autumn
穐 centers on the season of autumn. It is a variant form of 秋, carrying the same core meaning.
穐 is a variant form of 秋 (autumn), replacing the fire component 火 with 亀 (turtle). The reason for this substitution is uncertain, but it may be a graphic variation or a way to emphasize a different aspect of the season.
The left side 禾 is grain, and the right side 亀 is a turtle. Picture a turtle gathering grain in the autumn, storing food for the coming winter.
For シュウ, imagine a turtle shooing away falling autumn leaves: shoo -> シュウ, and the turtle is busy with the harvest.