Important note: interpreting the guidelines framework
The following guidelines provide a recommended approach that is considered best practice for many management circumstances given current understanding of cyanobacterial risks in New Zealand fresh waters. However, local decisions about whether to follow the guidelines’ approach should ultimately result from consideration of site-specific factors (such as resource availability, management priorities and historical understanding of local bloom conditions) as well as the guidance offered in this document. Monitoring agencies may have reason to depart from the methodologies suggested in these guidelines.
Part A: Planktonic cyanobacteria
3.1 Planktonic cyanobacteria: an introduction
Cyanobacteria inhabit all natural waters and usually only become a problem when they increase to high concentrations, forming ‘blooms’. Cyanobacterial blooms have been a regular occurrence in many New Zealand lakes since the 1970s. However, they have become increasingly prominent in recent decades, possibly in association with anthropogenic eutrophication and climate change (Wood, Jentzsch et al, 2008). The growth of cyanobacteria and the formation of blooms are influenced by a variety of physical, chemical and biological factors. Key variables within New Zealand water bodies that can lead to bloom formation are detailed in Section 4.6.
Planktonic cyanobacteria in New Zealand are now known to produce the following cyanotoxins: microcystins, nodularin, anatoxin-a, cylindrospermopsin, deoxycylindrospermopsin and saxitoxins (Stirling and Quilliam, 2001; Wood, Stirling et al, 2006; Wood, Selwood et al, 2007; Wood, Rasmussen et al, 2007). (See Appendix 1 for information on cyanotoxin distribution in New Zealand). The health risks associated with cyanotoxins are greatest during bloom events. The highest concentrations of cyanotoxins are usually contained within the cells (intracellular), and toxin concentrations dissolved in the water (extracellular toxins) are rarely reported above a few parts per billion (Chorus and Bartram, 1999). People using water bodies for recreational purposes are most likely to experience maximum exposure when a cyanobacterial bloom develops or forms surface scums near water entry points. Wind-driven accumulations of surface scums can result in toxin concentrations increasing by a factor of 1000 or more, and such situations can change within very short time periods (hours).
In New Zealand, research is still underway to determine which species produce the various cyanotoxins. It is therefore recommended that when a species is known to be a toxin producer elsewhere in the world, it should be regarded as potentially toxic in New Zealand until proven otherwise. These guidelines are aimed at protecting human health during recreational activities. Details on the methods use to derive the action alert-level values are given in Appendix 2. For a detailed risk assessment of the risks posed by frequent occupational exposure (eg, daily contact), see de Wet, 2008.
3.2 Alert-level framework: planktonic cyanobacteria
|Alert level||Actions |
(See section 2.4 for the recommended framework for roles and responsibilities relating to actions, and the text box at the beginning of Section 3 for advice on interpreting the guidance in this table.)
|Surveillance (green mode) |
Situation 1: The cell concentration of total cyanobacteria does not exceed 500 cells/mL.a
Situation 2: The biovolume equivalent for the combined total of all cyanobacteria does not exceed 0.5 mm3/L.
|Alert (amber mode) |
Situation 1: Biovolume equivalent of 0.5 to < 1.8 mm3/L of potentially toxic cyanobacteria (see Tables 1 and 2); or
Situation 2c: 0.5 to < 10 mm3/L total biovolume of all cyanobacterial material.
|Action (red mode) |
Situation 1: ≥ 12 μg/L total microcystins; or biovolume equivalent of ≥ 1.8 mm3/L of potentially toxic cyanobacteria (see Tables 1 and 2); or
Situation 2c: ≥ 10 mm3/L total biovolume of all cyanobacterial material; or
Situation 3e:cyanobacterial scums consistently present.
a) A cell count threshold is included at this level because many samples may contain very low concentrations of cyanobacteria and it is not necessary to convert these to a biovolume estimate.
b) In high concentrations planktonic cyanobacteria are often visible as buoyant green globules, which can accumulate along shorelines, forming thick scums (see Appendix 3). In these instances, visual inspections of water bodies can provide some distribution data. However, not all species form visible blooms or scums; for example, dense concentrations of Cylindrospermopsis raciborskii and Aphanizomenon issatschenkoi are not visible to the naked eye (see Appendix 3).
c) This applies where high cell densities or scums of ‘non-toxigenic’ cyanobacteria taxa are present (ie, where the cyanobacterial population has been tested and shown not to contain known toxins).
d) Bloom characteristics are known to change rapidly in some water bodies, hence the recommended weekly sampling regime. However, there may be circumstances (eg, if good historical data/knowledge is available) when bloom conditions are sufficiently predictable that longer interval sampling is satisfactory.
e) This refers to the situation where scums occur at the recreation site for more than several days in a row.
f) Cyanotoxin testing is useful to: provide further confidence on potential health risks when a health alert is being considered; enable the use of the action level 10 mm3/L biovolume threshold (ie, show that no toxins are present; and show that residual cyanotoxins are not present when a bloom subsides).
