red tide

Short definition from Wiktionary

Noun

1. 1
   An algal bloom of sufficient quantity to cause discoloration (often red) in a body of water; red tides can potentially cause irritation or death to exposed creatures.
2. 2
   An algal bloom of sufficient quantity to cause discoloration (often red) in a body of water; red tides can potentially cause irritation or death to exposed creatures: specifically, one caused by Karenia brevis.