In marine phytoplankton assemblages from waters relatively poor in dissolved Fe and Si, siliceous groups (diatoms, radiolarians) are suppressed while non-siliceous/mixotrophic groups dominate.
Diatoms and radiolarians need silica (and diatoms need Fe as a co-limiting micronutrient) to build their frustules/tests, so in Fe/Si-depleted waters they become scarce. Dinoflagellates...
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