3.3 Details of the framework: planktonic cyanobacteria
Three levels of monitoring have been identified: surveillance (green mode), alert (amber mode) and action (red mode) (see Decision Chart 1). There are some important points to note in relation to sampling and cell concentrations for these different alert levels.
For a start, the cell concentrations, or biovolumes, that define the levels apply to samples of the recommended type (ie, composite 50 cm hose-pipes, see Section 4.3.2) that are taken at a representative location(s) in the water body (ie, the likely or designated recreational areas). A single site that is representative of the recreational area is the absolute minimum, but multiple sites are warranted if the area is large, due to the potential for large spatial variations from buoyant cyanobacteria that aggregate under specific physical conditions (eg, calm and still water). Cyanobacteria can still form surface scums at low population densities, particularly if the wind pushes the cyanobacteria to one side of a water body. It is good practice to visually inspect waters regularly under calm conditions from multiple viewpoints. The number of samples taken depends on factors such as the size of the water body and the degree of use of different recreational sites (see Section 4.3.1).
The rationale for the use of biovolumes (as opposed to cell concentrations) is given in Appendix 4. Table A4.1 in Appendix 4 gives biovolumes for common problematic New Zealand species. In many instances this will enable a direct conversion of cell concentrations to biovolumes. For species not listed in Appendix A4.1 it will be necessary to establish their biovolume by undertaking cell measurements. Formulas for calculating biovolume for common geometries of cyanobacteria cells are given in Table A4.2 in Appendix 4.
It is also important to note that in some circumstances monitoring agencies may have good reasons to depart from some of the recommended actions in the three-tier framework. For example, if there is a long history of monitoring and management for a particular water body, a monitoring agency may not consider it necessary to adopt high-frequency sampling (eg, weekly) or to undertake toxin testing in order to confidently characterise recreational health risks.
3.3.1 Surveillance: green mode
Surveillance (green mode) is triggered when cyanobacteria are first detected at low levels in water samples, signalling the early stages of a possible bloom. A lower limit of up to 0.5 mm3/L or 500 cells/mL is given, because the presence of some cyanobacteria in water samples is common and does not indicate the early stages of bloom formation. Sampling and cell counts should be undertaken weekly or fortnightly (from spring to autumn) where cyanobacteria are known to proliferate. Fortnightly sampling frequency may be appropriate for the surveillance level where non-toxigenic species are present and the risk is perceived to be lower (eg, in a low-usage recreational water body).
3.3.2 Alert: amber mode
Alert (amber mode) is triggered when either there is a biovolume equivalent of 0.5 to < 1.8 mm3/L of potentially toxic cyanobacteria (see Tables 1 and 2 below) or the biovolume range is 0.5 to < 10 mm3/L for the combined total of all cyanobacterial material where known toxins are not present. This accommodates the transition to the action level (red mode) – situation 2 (see Decision Chart 1, ie, > 10 mm3/L biovolume). (See Appendix 2 for further explanation on how the values are derived.)
The alert level (amber mode) requires notification and consultation with public health units and ongoing assessment of the status of the bloom (see Section 2.4). This consultation should start as early as possible and continue after the results of toxin analysis become available (if used). The requirement for information on toxins will depend on advice and discussion with public health units, and on circumstances such as whether the cyanobacteria are known toxigenic species or whether there is a past history of toxin production. The sampling frequency depends to some extent on the sensitivity and usage of the area, as well as historical knowledge of the site. For example, twice-weekly sampling may be justified where there is a pressing need to issue advice for ongoing use if the site is being used heavily by recreational users or a special event is imminent. In most circumstances weekly sampling provides sufficient information to assess the rate of change of cyanobacterial populations, and to judge the population growth rate and spatial variability – and therefore the hazard.
