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Sediment is soil that has been washed off the land by rain water, taking nutrients with it. Sediment has damaging impacts on water quality and the health of streams, rivers, estuaries and coastal waters.
Soil is the basis of all farming profitability and takes thousands of years to be created, so farmers benefit by doing all they can to hold soil on their land.
Sediment is no good for stream habitats and fish life. As an example most of the Tukituki Catchment has high levels of riverbed sediment. The muddy water irritates the gills of fish and invertebrates, and makes it harder for fish to find food; imagine trying to eat a sandwich sitting in a sand storm. When sediment settles, it fills the nooks and crannies that bugs and fish live in; basically taking that sandwich away! An experiment near Hamilton showed that 90% of fish left a channel after it received fine sediment deposits.
Rivers eventually flow to the sea and carry that sediment out to the coast. The Regional Council’s 2016 State of the Environment Report shows estuaries around Hawke’s Bay becoming ‘muddier’. Fine sediment accumulation can be detrimental to coastal fisheries, so reducing erosion and preventing sediment from reaching waterways in the first instance is one of the regional council’s top priorities. We are also researching the impact of sediment on our coastal fisheries.
Good margins (buffers/gaps) between waterways and productive land are critical for ecosystem health. Fencing off waterways to prevent stock entering them is essential to ensure a healthy plant barrier and also stop animals excreting in the water.
Fencing off streams, planting trees and improving ground conditions along waterways is good operational practice and land stewardship, and will help fish and bugs live happily. An experiment near Taranaki showed that removing riparian habitat resulted in 75% less inanga (the main whitebait species) in a waterway.
Nitrogen and phosphorus are both critical for plants. Farming systems aim to make the most of them because obviously it costs money to apply fert. Unfortunately, nutrients don’t always stay where they are intended and can be attached to sediment in water.
Once in waterways, nutrients can trigger excessive algal growth in both tributaries near the source and further downstream in the main river channel. Ultimately, these nutrients find their way to estuaries and the ocean which causes other problems.
Although algal growth is naturally occurring, too much algae in the water smothers habitat and reduces life-supporting capacity. It looks slimy, may smell bad and may result in Phormidium (black mat) growth, which can be toxic to people and animals.
Currently, either phosphorus or nitrogen, or both, are above target levels in waterways in most of the Tukituki catchment.
Photos: 1) long green filamentous algae 2) phormidium
E. coli in water is an indicator of disease-causing organisms associated with faeces in our waterways. People can become sick after swimming in water with bacteria and other pathogens from faeces. The regional council uses faecal source tracking as a way of figuring out what type of beast (eg. human, bird, cow) E. coli comes from.
In Hawke’s Bay faecal contamination mainly comes from livestock, and cows in particular. E. coli and other faecal bacteria can live for several weeks in water, and stay alive while being carried a long distance downstream from its source. So stock accessing and excreting in small streams a long way from swimming holes in rivers or from coastal beaches still poses a risk to human health.
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