"Algal blooms are a sign of a wildly imbalanced ecosystem. Before modern industrial agriculture came along and started polluting the lake, algae were kept in balance and didn't mass-produce the poisons that now contaminate the Toledo water supply.
What's happening is that all the fertilizer being dumped on nearby farm fields causes phosphorus runoff. This phosphorus runoff winds up in Lake Erie where it feeds the algae, resulting in algal blooms that produce the toxins. These toxins contaminate the drinking water supply and have led directly to the current water crisis."