3.3.3 Action: red mode
The action level (red mode) is triggered when representative samples exceed either:
- situation 1: a concentration of > 12 μg/L total microcystins, or a biovolume estimate of 1.8 mm3/L of potentially toxigenic cyanobacteria (see Tables 1 and 2 below); or
- situation 2: > 10 mm3/L for total biovolume of all cyanobacteria where known toxins are not present; or
- situation 3: cyanobacterial scums are consistently present.
(See Appendix 2 for further explanation on how the values are derived.)
In the action level (red mode), public health units (see Section 2.4 for details on the role and responsibilities of each agency) should warn the public of the existence of potential health risks. This should be done (although not exclusively) by media releases and by requesting territorial authorities to erect signs at affected water bodies. An example of information that should be included in a media release is given in Appendix 5. Appendix 6 provides a warning sign template. Warning signs should provide the public with information that enables them to make informed decisions about appropriate use of the water body. Also, local doctors should be encouraged to report any illness that may be linked to contact with water containing cyanobacteria to the public health unit.
The action level (red mode) situation 1 guideline is designed to protect against health effects of repeated exposure to cyanobacterial toxins ingested during recreational activity. Situations 2 and 3 guidelines apply where there is an increased probability of respiratory, irritation and allergy symptoms from exposure to very high cell densities of cyanobacterial material, irrespective of the presence of toxicity or known toxins.
The biovolume threshold of action level (red mode) situation 1 may be used to trigger the action level (red mode). If subsequent toxin analysis is undertaken and is negative, the mode may revert to alert (amber mode) in the biovolume range 0.5 to < 10 mm3/L. If cell numbers continue to increase to < 10 mm3/L, action (red mode) situations 2 and 3 guidelines definitions apply; in other words, either the total biovolume of all cyanobacterial material exceeds 10 mm3/L or cyanobacterial scums are consistently present.
3.3.4 Changes in alert levels over time
In slow-moving water bodies toxin testing is usually only warranted at 7- to 10-day intervals. Research has shown that toxin concentrations in a cyanobacterial population can change, but that it is unlikely to become completely non-toxic within a few days. It is therefore recommended that the alert level not be changed from a higher to a lower level (eg, from action to alert) until two successive results (biovolumes) from representative samples have been recorded. The sampling interval between these should be greater than seven days.
Note that cell counts and biovolumes may not give a true indication of toxin levels in a water body. As cyanobacteria die their cells break open, releasing the toxins contained in them. It is therefore possible to have elevated levels of dissolved cyanotoxins corresponding with low cell counts. This was shown in a study at Lake Rotoiti (Rotorua), where biovolumes were below 1.8 mm3/L but total microcystin concentrations were over 12 µg/L (Wood, Briggs, Sprosen et al, 2006). Microcystins have been shown to persist dissolved in water (ie, extracellularly) for up to 21 days in a water body during post-bloom decline (Jones and Orr, 1994). For greatest confidence in decisions to downgrade alert levels, toxin testing should be considered to ensure concentrations are below the recommended thresholds. Some species, particularly Cylindrospermopsis raciborskii, actively transport toxins out of their cells (Chiswell et al, 1999), resulting in high extracellular toxin content, and so toxin testing is recommended when C. raciborskii is present.
* New toxic species continue to be identified, and all cyanobacteria should be regarded as potentially toxic until proven otherwise.
Table 2: Summary of known cyanobacteria genera that produce each cyanotoxin*
|Anabaena1**||Anatoxin-a, anatoxin-a(S), cylindrospermopsins, microcystins, saxitoxins|
|Aphanizomenon 2||Anatoxin-a, cylindrospermopsins, microcystins, saxitoxins|
|Cylindrospermopsis 3||Cylindrospermopsins, microcystins, saxitoxins|
|Lyngbya sp.||Anatoxin-a, cylindrospermopsins, saxitoxins|
|Microcystis 1,4||Anatoxin-a, microcystins, saxitoxins|
|Oscillatoria 6**||Anatoxin-a, anatoxin-a(S), microcystins|
|Phormidium 1,7**||Anatoxin-a, homoanatoxin-a, microcystins|
|Planktothrix 1,8**||Anatoxin-a, homoanatoxin-a, microcystins, saxitoxins|
|Raphidiopsis||Anatoxin-a, cylindrospermopsins, homoanatoxin-a|
* This is a compilation of worldwide information, and the toxins are not produced by all species of the particular genus. Species from the genera in bold type are known to produce the associated toxin (in bold type) in New Zealand.
