red tide
A harmful algal bloom that discolors seawater, often causing marine life die-offs. Common in news and environmental contexts.
This summer, a red tide occurred along the coast.
Many fish died due to the red tide.
Standard kanji spelling; the reading あかしお is unambiguous.
Compound of 赤 (あか, 'red') and 潮 (しお, 'tide'), referring to the reddish discoloration of seawater caused by algal blooms.