Red tide is a common name for algal blooms, which are large concentrations of aquatic microorganisms, such as protozoans and unicellular algae.
The upwelling of nutrients from the sea floor, often following massive storms, provides for the algae and triggers bloom events
A change in the color of a substance -- this one is indeed a chemical change. Think of iron, if iron turns reddish, it's a completely different substance.
