Toxic algal blooms in NZ – Expert Q&A

UPDATED: Last week, the New Zealand Government announced its intention to make 90 per cent of NZ’s waterways ‘swimmable’ by 2040.

Example of a planktonic cyanobacterial bloom in Lake Horowhenua (Levin). The wind has blown the buoyant cells to the edge of the lake where they form a thick scum. Susie Wood/Cawthron Institute.

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.

UPDATED with Q&A with Niwa Chief Scientist for Freshwater and Estuaries Dr John Quinn.

Please feel free to use these comments in your reporting – supplied photos are available on

Dr Susie Wood, Senior Scientist, Microalgae and Algal Biotechnology, Cawthron Institute:

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 1980’s 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.


“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.”


“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.”

Dr John Quinn, Chief Scientist for Freshwater and Estuaries, Niwa

What causes algal blooms? Are they an inevitable occurrence, or are there factors that make them more common?

“Algae are essential parts of freshwater ecosystem foodwebs. Algae use light and nutrients to create plant biomass by photosynthesis and fuel food chains (via algal grazing invertebrates, eaten by predatory invertebrates and fish eaten by larger fish and birds).

“But you can have too much of a good thing (as discussed in relation to periphyton).

“Algae become a nuisance when their abundance reaches high levels that result in the degradation of other freshwater values, such as water clarity and bottom water dissolved oxygen in lakes, or night-time dissolved oxygen in rivers.

“Harmful effects of algal blooms on swimming, drinking water, and other recreational water contact, are increased when they consist of toxin-producing cyanobacteria (also known as blue-green algae). These can make water contact or ingestion (during swimming or drinking) dangerous.

“People are most likely to be exposed to highly toxic blooms when they are concentrated in surface floating scums and/or are by being blown towards the down-wind shore of a lake. Dogs are susceptible to cyanobacterial poisoning, likely due to the combination of sensitivity and their behaviour of eating smelly material, including floating or stranded clumps of cyanobacteria mat or scums.”

Where do they happen – are there areas that blooms are more common?

“There are two types of algal blooms – those caused by phytoplankton, that are in the water column (predominant in lakes and the ocean), and by periphyton, that are attached to the bed (predominant in rivers).

“Like all plants, they need light and nutrients (particularly nitrogen and phosphorus) and growth rates also increase with water temperature. Many lakes separate into layers in summer (i.e. stratify) due to surface water warming. This increases the duration, intensity and exposure to sunlight of phytoplankton in the surface layer. All these factors tend to result in phytoplankton algal blooms being generally greatest in summer in stratified, nutrient-rich lakes.

An international review showed that the risk of cyanobacteria dominance of lake phytoplankton increased markedly as the concentration on total nitrogen increased above 1000 mg/m3 and total phosphorus increased above approximately 40 mg/m3.

“Periphyton in rivers also tends to be more common in summer due to influences including long day-length, high light, warm temperatures and longer gaps between high flows that scour growths from the river bed. Cyanobacterial periphyton mats have been associated with moderate to high levels of dissolved nitrogen (as nitrate plus ammonium). These mats are particularly adept at trapping fine sediment and appear to be able to scavenge phosphorus in this sediment to supplement phosphorus in the water column, enabling them to develop high biomass where the water has relatively low dissolved phosphorus.”

Are we seeing any trends in the number of blooms or when they appear?

“There have been no analyses of trends in blooms per se. However, there have been several analyses of trends in lake and stream algae across the entire range of biomass levels; none of these analyses distinguishes between blooms from other algae abundance levels.

“A recent analysis of water quality trends in NZ lakes between 2004 and 2013 included trends in phytoplankton biomass in 39 lakes. Phytoplankton biomass was increasing in three of the lakes, decreasing in 15, and the trend direction was indeterminant in the remaining 21 lakes. The median trend magnitude across all 39 lakes was very close to zero. Although the sample size was very small compared to the large number of lakes in New Zealand (nearly 4000 lakes larger than 1 ha), these observations provide circumstantial evidence for the prediction that phytoplankton blooms are not increasing in frequency in most lakes.

“Periphyton abundance in streams is most often measured as the proportion of periphyton-covered stream bed, rather than biomass. A national-scale analysis of trends in periphyton from 1990-2009 at 73 monitoring sites used annual maximum cover, which can be seen as an estimate of bloom potential. In that study, trends in annual maximum periphyton cover were negative (decreasing) at 14 sites, positive (increasing) at 4 sites, and indeterminant at the remaining 55 sites. As with lakes, these observations provide circumstantial evidence for the prediction that periphyton blooms are not increasing in frequency in most stream reaches.

