According to Ben Urbonas, president of the Urban Watersheds Research Institute, these nutrient contaminants tend to be more of a problem in closed water systems,like lakes, where concentrations can build up.

VOA: standard.2010.08.04

So a closed system would be, for instance, a glass of ice water with an ice cube in it, with the lid on top.

因此闭系可以是，比如说，一杯放了,一个冰块的冰水，顶上加了盖子。

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