Nutrient Enrichment (Eutrophication)
Intensive farming practices and fertiliser applications can significantly increase the concentrations of nutrients entering our waterways.
Most of these nutrients eventually reach estuaries and coasts. Of these, nitrogen and phosphorus particularly stimulate the growth of phytoplankton (algae suspended in the water) and seabed algae. These growing algae can then start a cascade of effects through the ecosystems.
For example, phytoplankton blooms reduce the amount of light reaching the seabed, and this in turn can lead to reduced growth or die-off in subtidal seagrass and seaweed communities.
As the phytoplankton die and settle on the seafloor they rot, creating low oxygen conditions in the overlying waters. In extreme cases, this can suffocate fish and invertebrates that need high oxygen levels to survive.
A number of overseas studies have looked at the effects of nutrient enrichment on coastal ecosystems. The researchers found that early on, when nutrient levels first increased, there was a general increase in productivity but decreased diversity of species in the ecosystem. However, once nutrient levels tip past a certain point, systems collapse, creating biological ‘dead zones’.
In one Scottish study, nutrient enrichment transformed inter-tidal seagrass meadows, rich in crustaceans and wading birds, into an algal-mat with few invertebrates and no wading birds.
Little work has been done on the effects of nutrient enrichment on coastal fisheries or ecosystems in New Zealand. However, nutrient impacts here are thought to be modest, relative to some other parts of the world.
Where enrichment occurs, this may be reduced by tidal flushing and current flows. Large beds of filter-feeders like mussels, cockles and pipi can also help – these shellfish filter out phytoplankton, increasing the clarity of the waters they live in.