Nutrients and agriculture
What are the potential sources of nutrients from agricultural activities?
Nutrients on farms can come from animal urine and manure, fertiliser, milk residue, and wastewaters. They may enter waterways from animals entering streams without adequate fencing and riparian vegetation , oxidation pond discharges, surface water runoff from the land during periods of heavy rain, or from leaching into groundwater.
Excess nutrients are more likely to enter waterways when vulnerable areas on the farm are not carefully managed. Nutrients hotspots on farms include areas where animals walk or 'camp' frequently, road and track runoff, eroding soils (nutrients can be bound to sediment), stream crossings, and effluent from dairy sheds, silage pits, oxidation ponds, and other wastewater discharge and storage areas that have been inadequately treated.
Potential impacts of high nutrients on water quality and mahinga kai
- Eutrophication - excess nutrients in lakes, estuaries, or slow-moving streams and rivers can lead to an increase in primary productivity (excessive plant and algal growth) that degrades water quality.
- Loss of species - an increase in plant growth, sometimes called an algal bloom, reduces dissolved oxygen (DO) in the water when dead plant material decomposes and can cause organisms (fish and invertebrates) to die. If this cycle happens repeatedly, species may be lost from the lake or waterway.
- Loss of habitat - eutrophication of the water can kill off plants that fish depend on for their habitat and alter the lake bed habitat for invertebrate species.
- Increased turbidity and decreased visibility - when algae increase in response to nutrients this reduces water clarity, visibility, and recreational suitability. It also reduces the ability of some fish to see prey or predators.