The Nitrogen Cycle

The element nitrogen is fundamental to life on Earth, including freshwater life. Catchments are a dominant source of nitrogen to freshwater, and human activities have accelerated the scale of these inputs. The atmosphere is also increasingly a source of nitrogen to freshwater. Within freshwater systems, nitrogen transformations are complex and challenging to measure but many of the same key processes dominate throughout lakes, reservoirs, rivers, and streams globally. Ultimately nitrogen is lost from freshwaters, primarily to the atmosphere, the ocean, and via burial within aquatic systems. In this chapter the focus is on the key nitrogen processes within lentic systems (lakes and reservoirs) and lotic systems (rivers and streams). Finally, the impact of humans on the aquatic nitrogen cycle, via eutrophication and climate change, is outlined.