Excess Nitrogen

Nitrogen is a nutrient that is present in the natural environment and is needed by marine plants and animals to grow. Nitrogen from various sources (both human and natural) enters Puget Sound via different pathways. People increase the amount of nitrogen entering the Sound above natural levels. For example, nitrogen is present in human wastewater and in plant fertilizers. 

Excess nitrogen can fuel algae blooms that eventually decompose and sink through the water column. This organic matter decomposition process decreases dissolved oxygen levels, typically near the bottom of Puget Sound waters. 

Fish and other marine organisms depend on oxygen to survive and thrive. Oxygen levels in many parts of Puget Sound are below levels that are needed for these organisms to thrive successfully.

The Nitrogen Cycle

Nitrogen has many components and is present in the environment in inorganic and organic forms as well as in dissolved and particulate forms. Nitrogen released into the environment in one form can be transported or transformed into other forms. It can also move from one compartment of the ecosystem into another (e.g. land, sub-surface, vegetation, groundwater, surface water, and marine waters). The nitrogen cycle is illustrated in this graphic.

Many of the estimates and plots presented in this story map focus on dissolved inorganic nitrogen (DIN). Of all forms of nitrogen, DIN is the form of greatest interest, since it is the most bio-available form of nitrogen used by marine algae. DIN is the sum of nitrate, nitrite, and ammonia nitrogen – three different forms of inorganic nitrogen. Organic forms of nitrogen are also present but in a much smaller amount. 

The graphic below illustrates the partitioning of total nitrogen into the various forms found in the environment.

Algal Blooms

Algal blooms are a common feature in marine waters, but we are observing a higher frequency of blooms in Puget Sound. This may be because of a combination of both 1) an increase in the actual frequency of algal blooms and 2) an increase in observations from more recent monitoring data and information.

This graphic illustrates how excess nitrogen loading contributes to low dissolved oxygen when algal cells decompose after a bloom occurs. Other factors also enhance or inhibit algal growth and decomposition, including:

• circulation, temperature, and other weather conditions
• different species of algae respond differently nitrogen inputs
• levels of other nutrients e.g. total organic carbon
• changes in the food web and at other trophic levels e.g. presence/absence and type of zooplankton, which feed on algae

Dissolved oxygen levels are low

Low dissolved oxygen levels have been observed in Puget Sound for a number of years, but were also present historically. Extremely low dissolved oxygen levels (below ~2 or 3 mg/L) are referred to as hypoxia, which means that there is not enough oxygen for many marine organisms to thrive. Washington State’s water quality standards define minimum levels of dissolved oxygen that marine organisms, such as salmon, need to thrive; these levels are several mg/L above hypoxic levels.

This map illustrates areas in Puget Sound where measured dissolved oxygen levels are either below the water quality standard (impaired) or close to being at this minimum threshold (waters of concern) based on the 2014 Water Quality Assessment. The minimum dissolved oxygen water quality standard in most of Puget Sound is set to 7.0 mg/L. A few smaller inlets and bays of Puget Sound have a minimum dissolved oxygen standard of 5.0 or 6.0 mg/L.

In addition to dissolved oxygen depletion due to algal and other organic matter decomposition, a number of other factors also affect oxygen levels. These include:

• circulation and stratification patterns
• bathymetry of Puget Sound
• chemistry of incoming Pacific Ocean water
• water temperature, salinity, and density
• timing and magnitude of freshwater flows
• biochemical or ecosystem-induced changes in the sediment layer
• local weather and regional climate

Modeling Dissolved Oxygen

The Salish Sea Model takes into account the physical, chemical, and biological processes that affect oxygen levels, to simulate nutrient and oxygen levels in Puget Sound. This video shows an animation of results from the model, illustrating levels of dissolved oxygen in the surface (left) and bottom (right) of Puget Sound in 2008. The model results show that:

• Oxygen levels are significantly lower at the bottom than at the surface of the Sound
• Oxygen levels are lowest in the late summer/early fall period (August – September)
• Oxygen levels vary spatially within Puget Sound

One of the questions that we are trying to answer using the Salish Sea Model, is: How much of the observed dissolved oxygen problems in Puget Sound are a result of human sources of nitrogen from this region?

Puget Sound Nutrient Reduction Project

Ocean Acidification

Ocean acidification is the scientific term used to describe how seawater chemistry is changing primarily due to increased amounts of carbon dioxide (CO2) in the ocean from human activities. This extra CO2 has changed the chemistry of seawater, making it more acidic. Puget Sound is affected by global ocean acidification particularly when deep ocean water upwells into the coastal zone and enters the Sound and coastal inlets. As seawater becomes acidic, it contains fewer carbonate ions that are available for marine organisms. This shift in chemistry affects marine organisms that need calcium carbonate to form skeletons and shells. 

In Puget Sound, nitrogen loading and organic-matter inputs from various local sources increase acidification in some areas (e.g. in the main basin of Puget Sound) and reduce acidification in other areas (e.g. the shallow inlets and bays of South Puget Sound). The mechanisms for this include:

1. An increase in primary productivity, which increases the uptake of CO2 and increases pH, primarily in the surface layers (makes water less acidic).
2. Subsequent release of CO2 when the algae and organic matter decompose, which then reduces pH levels (makes water more acidic) in the bottom waters.

This 2017 report found that increased dissolved inorganic nitrogen and non-algal organic carbon contribute to acidification in Puget Sound. Non-algal carbon includes detritus and dissolved pools of organic carbon, and does not include organic carbon that is in living phytoplankton cells.