Nitrogen Trends in Puget Sound Marine Waters

Nitrate Concentrations Are Increasing in Puget Sound

Marine nitrate concentrations in Puget Sound have increased over the period from 1999-2012 in many regions of Puget Sound, a trend which cannot be explained by oceanic influences or river inputs alone. This trend is seen across all monitored stations.

This heat-map table shows trends over 14 years in shallow-water marine nitrate concentrations at Ecology’s monitoring stations in Puget Sound. The left-hand side of the table shows the anomalies in monthly nitrogen between 1999 and 2012. Black indicates concentrations that were about the same as baseline conditions, red indicates concentrations higher than baseline conditions, green indicates concentrations lower than baseline conditions, and gray indicates missing data. The red-colored squares increase markedly over time.

Data aggregated across different stations in Puget Sound for each year from 1999-2013 shows that marine nitrate concentrations increased approximately 3 µM. This figure is based on yearly averaged anomaly data across all Puget Sound stations. In 2013, nitrate concentrations were lower than expected, interrupting the long-term increasing trend.


This increasing trend could either be because more nitrate is entering Puget Sound marine waters, or that available nitrate is not being used up as effectively by marine organisms. The latter case is supported by decreasing chlorophyll biomass over the same time period.



The poster, S. Puget Sound 2011 and 2012 in review: Aerial and water column observations from Ecology’s long-term monitoring program summarizes many of these findings and contains the nitrate-concentrations chart.

Nutrient Cycling in Puget Sound is Changing

Oceanic water that enters Puget Sound contains nitrogen and silicate concentrations that are at a fairly stable and known ratio. When nitrogen from non-oceanic sources enters marine waters, the dissolved-nitrogen-to-silicate ratio changes, and can therefore be used as a proxy to indicate a change in the balance of nitrogen from different sources. Over the past 15 years, the ratio of Silicate (Si) to DIN has been decreasing in Puget Sound by about 10 units per decade.

Click to enlarge.
Significant negative trends in the ratio of Si:DIN have been observed in Puget Sound. This figure is based on yearly averaged de-seasonalized data across all stations in Puget Sound. (Source: Krembs et al., 2012 and personal communication). The observed declines are regionally strongest in South Puget Sound and during the summer season. The declining trend was interrupted in 2013 due to lower nitrate concentrations.

Declines in the Si:DIN ratio can be an indication that human induced nitrogen inputs are increasing. However, as discussed further below, neither river or wastewater treatment plant nitrogen sources have been increasing, so we still do not have a complete explanation of why the Si:DIN ratio is changing.

Increases in nitrate concentrations are also paralleled by increases in phosphate concentrations, indicating a changing nutrient balance. These shifts change nutrient cycling, which in turn can affect the Puget Sound ecosystem and food web e.g. the species composition of algae or the timing and extent of algae blooms. Scientists from the marine monitoring unit have proposed a potential hypothesis of how changing nutrient ratios drive changes in the Puget Sound food web, which you can read more about in this poster:

Changes in nutrient ratios drive changes in pelagic and benthic assemblages, and benthic-pelagic coupling in Puget Sound: A compelling hypothesis linking water quality and the benthos explains the hypothesis.

Many of these chemical and biological processes still require further study in order to completely explain observed phenomena.

Watershed Trends discusses if the levels of nitrogen entering Puget Sound are changing over time.