| Summary: | <p> The impacts of a 20,993 L diesel and gasoline spill on the benthic macroinvertebrate (BMI) and diatom communities in Cold Creek and the Santa Ana River (below its confluence with Cold Creek) were assessed by comparing species richness, diversity and compositions between fuel-impacted and reference sites. BMIs and diatoms were sampled four times (13–26 months after the spill) in sites located upstream from the spill (reference) and in sites located 0.5, 1.3, 2.7 and 3.0 km downstream from the spill (impact). BMI communities up to 2.7 km below the spill and in Cold Creek had (1) lower species richness and diversity for at least 25 months after the spill and (2) dissimilar species compositions for up to 26 months after the spill when compared to the reference site. Diatom communities up to 2.7 km below the spill and in Cold Creek had (1) similar or higher species richness and diversity when compared to the reference site and (2) dissimilar species compositions when compared to the reference site for the entirety of the study. BMI and diatom communities located 3.0 km below the spill and in the Santa Ana River had similar species richness, diversity and compositions when compared to the reference site. These results provide evidence that the fuel spill had an impact on the BMI and diatom communities in Cold Creek and that these communities are still changing.</p>
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