potential or confirmed invasions occur. The present study reports the first occurrence of Marenzelleria neglecta (Annelida, Spionidae) and the spread of Laonome xeprovala (Annelida, Sabellidae) in the Danube Delta–Black Sea ecosystem. Spionidae is one of the most diverse families of annelid worms and is a dominant group in terms of the number of species that have been introduced to non-native areas, while the members of Sabellidae are among the most visible polychaetes commonly found in fouling communities and are colonizing new geographic areas. Based on 20 samples collected in 2021, we provide an overview of the distribution of the investigated species and possible arrival pathways for Marenzelleria neglecta. Specimens were identified based on morphological descriptions. Both species have invasive behaviour, colonizing large areas in relatively short time periods and reaching relatively high densities (M. neglecta—1400 ind.m⁻²; L. xeprovala—40 ind.m⁻²). Due to their distribution and high abundances, the biology and ecology of these species in the Danube River–Danube Delta–Black Sea system need to be investigated further in order to assess their impact on ecosystem structure and functioning. Full article

Different irrigation or ventilation strategies by macrofauna may provide a competitive advantage to tolerant species invading impacted benthic systems and alter benthic-pelagic coupling. To comparatively analyze the effects of an exotic and a native polychaete burrower on sediment-water exchanges, two laboratory experiments were performed. In the first experiment, the invasive spionid polychaete Marenzelleria neglecta was added to defaunated sediments and fluxes of the inert tracer (bromide, Br⁻) were measured to quantify the effects of irrigation by the worm on the tracer transport. In the second experiment, M. neglecta or the native polychaete Hediste diversicolor were introduced to a relatively diverse Baltic soft-bottom macrofauna community. The effect of species on fluxes of reactive solutes (ammonium, NH₄^+, and phosphate, PO₄³⁻) and transport rates of Br⁻ was estimated. The results indicate different invasion effects depending on the characteristics of the recipient habitat. In defaunated sediments, a single specimen of M. neglecta significantly enhanced originally low solute exchange rates. Total tracer flux was significantly enhanced over diffusive flux by a factor of 1.6 ± 0.14 (n = 3). In natural sediments, on the other hand, the addition of either M. neglecta or H. diversicolor had no statistically significant effects on benthic fluxes. Tracer flux estimates between control and treatment incubations differed by less than 10% on average, and both reactive solutes tended to increase by 10 to 40% after additions. One specimen of M. neglecta in cores with defaunated sediment generated approximately 20% of the tracer flux produced by the relatively diverse macrofauna community. Estimated net tracer fluxes in two experiments corresponded well with the number of adult polychaetes found in sediments (r² = 0.73, p = 0.005, n = 12). The invasive M. neglecta produced a small effect on fluxes in biodiverse sediments, comparable to those of H. diversicolor, but it may deeply alter porewater chemistry in azoic sediment. As M. neglecta tolerates chemically reduced and sulphidic conditions, its bioirigation may favor sediment reoxidation and ultimately the recolonization by less tolerant, native species. Full article