Although nutrient enrichment threatens many New Zealand estuaries, guidance on how to assess the extent of eutrophication (including indices and indicators that are useful for management) is limited. As a result, it has been difficult to:

  • Determine the current state of estuaries with regard to eutrophication;
  • Assess the effects of the recent landuse intensification and change on estuaries;
  • Gauge the consequences for estuaries of nutrient limits for freshwater (e.g., the National Policy Statement for Freshwater Management, NPSFM, 2014); and
  • Set nutrient load limits to achieve estuarine objectives.

In response, regional council coastal scientists sought advice via the coastal Special Interest Group (cSIG), with funding through Envirolink Tools Grant (Contract No. C01X1420), on the development of a nationally consistent approach to the assessment of estuary eutrophication, including nutrient load thresholds. The purpose of this project, called the NZ Estuary Trophic Index (ETI), is to assist regional councils in determining the susceptibility of an estuary to eutrophication, assess its current trophic state, and assess how changes to nutrient load limits may alter its current state. The project does this by providing tools for determining estuary eco-morphological type, where an estuary sits along the ecological gradient from minimal to high eutrophication, and providing stressor-response tools (e.g., empirical relationships, nutrient models) that link the ecological expressions of eutrophication (measured using appropriate trophic state indicators) with nutrient loads (e.g., macroalgal biomass/nutrient load relationships).

In terms of the regional council planning framework, the ETI provides vital supporting guidance for underpinning the ecological health component of regional plans by identifying relevant estuary attributes and outcomes for inclusion in plans, defining methods and indicators to measure ecosystem health attributes, and providing guidelines to assess whether or not the outcomes are being met.

The ETI provides three tools:

Details of when to use each tool and the knowledge underpinning the tools can be found on the Welcome and Background Information tabs, respectively, for each tool.

Concept diagram of the ETI

Figure 1. Concept diagram of the ETI, showing relationships between ETI Tools 1, 2 and 3. Tool 1 provides information on estuary susceptibility to eutrophication based on estuary type, its physical attributes and nutrient loading. Tool 2 provides scores for estuary trophic health based on measured trophic indicator values. Tool 3 provides trophic scores under under scenarios of changed land use or load limits, and/or when values of trophic state indicators are lacking.


Whilst NIWA has used all reasonable endeavours to ensure that the information contained in this website is accurate, NIWA does not give any express or implied warranty as to the accuracy of the information contained herein. This website has been reviewed internally by NIWA and meets NIWA standards for website delivery.

Two approaches are combined to enable the user to assess estuary susceptibility to eutrophication (i.e., the risk that an estuary will be eutrophic based on its physical characteristics), along with advice about when each approach is applicable. For more detailed information about each approach, please refer to the Background information and References tabs.

Nutrient loads and flows are needed and are supplied for most NZ estuaries in the provided default data. For other estuaries, these values need to be provided by the user, either from their own data or outputs from the Catchment Land Use for Environmental Sustainability model (CLUES, Elliott et al. 2016).

Dilution modelling approach

The dilution modelling approach predicts potential nutrient concentrations in New Zealand coastal waters by combining predictions of nutrient loads from catchments with simple dilution models to determine the mixing between ocean- and river- water in estuaries (see Glossary for definitions of words in bold). The tool chooses the appropriate dilution model to use based on estuary volume, tidal prism and freshwater inflow (Elliott et al. 2016, Plew et al. 2018). The dilution modelling approach calculates an ETI susceptibility band that combines macroalgae and phytoplankton banding, while considering the proportion of the estuary that is intertidal (Plew et al. 2020). If the user identifies an estuary as an ICOE, susceptibility bands are provided for both its open and closed state.

When to use the dilution modelling approach results

The dilution modelling approach toward assessing susceptibility can be used on any estuary for which the appropriate input data are available. Banding is provided for susceptibility to both macroalgal and phytoplankton blooms. However, the main effects of phytoplankton eutrophication are oxygen depletion and high light attenuation in deeper and often stratified estuarine systems, which typically do not occur in New Zealand SIDEs when they are permanently open. Phytoplankton effects are more likely in SSRTREs, particularly those with longer flushing times. Using the Tool 1 database, we have found that the great majority of estuaries with intertidal areas less than 20% are SSRTREs, while the great majority of SIDEs have intertidal areas greater than 40%. Therefore, we also provide an overall ETI susceptibility band that takes into the proportion of intertidal area into consideration. For estuaries with intertidal areas greater than 40% (e.g., SIDEs), ETI susceptibility is banded using the macroalgae band. For estuaries with intertidal areas less than 5%, ETI susceptibility is banded using the phytoplankton band. If the intertidal area is between 5% and 40% or the estuary is an ICOE, ETI susceptibility is banded using the worst of the macroalgae and phytoplankton bands.

ASSETS approach

The ASSETS approach to gauging estuary susceptibility to eutrophication is adapted from the Assessment of Estuarine Trophic Status (ASSETS) protocol developed in the United States (Bricker et al. 2003). This approach is based on physical characteristics of estuaries and nutrient input load-estuary response relationships for key New Zealand estuary types. The tool produces a single physical susceptibility score that can be used to classify either physical susceptibility (e.g., very high, high, moderate, low susceptibility) or be combined with nutrient load data to produce a combined physical and nutrient load susceptibility score.

