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The non-indigenous round goby (Neogobius melanostomus) entered the Flint River, Michigan, USA around 1996 while zebra mussels (Dreissena polymorpha) invaded in late 1998. We wanted to identify if there were round goby or darter (Etheostoma/Percina spp.) diet or density alterations by comparing 1998 data with our 2002 data after mussel colonization. Chironomids dominated the round goby's pre-zebra mussel diet in August 1998 (89% by number), followed by hydropsychids (10%). After zebra mussels arrived, diets diversified; chironomids declined to 52%, hydropsychids stayed the same, gastropods were more prominent (22%) and 4% were zebra mussels. Data from a site upriver (with no round gobies or zebra mussels) showed darters consumed mostly chironomids (49%), mayflies (11%) and hydropsychids (9%), suggesting reliance on chironomid prey and other interactions compromised the ability of darters to coexist with round gobies downstream, since only one of three species present during 1998 was collected in 2002. Benthic assemblages on rocks changed dramatically (R-values =0.95) from 1998 to 2002. Blackside darter (Percina maculata) density in the presence of round gobies increased eightfold compared with 1998. We hypothesized zebra mussels fostered the growth of macrophytes, resulting in odonates composing 42% by volume of blackside darter diets in 2002 compared with 10% in 1998. Diet overlaps between small and large round gobies and blackside darters were high (Schoener Index =0.55-0.59, R-values =0.11-0.24), suggesting potential negative competitive interactions. Zebra mussel-mediated ecosystem changes may have decreased interspecific competition for food with blackside darters, allowing coexistence with round gobies. Native darters with varied diets feeding in mid-water, are most likely to coexist with round gobies, especially where dreissenids potentially mediate competitive interactions. [ABSTRACT FROM AUTHOR]