STATE OF THE GREAT LAKES 2005
WHAT IS THE STATE OF THE GREAT LAKES FOOD WEB?
Introductions and invasions of non-native aquatic species, harvest and stocking of top predator fish, elevated
nutrient levels, and presence of contaminants can disrupt the Great Lakes food web, impacting fisheries,
wildlife, and ecosystem health.
The Issue
The Great Lakes comprise the world's largest
freshwater ecosystem, and a healthy, complex food
web is critical to the overall health of this ecosystem.
Disruptions to the Great Lakes food web have
occurred due to introductions and invasions of non-
native aquatic species, harvest and stocking of top
predator fish, elevated nutrient levels, and
contaminants.
Major features of the Great Lakes food web
A food web is composed of all the interconnected
feeding and habitat relationships in an ecosystem.
Green plants and algae (phytoplankton) transform
solar energy into organic matter, which can be
transferred to top predator fish and birds through
many pathways. Phytoplankton are consumed by
plant-eating animals such as crustaceans and fish
larvae (types of zooplankton). Zooplankton are
consumed by other animals, including predatory
zooplankton and preyfish. Preyfish are food for
predator fish such as walleye, salmon, and lake
trout. Another key component of the food web is the
bottom-dwelling crustacean Diporeia, the dominant
prey item of whitefish and various preyfish species
in offshore waters. The burrowing mayfly,
Hexagenia, is also an important food web link to
nearshore fish species, such as yellow perch and
walleye.
Lake trout. Photo: Fisheries and Oceans Canada.
Disruptions of the Great Lakes food web
The species distributions and the feeding and habitat
interactions in the Great Lakes food web can be
disrupted in many ways, including:
Non-native aquatic species. The 169 known non-
native aquatic species in the Great Lakes have been
disrupting the Great Lakes food web for decades.
Non-native alewive and rainbow smelt preyfish
impact native preyfish through predation
and competition for food. Sea lamprey prey on lake
trout. Zebra and quagga mussel feeding activities
impact phytoplankton and zooplankton in the water
and invertebrates in the sediments. Non-native
predatory zooplankton prey on native zooplankton.
Harvesting and stocking of top predator fish.
Overharvesting of lake trout contributed to the
decline of this native top predator by the 1950s and
allowed non-native alewife to become the dominant
preyfish. Coho salmon, Chinook salmon, rainbow
trout, and brown trout were introduced to the Great
Lakes in the 1960s as top predators primarily to
control alewife and rainbow smelt populations. The
non-native top predator fish can compete with native
fish for food and habitat.
Elevated nutrient levels. Phytoplankton growth in the
Great Lakes is controlled, in part, by the naturally-
occurring nutrient phosphorus. Municipal, industrial,
and agricultural wastewater inputs can elevate
natural phosphorus levels, resulting in algae blooms.
Contaminants. Once introduced into the waters of the
Great Lakes, some chemical contaminants can be
accumulated by algae and subsequently transferred
through the food web to top predator fish and birds.
Contaminants can also directly affect organisms.
Phytoplankton (left), zooplankton (center), and Diporeia (right).
Photos: U.S. EPA Great Lakes National Program Office.
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WHAT IS THE STATE OF THE GREAT LAKES
FOOD WEB?
The Indicators and Assessment
Phytoplankton and zooplankton
Phosphorus load reductions in the 1970s and 1980s
contributed to substantially reduced algae blooms
and improved spawning and nursery habitat for
many Great Lakes fish. As a result of improvements
to sewage treatment plants, use of reduced
phosphorus detergents, and changes in agricultural
practices, average phosphorus concentrations in
most open waters of the Great Lakes in 2003 were at
or below target levels set in 1978. Zebra and quagga
mussels also appear to have reduced phytoplankton
populations in some areas of the Great Lakes. Non-
native invasive species pose a threat to the Great
Lakes zooplankton community.
Macroinvertebrates
Diporeia populations have declined dramatically in
Lakes Michigan, Huron, and Ontario over the past
12 years and are now rare in Lake Erie. Declines in
slimy sculpin and lake trout abundance and
deteriorating whitefish health are linked to the
scarcity of Diporeia. In the early to mid-1900s,
Hexagenia abundance declined in many Great Lakes
habitats impacted by excess nutrients or
contaminated sediments. Hexagenia populations in
areas of Lakes Erie and Ontario are recovering.
Preyfish
Native preyfish populations in all of the Great Lakes
except Lake Superior are declining, attributable to
dominance of non-native alewife and rainbow
smelt, Diporeia declines, and perhaps negative
interactions with zebra and quagga mussels.
Top predators
Controls on sea lamprey populations contributed to
restoration of lake trout to most areas of Lake
Superior. Improved water quality and habitat
quality, along with fishery management programs,
led to recovery of walleye in many areas of the
Great Lakes in the 1980s. Populations generally have
declined from the mid-1990s to present,
however. Levels of legacy contaminants such as
PCBs, DDT, and mercury have declined in Great
Lakes lake trout and walleye since the 1970s, but may
still be high enough to impair fish-eating birds such as
the bald eagle.
Current Actions
After the ballast water-introductions of Eurasian ruffe
and zebra mussels in the 1980s, voluntary ballast
management measures for ships in the Great Lakes
began, followed by mandated regulations in 1993.
Monitoring of phosphorus concentrations in the Great
Lakes continues. Monitoring and research programs
are investigating the declines of Diporeia and preyfish
stocks. Development of alternative sea lamprey
controls is underway. Stocking of Atlantic and
Chinook salmon, lake trout, and walleye is one of the
principal tools used by Great Lakes fisheries
management to ensure that top predator fish and
preyfish populations are balanced.
Actions Needed
To restore the Great Lakes food web, the following
actions are needed:
• Prevention of further non-native species
introductions and control of the abundance and
distribution of existing invasive species
• Research to determine the optimal stocking
amounts of non-native salmon and prey species to
support self-sustaining top predator fish
communities
• Protection or reestablishment of rare or eliminated
native preyfish to increase the diversity of
preyfish populations
• Maintenance of the capacity of existing sewage
treatment plants to control phosphorus loadings
to the Great Lakes in the face of growing human
populations
To Learn More
For further information related to the Great Lakes
food web, refer to the State of the Great Lakes 2005
report which, along with other Great Lakes references,
can be accessed at www.epa.gov/glnpo/solec.
02/06
EPA 905-F-06-902
IISG-05-28
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