In David Tilman's experiments with the diatoms Cyclotella and Asterionella, when the diatoms were grown at silicon/phosphorus ratios between 6 and 90, why did the two species coexist?

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The diatoms Cyclotella and Asterionella coexisted because one species was limited by phosphorus, and the other was limited by silicon. At lower silicon/phosphorus ratios, Cyclotella was favored because it was better able to compete for limited silicon. At higher ratios, Asterionella was favored because it was better able to compete for limited phosphorus. Therefore, the two species were able to coexist by partitioning their resource use.

In David Tilman's Experiments with the Diatoms Cyclotella and Asterionella