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.
Noctiluca bloom near Seattle June
17, 2013. Photo by Jim Devereaux of King County.
Excess
nutrients
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.
Red-brown
algal bloom in Ostrich Bay, Dyes Inlet (taken August 28, 2017).
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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 the 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.
More details about these trends can be found in the 2013
River and Stream Water Quality Monitoring Report.
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.
By: Jenny Robertson,
Ecology Environmental Specialist
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