This is the 3545th most frequent Chinese word.
水质
Water quality.
In this sentence, '水质' (water quality) is used as the subject to indicate a factor influencing ecological health.
水质影响生态系统的健康状况。
Water quality affects the health condition of ecosystems.
Here, '水质' is used as an object to refer to the water quality characteristic being subject to improvement.
科学家正在研究如何改善湖泊的水质。
Scientists are studying how to improve the water quality of lakes.
In this case, '水质' is used as the subject, described as being of a quality suitable for a specific purpose.