Sediment and horticulture
How may horticultural activities increase sediments in waterways?
Repeated cultivation (plowing, fertilising, and drilling) of the land can increase the amount of sediment that enters nearby waterways. Waterways near horticultural land are especially vulnerable when there is little riparian vegetation to act as a buffer for increased runoff from the land. Sediments can carry nutrients and chemical contaminants that may be washed into waterways from open soils during cultivation, especially after heavy rain.
Potential impacts of sediment on water quality and mahinga kai include:
- Decreased water clarity - increased sediment loading into a stream will decrease water clarity and reduce visibility for fish seeking food and places to live.
- Damage to fish gills and filter feeding apparatus of invertebrates.
- Changes to the benthic (bottom) structure of the stream/river bed - coarse substrates such as gravels and boulders are replaced/smothered by sand and silt.
- Decreased numbers of invertebrate species from smothering of habitat - invertebrates are a food source to some mahinga kai (e.g., kōura and fish) and diverse invertebrate communities are also an indicator of healthy stream systems.
- Decreased algal food supply at base of food chain - sediments can scour algae from rocks, make algae unpalatable, or reduce light to levels where algae cannot grow, because plants need light to photosynthesise.
- Increased contaminants from surrounding land - sediments can transport attached pollutants such as nutrients, bacteria and toxic chemicals from agriculture and horticulture into our streams.