** The results of cyanotoxin testing on environmental samples indicate that species from this genus produce the associated cyanotoxin in New Zealand. 1. Wood, Stirling et al, 2006; 2. Wood, Rasmussen et al, 2007; 3. Wood and Stirling, 2003; 4. Christoffersen and Burns, 2000; 5. Carmichael et al, 1988; 6. Hamill, 2001; 7. Wood, Selwood et al, 2007; Wood, Heath et al, in press.
Part B: Benthic cyanobacteria
3.4 Benthic cyanobacteria: introduction
The guidelines are designed to manage risks to recreational users. They have been designed to protect users from the risks associated with ingestion of and contact with water and mats. The levels given in the guidelines are not relevant for addressing risks to dogs that actively seek out and consume cyanobacterial mats. (Appendix 2 provides further explanation on how the values are derived.)
Benthic, mat-forming cyanobacteria are widespread throughout New Zealand rivers and are found in a wide range of water-quality conditions, including oligotrophic waters (Biggs and Kilroy, 2000). The most common mat-forming benthic cyanobacteria genus in New Zealand is Phormidium. During stable flow conditions Phormidium mats can proliferate, at times forming expansive black-brown leathery mats across large expanses of river substrate (see Appendix 7). Flow conditions, substrate, water chemistry and species composition can influence the macroscopic appearance of benthic cyanobacterial mats (see Appendix 7), and at times they may easily be confused with other algal groups (eg, diatoms or green algae). Microscopic confirmation should be undertaken by either competent regional council staff or a laboratory with micro-algae identification expertise (see Appendix 8).
Dog deaths associated with the consumption of benthic cyanobacteria have become increasingly common around New Zealand (eg, Hamill, 2001; Wood, Selwood et al, 2007; Heath et al, in press (a), in press (b)). In most instances these deaths have been associated with the presence of the neurotoxins anatoxin-a and/or homoanatoxin-a (Wood, Selwood et al, 2007), and this often results in the rapid death of the animal. The production of microcystins by benthic cyanobacteria (Nostoc. sp. and Pankthothrix sp.) in New Zealand has now been confirmed (Wood, Stirling et al, 2006; Wood. Heath, McGregor et al, in press), and in at least once instance a dog death was caused by microcystins (Wood, Heath, McGregor et al, in press). In other parts of the world benthic species are known to produce saxitoxins and cylindrospermopsins (Carmichael et al, 1997; Seifert et al, 2006).
Recent research suggests the presence of cytotoxic (toxic to cells) compounds affecting mammalian cells from multiple Phormidium species collected around New Zealand (Wood, Froscio and Campbell, unpublished data). Therefore health warnings should not rely solely on the presence of known toxins. In an in-depth study of the spatial and temporal distribution of Phormidium mats in the Hutt River (Lower Hutt) it has been shown that toxin concentrations within mats can vary markedly among sampling sites and over short time frames (eg, a week; Heath et al, in press(b)). It has also been demonstrated that the presence and concentrations of anatoxins within the mat are not related to the abundance of the Phormidium mats (Heath et al, in press (b); Wood, Heath, et al, in press). Therefore a negative toxin test does not guarantee the absence of toxins within a water body.
Under certain environmental conditions, or as they become thicker (and bubbles of oxygen gas become entrapped within them), mats will detach from the substrate and may accumulate along river edges (see Appendix 7). During these events the risk to human and animal health is higher due to the accessibility of the cyanobacterial mats to river users. The highest risks to users is likely to be via ingestion of and/or direct contact with these cyanobacterial mats. The risk associated with both types of contact is likely to rise as the abundance and/or number of detachment events increases.
It is unclear whether extracellular toxins (toxins in the water column) are released in substantial quantities from cyanobacterial mats, but these are likely to be rapidly diluted and pose a lesser risk. Traditional water-column sampling (ie,taking a grab sample) only provides a snap-shot from the flow continuum and may underestimate the risk posed by benthic cyanobacteria. A passive in situ methodology known as solid-phase adsorption toxin tracking technology (SPATT) is currently under development to assist in sampling benthic cyanobacterial toxins (Wood, Holland et al, 2008). This has the potential to be a useful and economical tool for early warning and for monitoring the presence of extracellular toxins in rivers. This methodology has yet to be validated in rivers.
