Chemical contamination and horticulture
What are the potential sources of chemical contamination from horticulture activities?
Horticultural activities include the heavy use of chemical contaminants such as pesticides, especially in geographical areas such as the Waikato, Bay of Plenty, Hawkes Bay, Auckland, and Canterbury. Approximately 34% of all pesticides used in Aotearoa are used for horticultural purposes and 16% for cropping. Chemical contaminants make their way into waterways via spray drift from arable land and runoff.
Potential impacts of chemical contaminants on water quality and mahinga kai
- Local loss of fish species - fish may be harmed by contaminated water. Discharges and runoff into rivers and streams can be lethal to aquatic life depending on the strength of the toxin and size of the waterway and causing fish kills.
- Local loss of invertebrate species - contaminants can be particularly lethal to invertebrates, e.g. kōura. Invertebrates are also food for fish and persistent discharges that kill invertebrates could cause fish to travel farther in search of food and expose them to greater risks and stress.
- Decreased dissolved oxygen (DO) levels - waste compounds released into waterways initiate biochemical reactions that use up oxygen as the stream bacteria break down the organic matter (Biogeochemical Oxygen Demand, BOD). Excess nutrients can also lead to algal blooms, and oxygen is used up when the algae die and decompose. Fish ‘breathe’ oxygen through their gills; a decrease in available oxygen (anoxia) in the water column threatens their ability to respire, which may lead to death. Fish that tolerate low levels of dissolved oxygen (such as the introduced species gambusia) may replace native populations that are less tolerant.
- Increased turbidity and decreased water clarity - water may become cloudy or discoloured with chemical contamination which reduces the ability of fish to see, prey and detect predators.
- Damage to species - repeated exposure to sub-lethal doses of some contaminants can cause physiological and behavioural changes in fish that have long term effects on the population, such as reduced reproductive success, abandonment of nests and broods, a decreased immunity to disease, tumours and lesions, impairment of the central nervous system, and increased failure to avoid predators.
- Some contaminants, such as mercury, may bioaccumulate in animal tissues and be carried to human consumers of the fish.