Welcome to our third installment of Puget Sound Nutrient Watch, an ongoing blog series that will focus on the excess nutrient problem in Puget Sound.
This post focuses on the Puget Sound’s excessive nutrients and how they negatively affect dissolved oxygen.
Nutrients such as nitrogen and phosphorus are an important part of a healthy and productive Salish Sea ecosystem. However, excess nutrients, especially nitrogen, can be problematic for water quality. When it enters marine waters, excess nitrogen causes what is called eutrophication. Nitrogen acts like a fertilizer causing algae to grow. Too much nitrogen results in excessive algae growth. When algae die and decompose, they consume oxygen out of the water column (especially at depth), in some cases depleting oxygen to low levels.
Excess nutrients can cause an explosion of algae growth called algal blooms. These can be foamy, scummy or a thick mat of slime on the surface of the water or along the beach. Rapid growth of algal blooms can starve the aquatic environment of light and dissolved oxygen, prompting shifts in the form and function of the ecosystem, and its ability to support aquatic life.
Trends in Puget Sound
Specific parts of the Sound, such as the shallow inlets and bays in southern Puget Sound, are more sensitive to eutrophication due to reduced flushing compared to more open marine waters.
With projected increases in the population surrounding the Salish Sea by 2070, local human nutrient sources will likely increase. The resulting changes in urbanization/land use, and increased wastewater effluent inputs, has the potential to change the timing and characteristics of freshwater entering the Puget Sound.
The flow of freshwater from rivers to the Salish Sea is important because it drives estuarine circulation- in which freshwater flowing out of a river at the surface causes denser seawater to flow into the estuary at depth. Reduced river flows result in reduced flushing or exchange of water. The relative timing of runoff from the Fraser River and other rivers, as well as coastal upwelling, impact Salish Sea water quality. Climate change can influence the timing of these processes.
It is critical to continuously collect monitoring data in order to understand current conditions, evaluate environmental trends, and provide an alert system for changing conditions.
Sources of nutrients
The Pacific Ocean is the largest single source of nitrogen to the Salish Sea. The ocean’s nutrient-rich upwelled waters have been the major driver of natural productivity throughout recorded history. However, model results show excess nutrients from human sources such as wastewater, stormwater, and animal wastes are adding to ocean inputs, resulting in depleted dissolved oxygen levels below what is allowed by our water quality standards in certain areas of the Sound.
Puget Sound is also sensitive to the influx of organic carbon which micro-organisms consume for energy. Decomposing organic carbon discharged to our marine waters reduces dissolved oxygen levels, just like the decomposition of dead algae.
Nutrient discharges to the Sound also play a significant role in ocean acidification. Salish Sea model estimates show that the biggest human derived nutrient impacts on acidification are seen in bottom waters in specific areas of the Sound. In our next blog, we will be discussing ocean acidification and how it contributes to a more unstable Puget Sound.
Explore more about excess nitrogen, nitrogen sources and pathways, monitoring nitrogen, river and marine trends in the interactive Nitrogen in Puget Sound story map.
Follow this Puget Sound Nutrient Watch blog series to stay current and learn about our work to reduce nutrients in Puget Sound.