Combined sewers collect sanitary sewage and stormwater run-off into a single pipe system. Combined sewers frequently overflow during heavy storms because they are not designed to handle the large volumes of stormwater runoff. These overflows result in the discharge of untreated sewage into surface waters.
Various forms of nitrogen in fresh water may stimulate algal growth and contribute to eutrophic conditions. Nitrogen input to the Great Lakes is represented by nitrate (+nitrite), which typically comprises the majority of total inorganic nitrogen.
Phosphorus runoff from watersheds is considered one of the most important drivers of eutrophication and the proliferation of nuisance algae because phosphorus often limits algal growth in freshwater ecosystems.