Image by Dan Meyers

Dams and other barriers create fragmented systems that reduce connectivity and overall range of habitat for aquatic and riparian communities. 

nutrient_enrichment_diagram.jpg

(Public domain)

Agriculture can be a source of nutrient enrichment when fertilizers (nitrogen and phosphorus) get carried into a water body by surface runoff. An increase in nutrients can lead to algal blooms which can temporarily increase the populations of organisms that feed on it. However, excessive algal growth can quickly overwhelm and kill off other aquatic plants by blocking sunlight. When the algae eventually dies, bacteria begin to decompose it and in the process use up a lot of dissolved oxygen. This can lead to a decrease in fish and macroinvertebrate populations that depend on dissolved oxygen.