Natural Sources of Nitrogen
Natural sources of nitrogen include some of the nitrogen in the atmosphere, instream sources (e.g. salmon carcasses and litter fall), and vegetation (e.g. from N-fixing plants). Nitrogen is also a by-product of many natural biogeochemical processes that occur in watersheds, rivers and streams, such as the decomposition of plants and organisms.
The table below compares the magnitude of nitrogen loading that is natural to that which is sourced from human activities (both point and nonpoint sources) – note that the natural nitrogen loads presented in this table only include watershed sources of nitrogen, and do not include oceanic nitrogen. The difference between human and natural loads reflects the influence of anthropogenic sources of nutrients, including changes in land use and development, increases in population, and loads from wastewater treatment plants.
The table below compares average annual natural and human DIN loads into Puget Sound (Source: Mohamedali et al., 2011a)
Another way to look at human vs. natural DIN load contributions is by percentages rather than absolute numbers. Human nonpoint sources dominate in Admiralty Inlet, Elliot Bay, and the Strait of Georgia (SOG), while natural load dominates in Whidbey Basin and Hood Canal.
Relative contributions of DIN load to different regions of Puget Sound from human point sources (WWTPs), human nonpoint sourced (via rivers) and natural sources (Source: Mohamedali et al., 2011a).
When the above loads are aggregated to the Puget Sound scale (rather than individual regions of Puget Sound), the total human contribution to DIN loading is 73%, while the total natural load is 27% - note again, that this does not include nitrogen from the ocean, but only from terrestrial sources. Human activity, such as changes in land use, development, agricultural practices, and wastewater discharges, contributes to increased DIN loads above natural levels.
Nitrogen from Spawning Salmon
One natural source of nitrogen to rivers and streams are salmon that return from the Pacific Ocean to their natal rivers to spawn. These salmon bring marine-derived nitrogen (MDN) to freshwater systems from the years they spent growing and feeding in the ocean. After spawning in Puget Sound rivers and streams, the salmon die, and nitrogen within the carcasses is made available to aquatic organisms. Fungi, bacteria, macroinvertebrates and mammals feed on the carcasses, thereby transforming this nitrogen into inorganic forms which are released into the water as waste products. Since salmon returns have declined drastically over the past century, there are fewer salmon carcasses and therefore a deficit of MDN (and phosphorus) in Puget Sound rivers and streams relative to historic levels.
Comparing historic (prior to European settlement) and current levels of salmon runs, Gresh et. al (2000) (PDF) estimated the magnitude of this nitrogen loss as a result of declining salmon runs for various regions around the Pacific Northwest, including Puget Sound. Two mechanisms have resulted in lower levels of nitrogen from returning salmon: fewer salmon runs, and reduced weight (biomass) of the salmon that are returning. The table below summarizes the results of their analysis for Puget Sound:
1Historic levels refer to data prior to European settlement, mostly from the early 1900’s.
The loss of 0.8 – 2.1 million kg/year of MDN translates to an estimated 2,300-5,800 kg/day of MDN that is not entering Puget Sound rivers and streams. MDN plays an important role in the ecosystem, providing nitrogen in a form that is different from the nitrogen contained in human sources – therefore, the deficit of naturally-sourced MDN cannot necessarily be offset by increases in nitrogen loading due to human activities in watersheds.
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