The Clean Water Act, which
became law in 1972, made it
illegal to discharge pollution
from a point source into navigable waters without a special
permit. That year also saw
the signing of the Great Lakes
Water Quality Agreement,
a treaty between the United
States and Canada to improve
management of the Great
Lakes for the benefit of both
countries. The resulting efforts
in the 1970s and 1980s succeeded in reducing phosphorus
inputs into the lakes and, as a
result, the frequency of algal
Since the mid-1990s, however, things have taken a turn
for the worse. Farming practices for applying fertilizer and
tilling the soil have shifted, and
climate change has brought
more intense spring storms to the Midwest.
“Farmers are switching to a technique called
conservation tillage, where they don’t turn the
soil over anymore. As farms get bigger, they
tend to apply fertilizer as pellets on the surface
of the soil,” Johnson says. “The phosphorus
from the pellets is only incorporated into the
top inch or so, the part of the soil that’s really
available to interact with rainwater.”
While the total amount of phosphorus
entering Lake Erie has not necessarily
increased, scientists who monitor the lake
have seen a shift from the particulate form
to the dissolved form, likely due to fertilizer
pellets interacting with rainwater at the soil’s
With plenty of dissolved phosphorus in
Lake Erie’s water, toxic algae populations
have been reaching dangerous new levels.
Microcystin concentrations in Toledo’s drink-
ing water during the 2014 algal
bloom reached 2. 5 parts per bil-
lion (ppb). The level that the U.S.
Environmental Protection Agency
has deemed safe to drink for
adults is 1.6 ppb. The threshold
is even lower for toddlers and
One recent study estimates
that for the lake to recover, the
amount of dissolved phosphorus
flowing into it would have to be
reduced by 58%, and full recovery would then take an additional
Beyond Lake Erie
Lake Erie’s situation is not
unique—algal blooms can happen anywhere nutrient-rich pollution meets warm water. They
cause millions of dollars in economic damage.
In 2015, the worst bloom ever recorded on
the U.S. West Coast contaminated the region’s
crabs with dangerous amounts of domoic acid
(C15H21NO6). The algae involved was a type of
diatom that naturally produces the powerful
neurotoxin. Similar to microcystin, domoic
acid can move up the food chain. Filter feeders such as clams eat the toxin-producing
diatoms. When crabs feed on the clams, they
also ingest the domoic acid through their prey.
How algae create ‘dead zones’
Perhaps the most infamous, recurring algal blooms
in the country are better known for the huge “dead
zones” they create in the Gulf of Mexico every year.
Fertilizers from agriculture across a huge swath of the country’s midsection wash into the Mississippi River and then into
the Gulf. Algae thrive on the nutrients, but their bacteria-driven
decomposition then sucks up
oxygen. This drives away or suffocates fish and other organisms.
Last summer’s dead zone was
the largest ever recorded, covering nearly 9,000 square miles,
an area the size of New Jersey.
What does this mean for you?
In addition to the harmful toxins
blooms can release, out-of-control algal growth can dampen
tourism in coastal areas, hurt fishing and shellfish industries,
and increase the cost of some seafood.
Organic matter decomposes,
and oxygen is consumed.
O2 O2 Algae thrive on utrients and
In addition to releasing toxins, algal blooms thriving on excess nutrients can cause trouble in other ways. After the algae die, they decompose through
an aerobic process, creating a low-oxygen “dead zone.”
Gulf of Mexico