Algae Bloom Health Warning Lifted
PRGI Lake Advocacy
The PRGI established a specific Lake Sub-Committee to advocate on behalf of the PRGI Members and Pegasus residents for improved water quality and reduction of algal blooms with stakeholders (Templeton Group, WDC, ECan). If you would like to join this group, please join the PRGI Committee. All the lake Sub-Committee and Templeton reports and updates are below.
Common Q&As on Lake Pegasus
It is important to accept that it is not possible to guarantee the lake water is suitable for swimming. Unless you build an entirely artificial construction and use chlorinated water. This is not the case at Pegasus where the lake is human-made but is open to natural processes, fed primarily by ground water, and directly connected to other waterways. This means the lake has and will continue to change over time like any other water body.
Pegasus Lake is officially only consented for "secondary contact", which excludes submersible activities such as swimming.
The water clarity will often improve in winter but darken in summer depending on the abundance of algae. Turbid, algal-rich water may be fine to swim in but also may not be appealing to some people.
There is always a risk of algal blooms occurring in waterways. Algal growth is a natural phenomenon in any water body and is driven by nutrient availability (bioavailable forms of nitrogen and phosphorus for example), water temperature, light and wind conditions. This is why algal growth is prevalent during summer when daylight hours are long and water temperatures are warmer.
Lake Pegasus is 90% fed by nine nutrient-rich natural groundwater sources. These sources have shown a worsening in water quality since the original consenting conditions were established.
It is primarily this contaminated water entering the lake which causes the blooms. Our lake conditions (depth, slow-moving flows) then provide an optimal environment that nurtures the development of the bloom, especially in warm weather periods.
The lake owner, Templeton Group (TG), has worked with a range of national and international experts to identify technical solutions to improve lake conditions, alongside ongoing lake maintenance. A trial of a potential solution is being arranged to be in place ahead of the summer 2022/23 season.
It is unlikely that year-long usage of the lake will be achieved while incoming groundwater quality continues to be poor, but TG will continue to work to reduce the period the lake is shut due to bloom each year.
It is important to note that not all algae is ‘bad’ or ‘harmful’; not all algal blooms are cyanobacterial blooms.
Cyanobacteria (Blue-Green Algae)
Cyanobacteria, also known as blue-green algae, are naturally occurring and have been present in the lake since soon after the lake was filled.
There are two main types of cyanobacterial blooms: benthic (mat forming - typically a black slimy mat on stones or stream/lake bed) or planktonic (suspended in the water column and characterised by the presence of a scum, often luminous green in colour).
Lake conditions can change from ‘normal’ to ‘bloom’ in a very short timeframe (over the course of days), if conditions (i.e., nutrients and climate) are right.
Not all cyanobacteria produce toxins. Of the cyanobacteria that do produce toxins, they do not necessarily produce toxins all the time.
Human exposure to cyanobacteria toxins may cause symptoms such as skin rashes, nausea, tummy upset and tingling/numbness around the mouth or tips of fingers. The toxins can have a particularly harmful and, in some cases, deadly effect on animals, including dogs.
Algal species counts (tests) are done regularly in summer; these results identify any cyanobacterial species and the numbers/concentrations are compared to the Ministry for the Environment guidelines to determine whether a bloom is occurring.
If the bloom is at a level of concern to public health, the Canterbury DHB’s Community and Public Health unit issues a health warning and signage is put in place around the lake warning people of the risk. The Waimakariri District Council also alerts the district and the PRGI also communicates this across their communication channels.
Pegasus Lake Flow
The lake water is sourced primarily from groundwater. The groundwater input will vary
seasonally according to the water table level. Inputs are likely to be highest in spring when groundwater levels are highest as a result of aquifer recharge from alpine snow melt and winter rains. Some water in the lake derives from rain directly into the lake and a small stormwater discharge from the commercial area after being treated by filters.
The lake discharges via two weirs into the ECMA, a natural wetland to the east of the lake. The ECMA has an outlet at its southern end. Water then flows north and discharges into the Ashley River at Waikuku.
The flow through the lake is determined by the natural ground water flow. The two weirs have
a small range of adjustability (approximately + or – 100mm) these can be raised or lowered to adjust the standing height of the water. Raising or lowering of the weirs does not provide a long term increase or decrease to the flow of water through the lake.
Essentially Lake Pegasus is flowing exactly as it was designed to do and there are no sustainable means to alter the flow of the lake from time to time.
The weed in Lake Pegasus is a common freshwater weed found in similar lakes and waterways throughout New Zealand and is not preventable.
Weed growth in the lake naturally increases during the spring/summer season and slows in winter. The weed growth absorbs nutrients from the water that would otherwise be available to algae and may assist in preventing the development of an algal bloom.
The lake have some lake weed physically removed from certain areas of the lake for aesthetic reasons from time to time.
Dog Leg Corner
Because of the wind direction and the shape of this part of the lake it tends to hold any debris that blows into it. The shallowness of this area means the water is warmer and calmer than other areas of the lake which tends to promote the growth of algae and weed.
Changing the shape of the lake is not a viable option because of the structural implication for the lake and surrounds. However as part of the ongoing review of the lake management plan we will investigate whether wetland-type planting in this area would be beneficial.
Routine removal of rubbish will also continue to be undertaken.
Eels have very little influence on the health of the lake. Smaller eels prey on insect larvae, worms, snails etc., while larger eels feed on fish and larger invertebrates. Other fish species can stir up bottom sediments and may cause the lake to become turbid; however, this does not occur with eels. Eels are simply part of the natural food web of Lake Pegasus.
Eels are a very long-lived species and their populations are considered to be in decline in New
Zealand. Eels do not spawn until the end of their lives, which can span between 40 and 100 years. In the interests of conservation and to avoid the risk of harm caused to lake users by fish hooks left in the lake we do not support fishing.
Eels are most active at night when people are unlikely to be using the lake and there have been no cases of bites reported to Todd Property. We do not have any information that suggests eel numbers should be reduced.
E. coli presence can be a health hazard and high levels of E. coli would result in lake closure the same as for a toxic algal bloom.