According to Toi Te Ora Public Health, it is “the rule of thumb” not to swim for at least 48 hours after heavy rain in order to avoid things like diarrhoea, vomiting, skin infections or ear infections.
“There is likely to be contamination from rural and urban run-off in waterways.”
Toi Te Ora’s health warning is reflected in environmental body Lawa’s (Land, Air, Water Aotearoa) recreational water quality data which, as of yesterday afternoon, showed all but one swimming spot in the region was unsuitable for swimming.
In late January before the latest heavy rain, 20 of the region’s 80 swimming spots were too unsafe to swim in.
Local residents called on visitors and boaties to stop ignoring health warnings and take greater care as health authorities issued warnings advising people to stay out of key areas in Rotorua lakes because of extensive algal bloom.
Lawa data is used by Bay of Plenty Regional Council and Toi Te Ora Public Health to determine whether certain areas are safe to enter.
Lake Rotoiti’s Okawa Bay and several locations around Lake Rotorua were among those “unsuitable for swimming” due to algal blooms.
Lake Rotoiti Community Association deputy chairman Jim Stanton said the health warnings issued around Lake Rotoiti over January had been “frustrating for everyone”, especially property owners and visitors.
“[They] come to enjoy a break over the summer holiday season only to find a blanket health warning against coming into contact with the lake water due to toxic algal blooms.”
Stanton said many lake users either ignored or were unaware of the warnings and continued with normal on-water activities such as skiing, using paddle boards and towing biscuits.
He said watercraft in particular needed to adopt “comprehensive” cleaning of equipment when moving between waterways.
“Unfortunately, entLake Rotoiti Classic and Wooden Boat Association commodore Richard Amery said algal growth happened once or twice a year and usually didn’t last more than a couple of days.
“These recent warnings were right on Christmas and the two areas mentioned, Okawa Bay and Te Weta Bay, turned green and people heeded the warnings.”
Amery said about four days after the warnings a storm meant the water reverted to normal colour.
“Despite no one lifting the algae warnings, everyone went back to water sports and swimming and as in the past, we have not heard of anyone suffering any aftereffects.”
Lake Rotoiti Classic and Wooden Boat Association's parade returns to Lake Rotoiti on February 4. Photo / NZME
Bay of Plenty Regional Council science manager Rob Donald said the criteria for deciding whether water was safe for swimming were based on microbiological guidelines in place since 2002.
“From late October through to the end of March, Bay of Plenty Regional Council collects weekly water samples from more than 70 popular coastal, river and lake recreation sites.
“The samples we collect are tested in our lab specifically for faecal contamination (E. coli or Enterococci bacteria) and for toxin-forming algae (cyanobacteria).”
Donald said the region had been “unlucky” this year.
“We have experienced a significant algal bloom in all four of the lakes we monitor. This resulted in all sites on these lakes (Rotorua, Rotoiti, Rotoehu, Ōkaro) being classed as unswimmable as a precaution because toxic algae can be moved rapidly across water by wind currents.”
Donald said the blooms were subsiding but the health warnings still stood.
“There are lots of factors that contribute to whether a site is swimmable or not, with the weather playing a significant role.”
Algae are essential to freshwater ecosystems but become problematic when they reach high levels. Photo / NZME
Bay of Plenty Regional Council Rotorua catchments manager Helen Creagh said algae were essential to freshwater ecosystems but became problematic when they reached high levels.
“Pollution from urban areas, excessive nutrients, lack of water circulation in calm conditions, extreme weather events and higher water temperatures can all alter the natural composition of algae in the lakes,” Creagh said.
“If the natural balance is disturbed an algae overgrowth or ‘bloom’ may occur, causing discolouration, scum, odours and even toxic effects.”
Creagh said algal blooms could happen suddenly.
“So a warning may not be in place when you’re out swimming. Remember to always check for signs, is the water pea green or discoloured? Does it have slime on the surface, like paint scum? Is it smelly? If in doubt, stay out.”
Creagh said the council’s goal for lakes with regular algal blooms was to reduce nutrient outputs from sewage and septic tanks and to reduce nutrients from land use.
“We support and encourage the retirement of riparian areas beside streams by fencing and planting trees, the conversion of land to native bush and forestry to remove livestock and to reduce fertilisers, and more environmentally acceptable farming methods with funding and advice.”
However, Creagh said these strategies could take years to have the necessary effect.
“To support water quality improvement some short-term interventions such as alum dosing can bring a much more rapid improvement in water quality.”
Creagh said alum restricted the available phosphorous algae needed to grow.
“[Using alum] requires resource consent, and mana whenua are involved in monitoring the activity to ensure they are comfortable with the dosing limits and any other environmental concerns.”
Te Whatu Ora medical officer of health at Toi Te Ora Public Health Dr Jim Miller said health warnings were issued after a risk assessment based on visual observation and laboratory sampling.
“It is not always possible to see the organisms that may be causing the risk. This is particularly the case with bacteria that can cause gastroenteritis.
“Further sampling is required to be reasonably sure that the event has passed. Until then it is wise to follow health advice and avoid contact with the water until the health warning is lifted.”
Miller said only people who had recreational contact with the water would be at risk.
“Algal blooms have the capacity to release toxins into the water which can cause significant ill-health to anyone who has contact with the water.
“The toxins can affect the nervous system causing numbness and difficulty with breathing, and also sparking asthma attacks. Contact with the water can also cause skin rashes, stomach upsets and visual problems.”
