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.

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Updated : 20 March 2009

Tracking nutrient enrichment in Southland

Work soon to be published shows evidence of reduced biodiversity and increased productivity in estuaries affected by higher nutrient loadings, compared with pristine sites.

Six estuarine and coastal inlets were studied in Southland between 2005 and 2007 to look at the effects of nitrogen enrichment. Two of these were pristine sites, the other four had inputs from agriculture and urban development.

The effects of this increased nitrogen in the water column were transmitted up the food web, particularly through marine algae. The growth of algae in suspension (phytoplankton) and on the seabed increased in response to increased nitrogen, but it appears that seagrass growth is not as enhanced.

Analysis of the gut contents of small estuary fish - spotties and triplefins - showed that those living in the pristine sites ate a wider variety of prey animals than those living in more nitrogen-enriched waters. The study also estimated growth rates of the spotties and triplefins, by comparing their size (weight and length) and age (estimated by counting growth rings in their earbone). This showed these fish generally grew quicker in nitrogen-enriched waters.

This general increase in productivity and decrease in biodiversity has also been found in a number of overseas studies that have looked at the effects of nitrogen enrichment on coastal ecosystems. However, overseas studies also show that when nitrogen increases past a certain point, the productivity stimulated by these excess nutrients can use up the oxygen in the water, creating biological ‘dead zones’.