Water - Life

A clean river nurtures plankton and water-weeds, which are the food and nourishment of small fish. This natural cycle of life becomes a feature of any stretch of water; even an artificially created piece of water like a pond or a canal will, if left alone, start its own "life cycle", beginning with things that wriggle in the mud. In a short time, small predators will move in, and a complex little food chain will have started. All water, whether still or flowing, is richly productive, for where there is water there is life. You often see people fishing in a narrow water channel; they fish in ponds the size of puddles, in streams that look like gutters. And they are nearly always rewarded with a catch. The combination of water and organic matter can always be counted upon to produce some kind of living matter. The one condition is that the water must have a certain proportion of oxygen, for all life needs some oxygen. In other words, it must not be polluted with chemicals. Which of the following statements can be supported by the text?