Introduction
Invasive seaweeds can rapidly
grow into an ecosystem and alter its structure and function, by displacing
native flora and reducing the overall diversity of species (Anderson, 2007).
This subsequently hampers socio-economic activities that are dependent on the
stability of living resources in that particular ecosystem (Doelle et al.
2007). With the increase in global trade, the rate of introduction of alien
marine seaweeds has been increasing over the past 20 years (Schaffelke et al.
2007).
One particular concern is the
impact of large quantities of debris that washes up on the beaches. During infestations seaweed
debris (a.k.a. wrack) wash up on beaches reducing the overall aesthetic value
of the beach, this especially true for these beaches used for recreational
purposes. Manual cleaning up of wrack tends to remove a lot of sand from the
beaches leading to more erosion (Fairweather et al. 2003). Beach users and tourists do not perceive
washed up wrack positively, according to Frampton (2010) this contributes to
reduced amenity value of the beach, which in turn can adversely impact the
economy of that area.
The objective of this article is to
review the management options against alien seaweed species that invades
intertidal ecosystems. It should be highlighted here that research in invasive
seaweed is sparse and options to manage, control or eradicate alien seaweed
infestation is limited (Schaffelke et al. 2006).
In contrast there are guides,
manuals and practical management plans and strategies for freshwater “weeds” (Anderson,
2007). For example the use of specialized
aquatic herbicides in Australia and New Zealand to manage aquatic weed
infestations in water ways (Chisholm et al. 2007). Therefore despite the apparent physiological
and ecological differences between invasive freshwater weeds and seaweed; it is
important to look into applicable techniques used in management of aquatic
weed.
The following sections will look
into successful strategies used to manage or eradicate seaweed along with some
applicable aquatic weed management techniques.
1. Tarpaulin
and chlorine treatment used to eradicate Caulerpa taxifolia in Southern
California (Anderson, 2003)
2. Aquatic
herbicides (Chisolm et al.
2007)
3. Mechnical
Methods (Anderson, 2007; Chisholm, 2007)
4. Biolgical
Control (Anderson, 2007; Chisholm 2007)
1. Tarpulin and Chlorine treatment
This method was used to eradicate Caulerpa
taxifolia infestation that was discovered in Agua Hedionda Lagoon (150 Ha
area) of Southern California (Figure
1), in
June 2000.
 |
| Figure 1 - Location of C. Taxifolia infestation Agua Hedionda lagoon (adapted from Anderson, 2003) |
Small
polyvinyl chloride (PVC) frames were placed over the Caulerpa taxifolia
plants and then covered with 20 mil black PVC sheeting. The sizes of the tarps
ranged from 500 m2 areas for the few large colonies initially discovered,
to about 1 m2 for small plants found in later surveys.
The sides
of the tarps were anchored and sealed to the bottom with gravel filled bags. An
overhang was provided between the edge of the colony and edge of the bagged area
to ensure that a margin of un-infested area was also covered and treated.
Initially, liquid sodium hypochlorite (ca. 12% stock solution) was injected
into the tarped areas via ports in the PVC tarps fitted with caps. Smaller
colonies were later covered with the PVC tarps without a frame, beneath which
several 2.5 cm dia. Solid chlorine-releasing tablets (‘pucks’) were placed.
 |
| Figure 2 - Underwater containment and treatment system used to apply chlorine (Anderson, 2003) |
Over the
three year period after the treatment it can be observed that the C.
taxifolia was completely eradicated from the river (Figure
3). The
success of this strategy also depended on early detection of infestation,
followed by timely response (tarps and treatment was deployed within 17 days of
discovery) that involved all key stakeholders and relevant agencies (Anderson,
2003).
 |
| Figure 3 - Progress in reduction in areal coverage of viable C. taxifoloa from 2000 to 20003 (adapted from Anderson, 2003) |
2. Aquatic Herbicides
The most widely used
herbicides to treat aquatic infestations in Australia and New Zealand are Diquat
and Endothal. These herbicides are safe for human and aquatic fauna and are not
persistent in the environment.
When applied properly they display high level
of phytotoxicity, and degrade rapidly in water. These herbicides are delivered
after mixing with an adjuvant such as Aquagel, marketed as Hydrogel®.