Although not specifically covered in these guidelines, benthic cyanobacteria do occur in lakes and ponds, where they have caused animal fatalities (Naegeli et al, 1997). These can detach and accumulate on shorelines (see Appendix 7). Where this occurs in recreational areas it is recommended that samples be collected for microscopic identification and cyanotoxin analysis, and the percentage of affected shoreline estimated.
3.5 Alert-level framework: benthic cyanobacteria
|Alert levela||Actions |
(See section 2.4 for the recommended framework for roles and responsibilities relating to actions, and the text box at the beginning of Section 3 for advice on interpreting the guidance in this table.)
Surveillance (green mode)
Alert (amber mode)
Action (red mode)
a The alert-level framework is based on an assessment of the percentage of river bed that a cyanobacterial mat covers at each site. However, local knowledge of other factors that indicate an increased risk of toxic cyanobacteria (eg, human health effects, animal illnesses, prolonged low flows) should be taken into account when assessing a site status and may, in some cases, lead to an elevation of site status (eg, from surveillance to action), irrespective of mat coverage.
b This should be assessed by undertaking a site survey as documented in Section 4.4.
3.6 Details of the framework: benthic cyanobacteria
3.6.1 Surveillance: green mode
This is triggered when cyanobacteria are first detected at low abundance (up to 20 per cent coverage), signalling the early stages of possible mat proliferation. Site surveys should be conducted as described in Section 4.4. Microscopic identification should be undertaken on samples to confirm the presence of cyanobacteria. Perform weekly surveys at representative locations along the river from spring to autumn and during peak recreational use periods. A single site that is representative of the recreational area may be acceptable, but multiple sites are warranted if the area is large. Fortnightly or monthly sampling frequency may be appropriate during cooler months and low use periods. Flow alerts (Section 3.7) can be used to trigger the surveillance level.
3.6.2 Alert: amber mode
The alert level (amber mode) is triggered when there is 20−50 per cent coverage of potentially toxic cyanobacteria (see Table 1) attached to substrate. The alert level requires notification and consultation with public health units for ongoing assessment of the status of the cyanobacterial proliferation (see Section 2.4). This consultation should start as early as possible and continue after the results of toxin analysis become available (if used). Testing for toxins can be undertaken to obtain a clearer indication of the health risks at a site. For example, if coverage is below 50 per cent and high concentration of cyanotoxins are detected, the risk level may be increased to action.
Weekly sampling should be undertaken. In most circumstances this will provide sufficient information to assess the rate of change of cyanobacterial populations, and to judge the population growth rate and spatial variability and therefore the hazard. The number of survey sites depends on factors such as the length of the water body and the degree of use of different recreational sites.
The alert level is also a good time to raise public awareness of the potential risk to water uses. Media releases (see Appendix 10) and information pamphlets (see Appendix 11) left at veterinary clinics, for example, are useful publicity methods. Information signs that provide the public with information on the appearance of mats and potential risk should be erected (see Appendix 9).
3.6.3 Action: red mode
The action level (red mode) is triggered when representative site surveys and sampling reveal either greater than 50 per cent coverage of potentially toxigenic cyanobacteria attached to substrate, or where up to 50 per cent of the available substrate is covered by potentially toxigenic cyanobacteria taxa (Tables 1 and 2) and these are visibly detaching from substrate, accumulating as scums along the river’s edge or becoming exposed on the river’s edge as river levels drop.
In action level (red mode), public health units (see Section 2.4 for details on the role and responsibilities of each agency) should warn the public of the potential health risks. This should be done (although not exclusively) through media releases and by requesting territorial authorities to erect signs at affected water bodies (Appendix 6 and Appendix 9).
Benthic cyanobacteria in New Zealand are known to produce toxic substances that have not yet been characterised. Also, based on data from planktonic cyanobacteria (eg, Pilotto et al, 2004; Stewart et al, 2006), there is an increased likelihood of respiratory, irritation and allergy symptoms from exposure to high abundances of cyanobacterial material, irrespective of toxicity or of the presence of known toxins. This is the rationale for action level (red mode) situation 1. As benthic cyanobacterial mats detach, they can accumulate along a river edge. Because of the increased availability of these mats, this is considered to be a period of high risk regardless of the percentage coverage in a water body (see action level − situation 2, Decision Chart 2, page 17).
