Indicator bacteria are bacteria types that are normally present in the human gut and when found in streams indicate that sewage is leaking in through failing septic systems or broken sewer pipes. Along with these come pathogenic bacteria, viruses and parasites.
Nutrients: phosphorus and nitrogen act as nutrients to algae growth. The resulting algae blooms result in depletion of oxygen that kills fish and other wildlife through a process called eutrophication. Some species of algae produce highly toxic compounds. These nutrients come primarily from sewage and fertilizer runoff from lawns and gardens.