The mixture can be applied to water from
a range of equipments such as gun and hose or a boat mounted boom etc. Diquant
mixed with such an adjuvant can be used to treat over 60 – 80 hectare area
infested with weed.
Hornwort
invasion Mouttere Stream, Nelson, South Island, New Zealand: the target was to eradicate
Hornwort from the location. Aquagel treatments to control Hornwort were first
made in March 2002. Aquagel was applied in strips (60 cm wide), over about 800
m of stream; 195 L of Hydrogel covered 0.7 ha. The cost of this treatment was
NZ $ 4500. After 6 weeks, all Hornwort had collapsed, and was no
longer noticeable in the stream. Spot treatments were conducted 12 months
later. Monitoring of the stream in November 2003 and February 2004 found no
Hornwort in the treated area.
Egeria
densa invasion of Geroge River: Hydrogel® was successfully used to eradicate Egeria
densa infestation of George River, Sydney, Australia in January 2007. The
area measured approximately 2500 m2. With a treatment of 15 L of
Hydrogel®, the infestation was completely eradicated within 2 months
(Figure 4 and Figure 5). The
total cost of this treatment was AUS $ 600 (Chisolm et al. 2007).
 |
| Figure 4 - Egeria densa infestation in section of George River, Sydney Australia (adapted from Chisolem et al, 2007) |
 |
| Figure 5 - Egeria densa controlled after 2 months of Hydrogel treatment (adapted from Chisolem et al, 2007) |
3. Mechanical Methods
Mechanical methods comprise a
range of physical treatments in or around infestations, with the objective of
removing, burying or killing established seaweed (Anderson, 2007).
Some of the mechanical methods such as hand weeding,
mechanical digging, rototilling, and small-scale dredging with their pros and cons
and estimated costs are discussed in this section.
Hand weeding: This method is used in cases of
small-scale infestation, with patches not exceeding 1 m2 area. On
the flip side hand weeding is labor intensive and costs may climb over US$
10,000 per ha (Chisholm et al. 2007).
Mechanical Digger: This method is most
effective in artificial canals and areas that are shallow and near the
shoreline that allows access for a digger. Cost of digging hinges on the width
and extent of weed infestation.
Disadvantages of mechanical digging include the indiscriminate
removal of benthic fauna and fish species. In addition this method can
potentially cause highly turbid and anoxic conditions in the water stream, and
may stimulate weed growth at areas that are widened and deepened by digging (Chisholm
et al. 2007).
Rototilling: This method has been used to
uproot Lagarosiphon in water depths of between 1.5 and 4 meters. The depth of
sediment penetration affects the results. Deep rototilling ( 3 – 5 cm sediment
depth) is more costly (US$ 5000 per hectare) than shallow rototilling (US$ 1000
- 2000 per hectare).
Rocks and hard substrate prevents effective
rototilling. Re-growth of aquatic weeds from distributed lakebeds can be
widespread. Rototilling is very costly and ineffective compared to other
methods (Chisholm et al. 2007).
Small-scale diver assisted dredging: Dredged
material is either pumped onshore, or into holding/settling tanks, or passed
through sufficiently fine mesh to retain algal fragments and other propagules.
This is suitable to small to moderate scale infestations (0.1 – 2 Ha) that are
fairly shallow (less than 5 m depth). Poorly
consolidated substrata may preclude this due to high turbidity generated during
dredging (Anderson, 2007).
4. Biological Control
Presently there are no programs
that utilize biological control against marine algae. However there are several
programs that utilize biological controls against invasive aquatic weeds. Sea Urchins and mollusks are being tested for
their efficacy in controlling invasive seaweed (Anderson, 2007).
Grass Carp: Ctynopharyngodon idella (Valenciennes)
is a widely used fish species to control aquatic weeds; which feeds on a broad
range of weeds (submerged and floating) in New Zealand. However they pose a
threat to native aquatic plants as they feed indiscriminately. It was estimated
that approximately 30 fish per hectare were required to provide adequate weed
control, which equates to a control cost of $750 per hectare (Chisholm et al.
2007)
Conclusion
Management and/or
eradication of alien seaweed invasion require thorough understanding of seaweed
biology and reproductive behavior. In addition success of these strategies
depends on communication and involvement of all stakeholders. The management or
eradication strategy also hinges on the funds available and how adverse impacts
infestation weighs against ecological as well as the socio-economic costs of
not taking action.
References
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