Harmful Algal Blooms

What are they?

Harmful algal blooms are caused by toxin producing blue-green algae species (Cyanobacteria) and basically characterise the advanced stage of eutrophication. Eutrophication is a process of nutrient enrichment that occurs naturally over centuries, but has been accelerated by human changes to land use that result in more nutrient runoff.

Algal bloom_eastern side of Lake Rotoroa

Inputs of nutrients to lakes occur as external loads exported from lake catchments and as internal loads released from bottom sediments. Nutrients stimulate algal/phytoplankton growth thus at high concentrations, phytoplankton blooms can occur. During blooms, dense, visible patches can be seen at or near the surface of the water. Decay of phytoplankton blooms can lead to the removal of oxygen from bottom waters through respiration of the bacteria that utilise the available dissolved organic carbon. This loss of oxygen in turn brings about release of bioavailable nutrients, phosphate in particular, from the bottom sediments. These nutrients are 'locked up' when there was plentiful oxygen. Nutrient release from sediments may again trigger algal blooms.

Some species of cyanobacteria produce potent natural toxins that can be transferred through the food chain where they affect and even kill the higher forms of life such as zooplankton, shellfish, fish, birds, mammals, and even humans that ingest them directly or indirectly.

How is LERNZ involved?

LERNZ aims to restore the indigenous biodiversity in New Zealand lakes by 2015 by developing new models and technologies to effectively manage harmful algal blooms.

This Intermediate Outcome focuses on enhancing the resilience of lake ecosystems through reducing the incidence of harmful algal blooms (HABs), applying integrated predictive models, and overcoming impediments to technology transfer, policy and planning.

Emphasis must be placed on control of nutrients from lake catchments to stabilise increasing oxygen consumption in bottom waters of lakes and subsequent nutrient release from sediments, thereby preventing seasonal loss of oxygen bringing about deterioration in lake water quality.