Historical clue formulations

• Like pond scum — 2019-07-24
• ___ bloom (result of fertilizer pollution) — 2016-12-11
• Like some pond growths — 2016-07-11
• Harmful ___ bloom (aquatic growth) — 2014-12-02
• Like some pond life — 2009-07-14
• Like pond scum — 2006-07-31
• Like some pond life — 2000-03-12