It's about nutrient imbalance, not simply excess of nutrients across the board. You can have, e.g., a high silicates and low phosphorus situation driving this

It's about nutrient imbalance, not simply excess of nutrients across the board. You can have, e.g., a high silicates and low phosphorus situation driving this particular diatom, simply because phosphorus is not the limiter (as I have already stated is the case for diatoms). If you had a different nutrient balance, there would be competition with other organisms, so no growth in the first place. Whether it's reproductive or vegetative growth is irrelevant, because the mechanism of competition is the same: it covers surfaces and outcompetes other colonies.