3.6.4 Changes in alert levels over time
It is recommended that the action level (red mode) not be changed from a higher to a lower level (eg, from action to alert) until the percentage cover falls below the action level on two successive surveying occasions (collected at weekly intervals). The regularity of flushing flow (Decision Chart 3) should also be considered when downgrading health alerts.
3.7 Benthic cyanobacteria and river flows
A correlation between benthic cyanobacterial mat abundance, water temperature and a lack of ‘flushing flow’ conditions has been observed in some rivers (Milne and Watts, 2007; Wood, Selwood, Rueckert et al, 2007; Heath et al, in press (b)). In some instances, the length of time since a flushing flow event can be used as an early warning of elevated risk of benthic cyanobacterial proliferations. However, the flow velocity required to shift cyanobacteria from the river bed will vary depending on factors such as the river bed substrate type and size. For example, a river with a sandy substrate will require a markedly smaller flow to flush benthic cyanobacteria compared to a river with a large cobble substrate. In addition, the length of time required for cyanobacteria to proliferate following a flushing flow event will vary. So, although there is no ‘one size fits all’ warning system, on a regional basis experts could use periphyton coverage records, flow data and local knowledge to develop warning systems for cyanobacterial proliferation risk.
The following is an example of an automated river flow-based warning system for benthic cyanobacterial proliferation risk that is currently used by the Greater Wellington Regional Council for selected rivers within their region (Milne and Watts, 2007). Such a system is recommended for recreational use rivers within New Zealand known to experience potentially toxic benthic cyanobacterial proliferations. Some in-depth research is needed to establish the appropriate ‘flushing flows’ in each river. For example, in the Otaki, Waikanae, Hutt, Mangaroa and Wainuiomata rivers a ‘flushing flow’ is defined as “three times the median flow”, consistent with field observations and the findings of Clausen and Biggs (1997).
The automated river warning system for cyanobacterial proliferation risk has two complementary alert levels based on flow conditions (see Decision Chart 3).
|Flow alert level||Monitoring suggestion|
|Alert mode 1 |
No flushing flow1 for 2 weeks.2
|Survey known problematic sites to assess cyanobacterial cover, as per surveillance level (green mode) (see Section 3.6.1).|
|Alert mode 2 |
No flushing flow for 2 weeks and river flows are low (set at lowest 10th percentile flow for each river).3
|Increase frequency (eg, to weekly) of surveys of known problematic sites to assess cyanobacterial cover, as per alert level (amber mode) (see Section 3.6.2).|
1 In the Wellington region a flushing flow is defined as three times the median flow.
2 Following an assessment of the events of spring 2005, two weeks was determined to be an appropriate (and conservative) duration.
3 The justification for a low flow alarm is that water temperatures may be elevated (promoting algal growth), and any cyanobacterial growths may become exposed or near exposed at the river edges.
Part C: Cyanotoxin accumulation
3.8 Cyanotoxin accumulation in aquatic organisms
Cylindrospermopsins, microcystins, nodularin and saxitoxins can accumulate in a variety of fresh water and marine organisms (Kotak et al, 1996; Vasconcelos, 1999; Saker and Eaglesham, 1999; Sipia et al, 2002). When this occurs, warnings to avoid consuming aquatic organisms should be included in media releases and on warning signs (Appendices 5, 6, 9 and 10).
Wood, Briggs et al (2006) showed that microcystins accumulate in rainbow trout (Oncorhynchus mykiss) and freshwater mussels (Hyridella menziesii) in Lakes Rotoiti and Rotoehu (Rotorua). Based on the microcystin levels found in their study, it is considered unlikely that eating trout flesh as part of a regular balanced diet would result in adverse health effects. However, concentrations of microcystins were significantly higher in rainbow trout liver, so it is recommended that fish be gutted and thoroughly washed in clean tap water before eating.
The downstream effects of water bodies containing cyanobacterial blooms should also be considered. For example, Lake Omapere (Northland) experiences blooms containing microcystins, and the outlet of the lake flows into the Hokianga harbour, where microcystins have been found in shellfish.
For further advice on appropriate levels of cyanotoxins in aquatic organisms, contact the New Zealand Food Safety Authority (www.nzfsa.govt.nz).