적조가 · 적조를
red tide (algal bloom)
Example
적조 때문에 양식장 물고기들이 떼죽음을 당했습니다.
Fish at the fish farm died en masse because of the red tide.
Word parts
적 red+조 tide
Literally 'red tide', just like the English term.
Good to know
Pronounced [적쪼]: ㅈ after ㄱ becomes tense.