FA grows on the bottom, floats up when it has accumulated enough (O2) to become bouyant.

Rain knocks the O2 bubbles loose so it can sink. Several posters have commented that rain "breaks up the FA" - my guess on that is the drops falling are strong enough to break the filaments (which would help release the trapped gas bubbles).

Does this make sense?

"Live like you'll die tomorrow, but manage your grass like you'll live forever."
-S. M. Stirling
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