Suspended Sediment and Sedimentation
Silt and mud washing down New Zealand rivers and streams is the most widespread of our land-based effects and the one that we know most about.
Mud and silt is suspended in the water column by rushing flood waters. When they reach the estuary or coast, these waters slow; and the silts and muds start settling to the bottom. There they build up, burying and smothering many creatures that live there.
Storms, tidal currents, and bottom fishing gear like dredges and trawls can stir up and re-suspend these sediments, compounding the original effects.
Currently, New Zealand rivers carry more silt and mud than almost anywhere else in the world. We have only 0.05 percent of the world’s land area, yet our rivers carry nearly 1 percent of the world’s mud and silt load. Some of this is quite natural, given our geology and climate. Much of New Zealand is hilly or mountainous and our position in the south-west Pacific Ocean means we get a lot of rain (including some extreme rainstorms).
However, human activities have increased both the rates and quantities of sediment arriving at our coast.
When Maori arrived, they burnt vegetation and established agriculture around coastal plains and near lakes and rivers. Sediment cores show that this led to increased amounts of silt and mud building up in lakes, estuaries, and harbours.
From the 1840’s onwards, Europeans began logging and clearing the steeper backcountry areas. And since the first and second world wars we have cleared more land and carried out increasingly extensive earthworks to build roads, towns and cities. Our present phase of development is mainly characterised by further intensification of these activities.
Keeping step with this development has been ongoing erosion and sedimentation. Most of this silt and mud eventually reaches our coasts, making the waters murky at times and gradually building up layers of fine sediments on the seafloor.
Effects of sediment on fisheries and ecosystems
Recent research indicates that some of our fisheries and fisheries habitats are not in good shape – a warning that something is wrong in our coastal environment.
As soon as suspended sediment flows into coastal waters it can have immediate, harmful effects on the plants and animals living there.
Sediment in the water can clog the gills of fish, making it harder for them to ‘breathe’, and the reduced visibility can make it harder for some fish to find food. Suspended sediment also makes it harder for filter-feeding shellfish to feed; so cockles, pipi, mussels and scallops can die off if the waters where they live are silty.
Silty water can also reduce the amount of light reaching the seabed and cause seafloor plants like kelp and subtidal seagrass to die off if waters stay silty for too long.
Suspended silts and muds – especially those washed down in floods - may eventually settle to the seabed and the waters will become clear again. However when this happens, these sediments can smother shellfish beds and other living creatures on the seabed, particularly if the silt/mud builds up in thick enough layers.
Layers of silt and mud also prevent shellfish like green lipped mussels attaching to the seabed, because these species need something firm to attach to. Extensive mussel beds in the Firth of Thames are now gone. Initially, this was probably the result of over-fishing. But since fishing has stopped, they have not returned to what is now a very muddy sea bottom.
Sediment settling also affects rocky reef areas because, like the Firth of Thames mussels, kelp spores need a hard surface to attach themselves to. Absence of spores means absence of kelp forests. Baby paua also find it hard to establish on surfaces covered by a fine layer of mud or silt.
But it doesn’t stop there – the direct affects of sediment on marine life can flow-on into other parts of the ecosystem, causing more widespread harm.
For instance, rocky reef ecosystems are supported by the growth of kelp and other seaweeds there. Marine algae are the start of the foodchain – they provide food for many animals that in turn are eaten by larger animals.
By preventing attachment of kelp and seaweed spores, sediment can reduce the growth and extent of kelp forests. Where this happens, the whole productivity of the rocky reef ecosystem is reduced. Lower productivity means fewer of the much sought-after paua and crayfish in some areas, as well as many other popular and biologically important fish species.
Sometimes these indirect (or flow-on) effects of sedimentation become felt over an extensive area. This can happen to species that have a life stage (or stages) that relies on a habitat that is vulnerable and in nearshore waters.
Snapper, trevally, tarakihi, John dory, garfish, parore, blue cod and mullet all rely on habitats that are sensitive to sediment – places like mussel beds, sub-tidal seagrass meadows, sponge gardens and bryozoan and tubeworm mounds. These provide important nursery and rearing areas, rich feeding grounds and safe havens from predators. They also frequently occur in sheltered bays or harbours – the very places most affected by sediment from the land.
When a fishery gets stressed by sediment or other environmental effects, it becomes more vulnerable to further pressure – from fishing, for instance. Sometimes the environmental stress reaches the point where even closing the fishery will not bring it back. This may explain the fact that previously relatively abundant shellfish beds at beaches around the Auckland region have shown no sign of recovery even though harvesting has been banned for many years.
Such declining or closed fisheries can be symptoms of problems in the ecosystem – canaries in the coalmine of our coastal zone.