The emergence of mega-journals (MJs) has influenced scholarly communication. One concrete manifestation of this impact is that more citations have been generated. Citations are the foundation of many evaluation metrics to assess the scientific impact of journals, disciplines, and regions. We focused on searching for citation beneficiaries and quantifying the relative benefit at the journal, discipline and region levels. More specifically, we examined the distribution and contribution to citation-based metrics of citations generated by the five discipline-specific mega-journals (DSMJs) categorized as Environmental Sciences (ES) on Web of Science (WoS) from Clarivate Analytics in 2021: Sustainability, International Journal of Environmental Research and Public Health, Environmental Science and Pollution Research, Journal of Cleaner Production and Science of the Total Environment. Analysis of the distribution of citing data of the five DSMJs shows a pattern with wide coverage but skewness by region and the WoS category; that is, papers in the five DSMJs contributed 26.66% of their citations in 2021 to Mainland China and 22.48% to the ES. Moreover, 15 journals within the ES had their JIFs boosted by more than 20%, benefitting from the high citing rates of the five DSMJs. More importantly, the analysis provides clear evidence that DSMJs can contribute to JIF scores throughout a discipline through their volume of references. Overall, DSMJs can widely impact scholarly evaluation because they contribute citation benefits and improve the evaluation index performance of different scientific entities at different levels. Considering the important application of citation indicators in the academic evaluation system and the increase in citations, it is important to reconsider the real research impact that citations can reflect.