Within the lake environment live microscopic plants commonly known as algae or phytoplankton. Phytoplankton exist as single-celled forms, multicellular colonies and filaments. Most are invisible to the naked eye except to give the water a green colour. Generally algae drift freely through water, carried by currents. Some phytoplankton are able to control their depth by altering their buoyancy, i.e. how much they float.
Cyanobacteria (commonly called blue-green algae) seem similar to real algae but are actually aquatic bacteria that can photosynthesise like algae. They are usually unicellular and often grow in colonies large enough to see. They are able to fix nitrogen from the air, so can become more populous when nitrogen levels are low and competing algae species are limited. Cyanobacterial blooms are now common in many of the Rotorua lakes. Since 1997 Environment Bay of Plenty has monitored cyanobacteria in the lakes, especially over summer when blooms are more likely. Cyanobacteria blooms are particularly common in nutrient rich lakes.
Positive effects
Just as in the sea, phytoplankton function as the primary producers in a lake. They form the base of the lake food chain like grass on a farm. Like land plants, phytoplankton produce organic material from sunlight, carbon dioxide, water and inorganic nutrients (such as nitrogen and phosphorous) via photosynthesis. Phytoplankton also produce oxygen, most of which is dissolved in lake water for other lake species to use.
Negative effects
Problems occur when high nutrient levels promote high algal growth rates and excess biomass, known as a bloom. Blooms can adversely affect water quality by:
- decreasing the clarity (or clearness) of the water
- turning the water a green or brownish colour· forming layers of scum or foam on the lake surface
- causing unpleasant smells
- reducing light penetration through the water column
- altering the pH of the water
- depleting the oxygen in the water as algae decompose
- clogging water filters, chlorination and flocculation processes
- disrupting the ecology of the lake system
Cyanobacteria can also bloom, and these blooms have been choking many of the Rotorua lakes. They are encouraged by warm, calm weather in summer and autumn, and high concentrations of nutrients in the lake water. Cyanobacteria have gas vesicles that allow them to float up and congregate around the surface of the lake to collect maximum sunlight. Wind can blow cyanobacteria clumps into bays and inlets, further concentrating their impacts, such as:
- forming layers of scum on the surface· causing unpleasant smells
- depleting dissolved oxygen in bottom waters when cells die, sink and decompose· turning the water a soupy yellowy green colour
- blocking sunlight required by good algae species that inhabit the lake stratum further down
- producing toxins
Between 25-50% of all cyanobacterial blooms are associated with toxin production. Of the blue-green algae types occurring in the Rotorua lakes, Anabaena and Apanizomenon are most commonly associated with the production of neurotoxins (which affect the nervous system) and contact irritants. Microcystis produces hepatotoxins (which affect the liver). Cyanobacterial toxins have been known to kill livestock, wildlife and humans.
Cell concentrations of >2,000 cells/mL can potential cause toxicity problems for human drinking water supplies. Cell concentrations of >15,000 cells/mL can potential cause toxicity problems for general contact recreational purposes and stock watering. In general, marked green discolouration of the water and surface scums typically coincide with cell concentrations in the order of several 100,000 cells/mL.
Bathing or showering in waters containing toxic algae blooms can result in:
- severe dermatitis
- burning or itching of the skin
- erythematous wheals
- redness of lips and eyes
- sore throat
- asthma symptoms
- dizziness
Drinking water containing toxic cyanobacteria blooms can result in:
- death of stock
- liver complaints in humans
- paralysis of skeletal and respiratory muscles (Anabaena species)
- irregular breathing and twitching (Aphanizomenon species)
- weakness, vomiting, cold extremities, hair standing on end, diarrhoea, heavy breathing and death due to circulatory shock (Microcystis species)
There has been some concern raised about chronic toxicity effects from eating trout from Rotorua lakes with cyanobacterial blooms. Research so far has indicated that unless trout is eaten every day, there should be no long-term effects from toxins. However the internal organs of the fish absorb much higher toxin levels and should not be eaten.
Environment Bay of Plenty passes on all blue-green algal monitoring results to the relevant health authorities, in this case Pacific Health. Pacific Health in conjunction with Rotorua District Council erect warning signs at relevant bays and lakes and issue press releases. If a lake or bay is no longer safe for contact recreation (or if a lake or bay becomes safe), Environment Bay of Plenty will update the Lake Health Warnings webpage.
The absence of signs does not necessarily mean that the waters are safe to swim in as blooms can occur very rapidly. It is advisable not to swim in waters that are distinctly pea-green in colour especially if there are green surface scums or films. If you think you may have been exposed to toxins from an algal bloom see your doctor and notify the health authorities.
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