​​​​​​​​​​​​​​​In the spring and summer, you may start to notice algae in streams, rivers, and bays in Maryland. There are many types of algae, and a great majority of them are not toxic and play a critical ecological role in Maryland’s waters. However, high concentrations of algae, or algal blooms, can sometimes be dangerous.

Blooms of algae and cyanobacteria can vary widely in appearance—from large, fibrous mats to discolorations in the water. Algae may become harmful if they occur in an unnaturally high abundance or if they produce a toxin. Harmful algal blooms can impact human or animal health, kill fish, or disrupt the ecosystem by blocking sunlight or reducing oxygen levels. Algal toxins must be ingested to cause illness making animals (including dogs) and small children most at risk.

Below are some examples of algal blooms found in Maryland. Can you tell which algal blooms are harmful?

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