Fate and Transport of Nitrogen

The Nitrogen Cycle

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 fate and transport of nitrogen, or the nitrogen cycle, is illustrated below in a generalized form.

The Nitrogen Cycle image from Jane Thomas IAN Image Library

Fate and Transport in Watersheds and Freshwater

There are many processes that change the forms and concentrations of nitrogen as it moves through the environment. Within a particular river system, nitrogen loads and concentrations can increase or decrease as a river flows from its headwaters in the mountains, through its watershed, and eventually into Puget Sound. Nitrogen is added to the river by different sources, both natural and human. Nitrogen is also removed by different processes, known together as "attenuation."

“Attenuating” or nitrogen-removing processes include:

  • Near-surface and nearshore denitrification—in the presence of bacteria, nitrogen can be released as a gas.
  • Uptake by algae and plants in the water column
  • Burial of organic matter into sediments and muck
  • Anaerobic (oxygen-less) bacterial processes, often generating ammonia

Ecology has not yet identified the factors responsible for attenuation of nitrogen within Puget Sound’s freshwater systems, but this work is underway through a USGS study funded by a National Estuary Program grant administered by Ecology to do just this. Results from this effort should be available in late 2014.

As mentioned earlier, the USGS has also developed a regional terrestrial SPARROW model of nitrogen for the Pacific Northwest. Though the model is not optimized for the Puget Sound region, it includes the Puget Sound region, and tracks various land-based sources of nitrogen within smaller watersheds and subbasins, and routes these sources to downstream waterbodies.

Fate and Transport in Marine Waters

A number of physical, chemical and biological processes within the marine waters of Puget Sound affect nitrogen dynamics in Puget Sound. Physical processes include circulation, temperature, salinity, tidal flow, as well as exchange with bottom sediments (discussed in marine sediments). Chemical and biological processes transform some of the nitrogen into different forms as it is assimilated by marine organisms (such as algae) and incorporated into marine biomass, but can re-enter the water column as organic matter during decomposition. These processes are intricately connected, and influence the amount of nitrogen in the water column – the illustration below captures some of the processes involved.

Source: adapted from Figure 7 of Konrad, 2014

Bivalve filter feeders affect nutrient dynamics

A recent USGS study explored approaches to evaluating the effects of bivalve filter feeders on nutrient dynamics in Puget Sound. Bivalve filter feeders remove nitrogen from the water column by feeding on phytoplankton and other particles in the water column that contain nitrogen. However, there currently are very limited measurements if nitrogen removal by filter feeders in Puget Sound. USGS identified several fundamental information gaps that must be filled to quantify both the existing effects and the potential effects based on future populations of native or cultured filter feeders.

The Pacific Shellfish Institute completed a unique and innovative Nutrient Bioextraction study in different parts of South Puget Sound (Quartermaster Harbor, Budd Inlet, and Thea Foss Waterway). This involved placing straps under the water at marinas on which mussels could attach and grow over several months. These mussels assimilate the nutrients (both nitrogen and phosphorus) from the surrounding marine water, effectively removing some of these nutrients from the water. The mussels were harvested and tested for nutrient content in order to calculate how much nitrogen these mussels removed from the water. After testing the harvested mussels, they were turned into a rich high-value organic compost product, and used in plant growth trials. Shellfish at Work: Nutrient Bioextraction (PDF) and Nutrient Bioextraction Trials in Budd Inlet & Thea Foss Waterway (PDF) describe the project.

Modeling Tools allow us identify important fate and transport processes

Modeling tools allow us to simulate many of the other physical, chemical and biological processes which cannot be measured everywhere at all times in the field, and evaluate the relative influence of these processes on nitrogen dynamics and other water quality variables. Several modeling tools are under development now to evaluate the fate and transport of nitrogen in the marine waters of Puget Sound, including Budd Inlet, South Puget Sound, and Puget Sound/Salish Sea.

These modeling tools have identified the following fate and transport processes to be influential to Puget Sound nutrient dynamics:

  • Tidal and estuarine circulation
  • Vertical stratification and mixing
  • Sediment-water exchanges
  • Primary production