**3. Conclusions**

Food chains and food webs describe the structure of communities and their energy flows, and they present interactions between species. Recently, diverse methods have been developed for both experimental studies and theoretical/computational studies. They improve our fundamental ecological knowledge and are e ffectively used for various applications, including the monitoring and assessment of ecosystems. In particular, ecological monitoring and assessment have advanced in recent decades. Along with the progress of molecular and eDNA techniques, the process of monitoring and assessment has become rapid and accurate. A wide variety of ecological disturbances associated with temperature and salinity changes and other environmental factors are being recognized as threats to the food chain functions of freshwater and marine ecosystems.

This Special Issue included empirical studies on food chains and food webs in aquatic ecosystems. They confirmed the usefulness of their methods including isotope, DNA-barcoding with gu<sup>t</sup> contents, and eDNA for biological monitoring and ecosystem assessment. In further studies, however, theoretical and computational approaches including food web modelling and network analyses are expected to characterize quantitatively the interactions among species as well as ecosystem structures and dynamics through the collaborative works between experimental and computational scientists.

**Author Contributions:** Conceptualization, I.-S.K. and Y.-S.P.; writing—original draft preparation, I.-S.K. and Y.-S.P.; writing—review and editing, I.-S.K. and Y.-S.P. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research was funded by the National Research Foundation of Korea (NRF) funded by the Korean governmen<sup>t</sup> (MSIP) (grant numbers NRF-2019R1A2C1087099 and NRF-2020R1A2C1013936).

**Acknowledgments:** We would like to thank all contributors in this Special Issue and all reviewers who provided very constructive and helpful comments to evaluate and improve the manuscripts.

**Conflicts of Interest:** The authors declare no conflict of interest. The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript, or in the decision to publish the results.
