Factsheet: Assessing state of our estuaries

What do we mean by 'state'?

The term 'state' is used to describe the current condition of a site. This assessment is usually based on a variety of indicators. For Estuary Health, the states of three indicators are considered to determine the health of the monitoring site:

  • Mud content

  • Contaminants

  • Estuary macrofauna


How is state calculated for our Estuary Health sites?

The state presented for mud content and contaminants for each Estuary Health site on LAWA is the average value of the monitoring results from the most recent year of sampling. If sampling occurs once per year, then this value is presented.

Estuary macrofauna data are summarised using a Benthic Health Model to calculate an Estuary macrofauna score. State is then simply the most recent monitored Estuary macrofauna score.

It is not necessary to combine data from multiple years to determine state, as the monitored indicators are generally not highly variable between years. Sampling is also designed to minimise the effect of spatial variation on monitoring results, with multiple samples (replicates) usually collected from across the monitoring site. See the Mud content, Contaminants and Estuary macrofauna  factsheets for more detail.

To provide the most complete picture of the data we have available, LAWA presents state based on the most recent data no matter how old they are. For the estuary health indicators, it is expected that data older than five years do not represent current conditions, so caution should be used when interpreting state based on data from more than five years ago. It is important to consider the specific indicator and site when making these judgements, however, as some indicators change very slowly over time and a less frequent sampling regime is appropriate. For instance, it may take decades for metal contaminant concentrations to decrease in muddy areas, in which case monitoring once every five years or less would be adequate to check on contaminant state.

How is state presented for Estuary Health sites?

State categories are defined for each indicator and colour-coded for presentation on LAWA. A traffic-light approach is used with state values that are expected to have negative impacts on estuary health in red and orange, and state values that are unlikely to impact estuary health in blue and green.

Mud content

National guidelines are not available to assess mud content against. However, many studies have investigated how mud content affects macrofaunal communities and ecosystem functions in Aotearoa New Zealand. We have used the knowledge generated by these studies to create five ecologically relevant categories for use on LAWA:

  • ≤3% (very low impacts on macrofauna)

  • 3−10% (low impacts on macrofauna)

  • 10−30% (moderate impacts on macrofauna)

  • 30−60% (high impacts on macrofauna)

  • >60% (very high impacts on macrofauna)

More information about mud content is available in this factsheet.



Contaminants can be assessed by comparing state to sediment quality guidelines. These guidelines use thresholds to indicate when contaminant concentrations have the potential to cause negative ecological effects. The guidelines commonly used and available for comparison on LAWA include:

  • Australian and New Zealand guidelines for fresh and marine water quality (ANZG)

  • Threshold Effect Level (TEL)

  • Effects Range Low (ERL)

  • Environmental Response Criteria (ERC)

Three categories are defined by each of these guidelines:

  • Ecological impacts unlikely

  • Ecological impacts possible

  • Ecological impacts probable

More information about contaminants are available in this factsheet.


Estuary macrofauna

The state of estuary macrofauna are assessed on LAWA using the Benthic Health Model (mud), which calculates an “Estuary Macrofauna Score”. We then translate the score into one of five categories. The thresholds between categories do not necessarily reflect ecological breakpoints currently, but categories can be described as indicating gradually increasing impacts on the macrofaunal community.

More information about Estuary macrofauna is available in this factsheet.


What do the Estuary Health state indicators tell us?

When plotted together on a map, the state results can contribute to a general picture of the state of an estuary. When multiple estuaries are viewed together, we can begin to understand regional and national patterns in estuary health. This helps us to identify areas where intervention may be needed.