red tide

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun A bloom of plankton, especially dinoflagellates, that causes a usually reddish discoloration of coastal ocean waters. Certain dinoflagellates produce toxins that contaminate shellfish, making them unsafe to eat, and can kill fish.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun 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

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun seawater that is discolored by large numbers of certain dinoflagellates that produce saxitoxin