CoachWater holds oxygen the way sweet tea holds sugar — invisibly, dissolved. The quantity is tiny: a litre of well-aerated water carries about of oxygen, some thirty times less than a litre of air. Every gill and every underwater respirer of Chapter 47 draws on this one thin stock (Proposition 46.8).
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Example 48.4 (Stream against pond)
Now the two waters read like ledgers. The mountain stream: cold (high ceiling), churned white over every boulder (fast refilling) — oxygen near the ceiling all year. The summer pond: warm (low ceiling), still (slow refilling), rich in mud and dead matter (hungry decomposers) — oxygen scraping along the floor, worst in the small hours before dawn, when the plants have been respiring all night with no daylight manufacture to offset them.