| Summary: | We studied the composition of non-indigenous macroinvertebrate species (NIMS) and biocontamination level in four major large Croatian rivers (the Danube, Sava, Drava and Mura) to establish which environmental parameters are the most important for the composition of NIMS assemblages and to determine how NIMS affect biological metrics regularly used in ecological quality assessment. We sampled benthic macroinvertebrates at 48 sites (44 lotic + 4 lentic), and among 236 taxa, 21 NIMS were identified, of which 9 were widespread and abundant. Only 14.6% of sites exhibited no biocontamination, 18.7% exhibited low or moderate biocontamination and 66.7% exhibited high or severe biocontamination. Higher biocontamination in the Drava may be due to both the proximity to the Danube as the main source of NIMS and the existence of three large reservoirs. We found significantly negative correlation between the number of NIMS and native taxa. The significant correlations between biocontamination indices and tested biological metrics were as follows: negative with %EPT, EPT-S, BMWP and IBE AQEM, while positive with HR-SI. This indicates that at sites where NIMS are abundant and native taxa scarce, standard biological metrics could provide unreliable results and compromise the assessment of ecological status of large rivers.
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