Marine Sediments

Nitrogen moves between the sediment-water interface at the bottom of Puget Sound in a process referred to as benthic sediment flux. This process typically results in a net supply of inorganic nitrogen to the water column from the sediments.

The nitrogen in marine sediments originally comes from algae, which take up inorganic nitrogen and convert it into organic forms. When the algae die and decompose, they settle onto the Puget Sound floor, supplying organic nitrogen to marine sediments. Microbes in the sediment degrade the organic nitrogen further, turning some of it back into inorganic nitrogen, which returns to the water column above. 

We currently have limited measurements of benthic flux of nitrogen (and other nutrients), and these measurements have mostly been taken in shallow bays and inlets of Puget Sound. 

Measurements of sediment nitrogen flux

The US Geological Survey performed an extensive literature review to summarize existing measurements of the benthic flux of inorganic nitrogen collected in different parts of Puget Sound. Fluxes were found to be influenced by a number of factors, including measurement methods, location in Puget Sound, depth of measurements, bottom temperature, and season. Sediment composition and organic matter are also important factors, but the effect remains unknown.

Overall patterns/results from this study included:

  • Ammonium fluxes are generally positive (out of the sediment and into the water column), and nitrate fluxes are generally negative (into the sediment from the water column).
  • Ammonium fluxes are greater in magnitude than nitrate fluxes.
  • Ammonium fluxes vary by season and are influenced by the timing of primary production.
  • Shallow areas (< 50 m) of Puget Sound have higher and more variable ammonium fluxes.
  • Ammonium fluxes generally increase where bottom temperatures are warmer, while nitrate and nitrite fluxes are less influenced by temperature – but some of these patterns may be due to limitations in available datasets.
  • Where sufficient data are available, calculations of net flux show that sediments act as a net source of inorganic nitrogen in 78% of locations and a net sink in 22% of locations.
  • Without more information on organic nitrogen fluxes, a complete nitrogen mass balance cannot be calculated.

Modeling sediment flux of nitrogen

This image shows results of average annual sediment fluxes of nitrogen in 2006, as simulated by the Salish Sea Model. Model results show that:

  • Ammonium fluxes are positive (out of the sediment and into the water column) and tend to be highest at the landward ends of inlets and bays.
  • Ammonium fluxes range from about 12 to 47 mg N/m2/day.
  • Nitrate + nitrite fluxes are negative (from the water to the sediment) because of denitrification and are highest in deeper areas of Puget Sound.
  • Nitrate + nitrite fluxes range from about -5 to -18 mg N/m2/day.