This study aims to explore how elementary students perceive and approach the issue of plastic debris in marine habitats by examining students' perspectives on the ecosystem and environmental solutions. The study was conducted to 143 Grade Four elementary school students in Seoul. After implementing two class-units on plastic waste, students' constructed responses on the problem of and solutions to plastic debris in marine habitats were collected. Data were analyzed through semantic network analysis and the keywords were visualized to reflect their relationships. Furthermore, students' responses on how they perceive environmental problems were further analyzed based on the following analysis criteria: students' perspectives on the ecosystem, the level of complexity of food chain(s), and the scope of their perspective. Also, student responses on environmental solutions were classified to be either at a personal or social level. Through semantic network analysis, keywords identified for students' perceptions on the problem were the sea, plastic, debris, animals, living things, humans, extinction, while keywords extracted for the solutions were plastic, debris, recycling, disposable, and I. Based on the analysis criteria, it was found that students were well aware of the food chain concept, could perceive the ecosystem as having comprised of both biotic and abiotic factors, and could approach the problem beyond the scope of the marine environment. Also, most students mentioned the solutions only at a personal level. Based on the findings, implications on how to move forward in educating environmental issues related to the ecosystem in science education is further discussed.