Algae blooms in the Gulf of Mexico use up the majority of the oxygen in the water, leading to massive "dead zones" that cannot support fish or other wildlife. The culprit? Nitrate, running off agricultural fields through tile drainage systems. But nitrate is only part of the problem. Algae in freshwater lakes and ponds flourishes when exposed to a different pollutant, phosphorus, and the tiniest amount is enough to trigger a bloom.