“More recent trend analyses have been carried out at the regional scale, using data from regional council river ecology monitoring programmes. One example comes from a recent state and trends report for the Wellington region that included trends for the period 2003-2011. In that report, sites with increasing trends in annual maximum periphyton cover outnumbered sites with decreasing trends. However, most of the monitoring sites had relatively low maximum annual cover regardless of trend direction; only three sites had strongly increasing trends (> 20% per year) in combination with high cover – these sites have high potential to form nuisance periphyton abundance blooms.

“A second example comes from an analysis of trends in periphyton biomass in the Horizons region for the period 2008-2015. In that study, sites with increasing trends in periphyton biomass also outnumbered sites with decreasing trends. However, increasing trends were limited to those sites with lower than average periphyton biomass, which would not qualify as blooms. These two regional examples suggest that, while periphyton abundance may be increasing at some monitoring sites, there is little evidence of increases in the incidence of periphyton blooms. Several regional councils have recently expanded their periphyton monitoring efforts, both by adding new sites and increasing the frequency of monitoring. Several years of periphyton data will be required to carry out rigorous trend analyses in these regions.

“Most trend analyses to date have focused on long-term (> five year) trends and effects of seasonal variation in these analyses is not accessed. However, seasonal variation in periphyton abundance has been reported, in association with seasonal cycles in flood frequency, water temperature, light and nutrient uptakes. Regular seasonal cycles are often masked by the effects of flood flows that can remove periphyton in any season. As noted above, both phytoplankton and periphyton abundance tends to increase in summer.”

What impact will climate change have on algal blooms?

“There is some evidence that warmer temperatures due to climate change will increase harmful algal blooms in lakes. Mechanisms include that (i) cyanobacteria tend to have a competitive advantage over other algae types at high temperature and (ii) warmer temperature strengthen lake stratification, which also favours cyanobacteria due to their ability to control their buoyancy using gas vacuole and so avoid settling out into the deep layer.

“In addition to direct of effects of climate change, indirect effects mediated through changes in nutrient supplies may alter the frequency and/or severity of algal blooms. For example, climate warning may lead to agricultural intensification at higher latitudes and elevations than at present. These changes in land-use and associated fertiliser and effluent application may in turn lead to nutrient enrichment in downstream streams and lakes, which can stimulate algal blooms. Independent of changes in agriculture, nutrient runoff from land to streams and lakes is predicted to increase in areas with forecasted increases in rainfall intensity, and this increased runoff may also stimulate algal blooms.

“For streams and lakes located in areas that are predicted to have reduced stream flows (i.e. east coasts of both North and South Islands and Northland), algal blooms may increase in frequency due to the decreased frequency in scouring flows in rivers and decreased flushing (increased residence time) in lakes. Conversely, in areas predicted to have higher flood frequencies (e.g., West coast of South Island and glacial areas characterised by meltwater floods), algal blooms are likely to decrease in frequency.”

The Government has just announced an initiative to make 90% of waterways swimmable by 2040, is this a practical and possible goal? Is this based on the science?

“The science indicates this is possible, but it is a stretch goal. Success will require substantial effort throughout catchments using a range of control technologies on land, along flow paths, and/or within lakes. The limit setting and collaborative community processes in the National Policy Statement on Freshwater Management provide a good basis for this challenge.

“Harmful algal blooms are generally a more important risk to swimming than pathogens (disease causing organisms, indicated by E. coli) in lakes, whereas pathogens are generally more important in rivers. This is because lake conditions promote the die-off of pathogens (via UV radiation, settling and predation) but promote growth of harmful algal blooms under the right conditions of nutrients etc. described above.

“The risk to swimming from pathogens is expected to be more readily controlled than risk due to harmful algal blooms because pathogens input is mostly in more controllable surface flow pathways, pipes and livestock access to streams, whereas influences on harmful algal blooms from nutrients (particularly nitrogen that travels mainly via groundwater) and climate change effects will be more difficult to control.

“Some NZ aquifers have decadal groundwater residence times (e.g., in the central North Island) that result in long delays between reduction in nitrogen leaching from improved land management and reductions in nitrogen levels in surface waters. Phosphorus also has legacy effects (reflecting the ‘ghost of pollution past’) because it accumulates in lake sediments, although this can be managed using in-lake phosphorus inactivation agents and bottom water aeration. A handbook on lake restoration in NZ to be published this year summarises many of the currently available tools for managing nutrients and harmful algal blooms.”