Measuring the concentrations of nutrients alone is not
sufficient to understand the causes of nutrient pollution
in a given water body. Whereas added phosphorus is
usually the cause of eutrophication in freshwater lakes,
additional nitrogen is in the culprit in most coastal marine
ecosystems. The reason for the difference is that algal
growth is limited by the nutrient that is in the shortest
supply, referred to as the “limiting nutrient.” In marine
environments, algal growth is usually held in check because
nitrogen is in limited supply relative to the other
essential nutrients in the water. When additional nitrogen
inputs enter these marine environments—for instance,
when heavy winter and spring precipitation wash fertilizers
and other nitrogen-containing compounds to the
coast—algal blooms can occur.