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Did anyone ever say you looked green when you were sick? This can happen to oceans, too! In fact, people sometimes observe the coastal waters of Ireland (and in many other parts of the world) turning green. So green that they call it a “green tide”. What is going on?

The green color comes from certain seaweed that grows so much that they can change the color on the beaches. These “seaweed blooms” occur after nitrogen or phosphate pollution from human activities such as agriculture, farms, factories or towns enters coastal waters.

We wanted to find out if one particular seaweed, the commonly found sea lettuce (Ulva rigida), could serve as a living (bio-)indicator of water quality at the coast. (It can.) We also looked for a clear connection between the amounts of these seaweed and certain harmful metals in the water (but did not find a significant one).

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