Toxic algal blooms in NZ - Expert Q
27 Feburary 2017
Last week, the New Zealand Government announced its intention to make 90 per cent of NZ's waterways 'swimmable' by 2040.
Yet with Summer hitting its peak, rivers and lakes are across the country are being closed to swimmers due to toxic
algal blooms.
The SMC asked Cawthron Institute's Dr Susie Wood about whether these toxic algal blooms are on the increase and if the
government's aims for swimmability are achievable.
What causes algal blooms? Are they an inevitable occurrence, or are there factors that make them more common?
"Cyanobacteria (also known as blue-green algae or toxic algae) are an ancient group of micro-organisms that require
sunlight and nutrients to grow. They are a natural and important part of all our freshwater habitats. Yet under certain
conditions some species can rapidly multiply and form blooms (large accumulations of cells) which can create water
quality problems.
"Many of these species also produce toxins, and contact with, or consumption of, water or organisms that have lived in
the water, poses a health risk to humans and animals.
"In our lakes, blooms are caused by cyanobacteria that float in the water (known as planktonic cyanobacteria). The
conditions that cause planktonic blooms are reasonably well understood and have been researched for many decades, both
internationally and in New Zealand.
"There are a number of causes but one of the key drivers is an increase in nutrients entering lakes. This is usually
caused by changes to the land around our lakes (e.g. removal of native vegetation and increases in agriculture, urban
settlements, or forestry).
"Introduced or invasive species can also promote blooms, for example, fish such as Perch eat small aquatic organisms
that would usually feed on and reduce cyanobacteria. Blooms are also more common in summer when water temperatures are
high and physical conditions within the lakes are favourable for cyanobacterial growth.
"In our rivers, cyanobacterial blooms form over rocks on the bottom (benthic blooms).This short video shows what benthic blooms look like.
"Benthic blooms in rivers are a relatively new problem. Cawthron scientists are among the world leaders in researching
reasons for these blooms and understanding the risks they pose. Unlike planktonic blooms, benthic blooms do not occur in
rivers with high levels of nutrients, rather in rivers with relatively good water quality.
"Our research indicates that increased concentrations of nitrogen and fine sediment promote these blooms; both factors
that intensify when land use is modified. Blooms are also more likely to occur in summer, when there are long periods
without rainfall which wash the cyanobacteria out of the rivers.
"Many Regional Councils do a good job monitoring lakes and rivers for toxic cyanobacteria, but it is not possible to
test all sites. We strongly recommend that the public familiarise themselves with what cyanobacteria look like, the
video above offers a good first resource to do this."
Where do they happen - are there areas that blooms are more common?
"Planktonic blooms usually occur in lowland lakes where the land surrounding them has been modified, and as a result there are high levels
of nutrients in the lake. Blooms have been reported in large and small lakes throughout New Zealand including well known
lakes such as Rotorua and Forsyth/Te Roto o Wairewa.
"High levels of toxins are often associated with these blooms, resulting in warning signs being required to stop
recreational water users from coming in contact with the water.
"Benthic blooms tend to occur in gravel lowland rivers, they are more common in rivers on the eastern side of New Zealand, but have been
reported from all regions.
"Our recent study identified blooms in over 100 rivers."
Are we seeing any trends in the number of blooms or when they appear?
"Both planktonic and benthic blooms are increasing in abundance and severity. A survey conducted in the 1980s identified
planktonic blooms in 33 lakes. A survey I undertook in 2000-2003 identified blooms in an additional 18 lakes, and I now
estimate that blooms occur in well over 100 lakes.
"Each year we receive new reports of blooms occurring, particularly in our lowland lakes.
"Historic records of benthic cyanobacteria in our rivers are lacking, but over the last 10 years reports of rivers with
blooms have increased from less than 10 to over 100. Each year we get reports of new regions and rivers experiencing
blooms.
"For example, this year there are reports of blooms in the Gisborne region - a region where blooms had not been reported previously."
What impact will climate change have on algal blooms?
"Climate changes will almost certainly increase both planktonic and benthic cyanobacterial blooms. Cyanobacteria grow
faster in warmer temperatures and are likely to out-compete other types of algae. More stable conditions in lakes and
rivers will also favour cyanobacteria.
"An increase in storm events may also promote blooms, particularly in lakes where rainfall events cause nutrients from
the surrounding land to be washed into the waterbody."
The Government has just announced an initiative to make 90% of rivers swimmable by 2040, is this a practical and
possible goal? Is this based on the science?
"This recent announcement is for lakes and rivers, although rivers appear to be receiving more attention.
"This is a very aspirational goal that will take commitment from many parts of society. I commend the funding increase
and focus on data collection. As a freshwater scientist, it’s pleasing to see our Government committing to improving and
protecting New Zealand’s precious freshwater ecosystems.
"Below I make some specific comments related to toxic cyanobacteria and swimmability.
Rivers
"Currently swimmability of rivers is based on levels of E. coli, and does not consider toxic cyanobacteria. This is due to a lack of robust datasets on cyanobacterial blooms in New
Zealand rivers and is an acknowledgement that there are still gaps in our knowledge as to why blooms occur and why they
have increased in abundance.
"Currently there is a risk that although a site might be swimmable based on E. coli levels (and the Ministry for Environment swimming maps), swimming should not occur due to toxic cyanobacterial blooms.
Examples where this could occur (in summer) include the Hutt River in Wellington, and the Maitai River in Nelson.
"While I encourage all actions to reduce E. coli concentrations and improve freshwater quality, there is a risk that in some rivers these actions might not reduce
cyanobacterial blooms. Further research is urgently needed to increase knowledge on why blooms form and the actions that
can be taken to reduce them.
"It is important to note that the swimmabilty maps on the Ministry for Environment website do show when a site has recorded a cyanobacterial bloom, but this information is not taken into consideration when a site
is ‘graded’ for swimmability."
Lakes
"Currently swimmability of lakes is based on levels of cyanobacteria – yet for most lakes in New Zealand this data is
lacking, and the current swimming maps are largely based on modelled data. For example, there is only data on cyanobacterial concentrations for three lakes in the entire South Island.
"There are 3821 (>1 ha) lakes in New Zealand, yet we have robust water quality data for less than 200 of these
(approximately 5%). There are also severe scientific biases among existing lake datasets. For instance, most monitored
lakes are in highly modified catchments and warmer regions, and oligotrophic (low nutrient) high-altitude lakes cannot
be readily monitored.
"This means that making accurate predictions of lake swimmability across New Zealand is extremely challenging.
"It is going to be challenging for the government to quantitatively measure improvements in the swimmability of New
Zealand lakes when currently the predictions are based on limited and biased datasets.
"E. coli also poses a risk to humans in lakes, but as the datasets are so limited, they could not be included in the current
assessment of lake swimmability. This knowledge gap needs to be addressed.
"In addition to funding restoration, more effort is needed to improve knowledge of water quality and ecology integrity,
and to further understand and protect New Zealand lakes. This information would also assist in informing effective
restoration."