A geochemical treatment aimed at slowing the eutrophication of lakes and the outbreaks of blue-green algae seems to fulfill its promises, at least for a lake in Montérégie.
Indeed, since it was the subject of this treatment five years ago,
Lake Bromont has posted
70% lower
phosphorus concentrations, the water is clearer and the
frequency of blue algae blooms -vert is 4 to 5 times lower.
Since the treatment applied in 2017, Lake Bromont has shown 70% lower phosphorus concentrations, the water is clearer and the frequency of blue-green algae outbreaks is 4 to 5 times lower."For certain types of lakes, this treatment would make it possible to alleviate the problems caused by the historical accumulation of phosphorus in the sediments", observes
Raoul-Marie Couture , professor in the Department of
Chemistry at
Laval University and holder of the Chair of
Sentinel
North research in the geochemistry of aquatic environments.
"However, for the effectiveness of the treatment to be maintained over
time , it is essential to continue to apply measures to reduce external phosphorus inputs and measures to
prevent shoreline erosion", insists- he.
Lac Bromont is a small body of water of 0.
which, due to human activities in its
catchment area , was showing signs of
rapid
eutrophication . Blooms (algal blooms) of blue-green algae were observed there a hundred
days a
year between May and October.
Research had shown that lake sediments were the main source of phosphorus to the lake during the summer.
This is what led the municipality of Bromont and the Ministry of the
Environment and the Fight against Climate Change to resort, in the
fall2017, to a treatment aimed at sequestering lake phosphorus in order to make it inaccessible to microorganisms.
About 174 tons of Phoslock were applied in this lake, an operation that cost $650,000.
Since then, Professor Couture's team and that of Professor Dolors Planas, from the
University of Quebec in
Montreal , have been studying how Lake Bromont reacts to this treatment.
Created in
Australia in the late 1990s, Phoslock contains
lanthanum , which binds to phosphorus in the water column and in the top layer of sediment.
The resulting inert compound
reduces the
amountof phosphorus available to microorganisms such as blue-green algae.
The monitoring carried out since 2017 by the Action conservation du bassin
versant du lac Bromont group points to a
clear improvement in the quality of the lake's water.
A
study recently published by Professor Couture's team in
Environmental Science: Processes & Impacts , however, brings some reservations.
This study, whose first author is
doctoral student Wessam Neweshy, shows that the treatment did indeed lead to the sequestration of some of the phosphorus in the sediments.
However, between 20% and 43% of the phosphorus present in the sediments is likely to be put into
circulationby
microbial
activity . “Some of the lanthanum has bound to other elements, including
iron and
carbon from dissolved
organic matter . This lanthanum is no longer able to bind to phosphorus,” Professor Couture explains.
In
theory , the Phoslock treatment was to be effective for several decades.
"If the sediment supply increases, the lanthanum will find itself buried more quickly and it will not be able to bind to phosphorus, warns the
researcher . The treatment will have to be started again, which
still represents significant costs."
Could this treatment help other lakes in Quebec struggling with blue-green algae?
"Phoslock treatment seems to give a good boost to lakes with a heavy legacy of phosphorus, but without conservation measures its effectiveness could quickly fade. The geochemistry of each lake must be taken into consideration in the analyses. The work we are conducting will help establish the appropriate
dosage of treatment for a given lake, its cost and its effectiveness over time, which could help in decision-making."
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