Name: _________________________________________ Per. _____ Date: ________________________
Effects of Nitrogen in the Puget Sound: Algal Blooms and Eutrophication
During the second half of the 20th century, synthetic
fertilizers became available to gardeners and farmers who
were looking to enrich their soil for plants. Fertilizer
manufacturers can now do the work of nitrogen-fixing
bacteria: they can convert unusable nitrogen gas from the
atmosphere into inorganic nitrogen compounds which
plants can use. As a result, it is now common practice to
add fertilizer to lawns, backyard gardens, and farms to boost
plant growth. But the benefits haven’t come without
environmental costs.
When it rains, the excess nutrients from fertilizer runs off
into waterways. Although the nitrogen itself does not cause
a threat to Puget Sound, too much nitrogen can cause algae
blooms. These are excessive growths of algae due to more
available nutrients. The algae will then die off and
decompose. When the decomposers are doing the work
required to decompose algae, they must go through cellular
respiration, which requires oxygen. Therefore, excess
decomposition results in decreased oxygen levels in the
water, typically near the bottom layers of Puget Sound
waters. The figure left illustrates this process, also known as
‘eutrophication.’ Low dissolved oxygen conditions are sometimes referred to as hypoxia, which means that
there may not be enough oxygen for marine organisms to thrive.
Low dissolved oxygen levels have been observed in Puget Sound for the past several years. The figures below
illustrates areas of Puget Sound where have
low dissolved oxygen concentrations have been
measured. One of the questions that
Washington’s Department of Ecology is trying
to answer is how much of the observed
dissolved oxygen problems are a result of
human sources of nitrogen.
Frequency of low dissolved oxygen
concentrations (hypoxia) measured in Puget
Sound from 2003-2008. The larger the dot, the
higher the occurrence of low dissolved oxygen
concentrations (Source: Puget Sound
Partnership, 2009)
Results from the 2012 Marine Water Quality
Assessment for dissolved oxygen in Puget Sound
In addition to algal blooms resulting in low
dissolved oxygen levels, some algal blooms are
called harmful algal blooms (HABs) because they
can be toxic and can affect human health either
directly by swimming in the water, or indirectly by
consuming shellfish that are grown in water which
has been exposed to a harmful algae.
Washington, Oregon, and California have been
affected by a large HAB in 2015, although it is not
due to nitrogen run-off. But the domoic acid
bloom (a toxin produced by a type of marine
plankton) has resulted in the shut-down of
recreational and commercial shellfish harvesting
(such as for crab and clams) and has also caused
seizures and other neurological disorders in sea
lions and other marine mammals who are at the top of the marine food chain.
Summary Questions
1. Why is Puget Sound receiving more nitrogen than is has in previous centuries? Where is the nitrogen
coming from?
2. Why does excess nitrogen cause algae blooms?
3. What happens to dissolved oxygen levels in the water as a result of algae blooms? How does this
happen?
4. How can an HAB affect human health?
Sources:
1. Scientific American, “How Fertilizers Harm Earth More Than Help Your Lawn”, July 20, 2009
2. http://www.ecy.wa.gov/programs/eap/Nitrogen/Effects.html
3. http://www.seattletimes.com/seattle-news/health/toxic-algae-bloom-might-be-largest-ever/