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With few exceptions, cyanobacterial
proliferation is most effectively controlled by measures in the catchment.
“Internal” measures, i.e. those managing the water-body itself, in
most cases have low success chances unless nutrient loads from the catchment
are reduced sufficiently so that – in the longer term – their
concentration in the water-body is too low to sustain a major cyanobacterial
biomass. However, responses of a water-body may take a number of years until a
new equilibrium is reached and cyanobacterial blooms actually do disappear,
e.g. until phosphorus stored in the sediments has been sufficiently flushed out
of the system by low-P inflow, and/or until reed-belts and other vegetation has
sufficiently recovered to bind a fair share of the phosphorus load. In such
situations, internal measures may speed up recovery towards a new equilibrium.
Also, in some settings, external loads cannot be
sufficiently reduced to control cyanobacteria. While biological measures such
as manipulating fish stock and nutrient loading from the sediments have NOT
proven effective in such settings, controlling physical factors such as light
availability or vertical mixing (to improve conditions for the growth of other
phytoplankton that outcompetes cyanobacteria) has worked because these act
independently of nutrient availability.
When developing your Water Safety Plan (WSP)
your WSP-team will have assessed the conditions in the water-body, and might
select control measures among those suggested below. Note that controlling
cyanobacteria by measures in the water-body is particularly tricky and requires
a high level of expertise in limnology, particularly in plankton ecology, and
even if this is available, the uncertainty of predictions is higher than for
control measures in the management of the catchment, drinking-water offtake or
treatment system. Wherever possible, preference should therefore be given to
controlling nutrient loading from the catchment, and if the load target derived
for the given water-body is met, allowing sufficient time for the water-body to
gain its new equilibrium may be more adequate than implementing (often
expensive) internal measures.
If measures in water-body management are under
consideration, the middle of the page on assessing the risk of cyantoxin
proliferation gives some targets that measures should meet. For each
control measure, your Water Safety Plan should document the reasons for its
choice and the targets it should achieve as well as how you validate that it is adequate for achieving the
targets you set. Furthermore, a management plan
should be developed which defines how performance of the control measure is
operationally monitored and which corrective action should be taken if
monitoring indicates poor performance, or if incidents occur.
The monitoring
and surveillance of such control measures is crucial to ensure that they
are in place and effective. This does not primarily imply cyanotoxin
monitoring, but rather checking whether controls are operating as intended,
i.e. operational monitoring as
well as surveillance over plans,
design and maintenance of structures.
Stakeholder involvement: Water-body management usually involves a number of
different stakeholders, among which conflicts of interest are particularly
common with anglers. Success in implementation therefore is more likely If they
collaboratively develop and define the control measures to be implemented in
the given system.
Note: this is
not a comprehensive catalogue of examples, but merely intends to trigger your
own setting-specific concept of control measures !
|
Process
Step |
Examples of
control measures for water-body management |
Operational
monitoring, surveillance and verification |
|
Planning
and design |
Develop water allocation plans that optimise
the balance between stakeholder interests and water-body protection, ensuring
sufficiently high minimal flows to avoid conditions conducive for
cyanobacterial growth – targeting at minimum 1-2 % water renewal per
day (see Padisak
et al.)
|
Monitor water offtake and water flow; ensure
restrictions are implemented (site inspection) |
|
Plan and design measures to control light
availability, targeting conditions less favourable for cyanobacteria,
conducive to the growth of their less noxious competitors |
Review plans and applications for permits in
relation to characteristics of the water-body; Validate that
measures are properly designed and meet their target |
|
|
Plan and design measures to control mixing
intensity in order to suppress buoyancy-regulating cyanobacteria (see Visser et al.
1997 and 1999 for a successful case study)
|
||
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Plan and design measures to control internal
loading with phosphorus stored in the sediments, targeting a reduction of
water-body concentrations below 10-25 µg/L of total P, e.g. through
sediment capping or sediment oxidation |
||
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Plan and implement biological measures such as
fostering macrophyte and reed-belt growth or stocking fish |
||
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… ? |
… ? |
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Operation |
Artificial mixing, designed for any of the 3
purposes above, i.e. for controlling light availability, for suppressing buoyant
species, or for oxidising sediments |
Any efficient monitoring system to check
whether aerators are in operation as planned, e.g. visual inspection, records
of pump operation |
|
Biological measures such as planting
reeds [c1] or stocking fish (“food chain
manipulation[c2] ”) |
periodic visual inspection or mapping of reed
growth and/or determination of fish population sizes |
|
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…
? |
… ? |
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On to Documentation and Management plans
[c1]Planting reeds and other macrophytes
has often proven unsuccessful, with rapid die-off, in very eutrophic
water-bodies, because dense algal blooms render the water too turbid and/or too
anoxic near the sediment for them to grow well. This measure is most successful
in mesotrophic water-bodies in which these plants can bind a fair share of the
total phosphorus available and thus effectively contribute to reducing the
share available for cyanobacteria.
[c2]Food chain manipulation by
introducing planktivorous fish requires competency in fisheries biology,
careful planning and surveillance. It is by no means a priori a cheap measure, as surveillance is labour-intensive. No
general target can be defined, rather, it is necessary to test case by case to
which extend an influence can be exerted on cyanobacterial populations,
particularly as these often are not very amenable to reduction through
zooplankton grazing. Food-chain manipulation also has proven most successful in
mesotrophic water-bodies.