Causes, Consequences and
Management of Nuisance
Cladophora
Project GL-OOE06901
Final Report Submitted to the
Environmental Protection Agency
Great Lakes National Program Office
by
Harvey A. Bootsma
Great Lakes WATER Institute
University of Wisconsin-Milwaukee
February 27, 2009
UNI VERSITYof WISCONSIN
IMMILWAUKEE
WATER
INSTITUTE
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Cover Photos: Top: Cladophora accumulation on Bradford Beach, Milwuakee; Center:
Cladophora at a depth of 9 meters in Lake Michigan; Bottom: Quagga mussels in
spring, with early Cladophora growth.
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Causes, Consequences and Management
of Nuisance Cladophora
Final Report
Harvey A. Bootsma
Great Lakes WATER Institute
University of Wisconsin-Milwaukee
600 E. Greenfield Ave.
Milwaukee, Wl 53204
February 27, 2009
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Executive Summary
This report contains the results of a study to determine causes of, and potential
management options for, excessive growth of the filamentous green algae, Cladophora
sp., in Lake Michigan. The problem was approached through a combination of in situ
measurements in the lake, laboratory experiments, and numerical models. In situ data
were used to validate a Cladophora model for Lake Michigan, and a graphical user
interface was developed which allows for simple data entry and manipulation of the model
to test management scenarios. The model simulate Cladophora growth, tissue
phosphorus content, and biomass on a daily basis using measurements of temperature,
irradiance and soluble reactive phosphorus concentration as inputs. The validated model
was used with historic (pre-mussel) and recent data to determine how changes in light,
temperature and dissolved phosphorus may have influenced Cladophora growth and
biomass in Lake Michigan. The model indicates that increased nearshore dissolved
phosphorus concentration is partly responsible for increased algal abundance, but the
most important factor has been the increase in water clarity resulting from filter feeding by
zebra and quagga mussels. Increased clarity has resulted in increased light intensities on
the lake bottom, allowing for more Cladophora growth at shallow depths and for a 2-fold
increase in the depth range of Cladophora, from a maximum depth of about 5 m prior to
the mussel invasion to a current maximum depth of 11 to 12 m. It is this depth extension
that is primarily responsible for increased algal biomass per unit of shoreline.
In addition to assessing the factors that influence Cladophora growth and biomass, this
study included the development of an empirical quagga mussel (Dreissena bugensis)
model that simulates soluble reactive phosphorus (SRP) excretion by mussels as a
function of water temperature, mussel size, and food concentration (the model focuses on
the quagga mussel because this species has now largely replaced the zebra mussel,
Dreissena polymorpha, in Lake Michigan). Laboratory and field studies indicate that
soluble reactive phosphorus (SRP) excretion by quagga mussels depends both on food
supply and water temperature, with excretion rates rapidly increasing when temperature
exceeds 12°C (59°F). As a result, moderate increases in summer nearshore
temperatures may result in large increases in P excretion by mussels. The mussel P
excretion model was used, along with data on mussel size distribution, mussel densities,
and nearshore distribution, to estimate the P loading to the Lake Michigan nearshore
zone resulting from mussel metabolism. For the Wind Point to Fox Point stretch of
shoreline, it is estimated that during the Cladophora growing season mussels excrete
SRP at a rate more than 4 times greater than the loading rate from the mouth of the
Milwaukee River. Therefore, efforts to control Cladophora growth through the reduction
of nearshore P concentrations must consider reducing the availability of food for mussels.
This food is provided both as particulate material that enters the lake directly from rivers,
and as plankton which grows in offshore waters and is mixed into the nearshore zone.
The relative importance of these two pathways is not yet known, but it will determine the
rate at which phosphorus concentrations and Cladophora biomass in the nearshore zone
respond to any decrease in phosphorus load.
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Although increased water clarity is the primary cause of increased Cladophora
abundance, the only practical management option is to reduce in-lake dissolved
phosphorus concentrations. In moderate to high-nutrient areas (i.e. where SRP
concentrations are frequently greater than 1.0 jig L~1), such as nearshore areas close to
river mouths, a 50% decrease in soluble reactive phosphorus concentrations will likely
result in modest Cladophora biomass reductions of 25% or less, because when
temperature and light conditions are optimal, Cladophora can grow at relatively low
dissolved phosphorus concentrations. In nearshore areas where soluble reactive
phosphorus concentrations are already low (less than 1 jig L"1), a 50% decrease may
result in Cladophora biomass reductions of as much as 74%, depending on depth, with
greater proportional reductions occurring at deeper depths. However, when setting
phosphorus management objectives, it must be remembered that Cladophora growth
responds to dissolved phosphorus concentrations in a depth zone of 5 to 15 cm above
the benthos, and concentrations within this bottom layer may not reflect those in the
overlying water column. For the purpose of Cladophora management, Cladophora P
content and near-bottom dissolved P concentration, rather than water column SRP
concentration, may be more relevant variables to use for monitoring programs and for
setting management targets.
Because many of the obvious steps to reduce phosphorus loads have already been taken
over the past three decades, further reductions will be a challenge. Agricultural sources
can best be managed by focusing on "hot spots" where phosphorus concentrations are
high and/or runoff and erosion are excessive. For urban centers there may also be
significant industrial point sources of phosphorus that can be considered. Much of the
phosphorus uptake by Cladophora occurs between May and early July, and uptake during
this period can support growth for several weeks into the summer. Therefore phosphorus
reduction efforts will be most effective if they focus on the April - June period. While
Cladophora in areas near river mouths may respond quickly to any changes in river
nutrient loads, large, lake-wide reductions in Cladophora abundance will only occur when
the rate of phosphorus flow through the mussel filter feeding - excretion pathway is
attenuated. This pathway is controlled by offshore plankton production and physical
nearshore - offshore exchange rates, which are currently not well quantified. Because
plankton production is influenced by dissolved phosphorus concentrations in offshore
waters, and because these concentrations respond slowly to changing river loads, a lag
time of 5 to 10 years between decreased phosphorus loads and significant Cladophora
response can be expected.
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