When to use the ASSETS approach results

The estuaries used in the USA ASSETS application were large and relatively deep, and consequently had high dilution of nutrient loads from land. The ASSETS banding system was developed for such estuaries. This presents a problem for ASSETS application in New Zealand where over half of estuaries are too small and shallow to be assigned rating categories in ASSETS. Consequently, its use is only appropriate for New Zealand’s larger volume estuaries (i.e., fiords, embayments, some larger tidal lagoons and larger tidal rivers; see Page 30, Robertson et al. 2016a). We do not recommend using ASSETS for NZ estuaries with an estuary volume of less than 2.8 million m^3.

ETI Tool 1 provides a default dataset containing information for 446 estuaries in New Zealand. The default data were sourced from Coastal Explorer, a database compiled using expert panels, including regional council staff, knowledgeable locals, university staff, and consultants. Information was mined from various sources including 1:50,000 topographic maps, aerial photographs, New Zealand Land Resources Inventory, the National Land Cover Data Base, the NZ Estuarine Environment Classification database, the New Zealand tidal model, wave hindcast models, RNZN Hydrographic charts, and numerous publications and reports. Councils may hold more accurate and recent information than that available from the Coastal Explorer database. The default data should be relied on only for coarse scale surveys of vulnerability to eutrophication.

The default data were updated on 1 July 2022 using CLUES loads calculated on 3 March 2022 (Semadeni-Davies et al. 2021). Properties of some estuaries were also updated. Please refer to the changelog for details.

You can calculate susceptibility scores using the default data (shown in the table below) or choose to analyse your own data by using the Choose which data to use buttons. If you choose to upload your own data, we recommend downloading the default data file to use as a data template. For information about each of the variables in the data template, including the correct units, please download the metadata file.

When you have selected your dataset, please navigate to 2. Map estuary data.

Map showing the selected data for analysis. Click on an estuary to identify it. If estuaries are not plotting on New Zealand as expected, please check that the coordinates are specified correctly in your uploaded data.

Click the Calculate susceptibility button to run the ETI Tool 1 Calculator. Susceptibility results can be downloaded using the Download Results button. Definitions and units for the results are described in the metadata, available for download from the 1. Download data tab under the Run Tool 1 menu. Plots of the results can be generated using the 4. Plot results tab under the Run Tool 1 menu.

Note that sites with missing data will have NA returned for their dilution modelling bands or ASSETS scores, with the reasons calculations were unsuccessful provided in the dilution modelling comments or ASSETS comments columns, respectively. ICOE results for the dilution modelling approach are only calculated for estuaries where values for Typical Closure Length have been provided. ASSETS calculations for ICOEs use the preferred method outlined on Page 19 of Robertson et al. 2016 and do not generate values for flushing or dilution potential.

Use the dropdown menu to select different ways to plot the results. These plots can be downloaded using the Download results report button on the previous tab.


Estuary Typologies

The morphological types are based on the original estuary classification of Hume et al. (2007) and its more recent revision (Hume et al. 2016). The ETI uses a simplification of the Hume typology into four ETI classes (Hume 2018): Shallow Intertidally Dominated Estuary (SIDE), Shallow Short Residence Time Tidal River Estuary (SSRTRE), Deeper Subtidal Dominated Estuary (DSDE) and Coastal Lakes. Intermittently closed/open lakes and lagoons estuaries (ICOEs) are classified as sub-types of SIDEs and SSRTREs. Robertson et al (2016a, 2016b) refers to ICOLLs - this terminology is no longer used and has been replaced by sub-types of SIDEs and SSRTREs called ICOEs.

Dilution modelling approach

The dilution modelling approach is derived from the GIS-based CLUES Estuary component of CLUES and uses the same model formulations (see Glossary for definitions of words in bold). This tool allows the user to run the dilution modelling approach without requiring access to CLUES. The tool currently gives a single time- and space-averaged concentration as a function of mean flow and nutrient input, with the capability to include seasonal nutrient and flow differences. It provides susceptibility bandings for macroalgal and phytoplankton eutrophication potentials, as well as an overall ETI susceptibility band that considers the intertidal area of the estuary. The tool has the potential to offer a relative comparison between different land-use scenarios (by virtue of CLUES), and to identify estuaries likely to be highly sensitive to current nutrient loads based on their physical attributes. See Elliott et al. (2016) and Plew et al. (2018) for a detailed description of CLUES Estuary and Plew et al. (2020) for details of how susceptibilities are calculated.

The dilution modelling approach requires unique tuning parameters for each estuary. These can be obtained through estuary surveys, using salinity to calibrate the parameters. As this has been done only for a few estuaries, estimated values for these tuning parameters are provided with the default input values. Our experience suggest that the default values are reasonable for SIDEs and SSRTREs but tend to underestimate dilution in large DSDEs such as coastal embayments, fiords and sounds. This may result in overestimates of flushing times and potential nutrient concentrations.

By default, the dilution modelling approach assumes that all estuaries are open to the sea (other than coastal lakes, which are considered permanently closed). For systems that intermittently close, (i.e., intermittently closed/open estuary states - ICOEs), predictions for the potential nutrient concentrations and bandings for macro-algae and phytoplankton susceptibilities are made for the closed state if the user provides an estimate of typical closure duration in days (Tl in the input file).

ASSETS approach

The ASSETS approach is a quick screening method for assessing estuary susceptibility that takes into account estuary morphology when producing its ratings. The ASSETS approach takes into account stratification, estuary volume, freshwater inflow, and tidal range, in two physical susceptibility indicators dilution potential and flushing potential. Daily nitrogen load per square metres is used for a third susceptibility indicator N load susceptibility. The dataset used to derive the susceptibility response relationships for ASSETS in the USA used large estuaries, which were primarily subtidal, open, 7.5m mean depth, long residence time embayments with moderate to high eutrophic symptoms. The most commonly occurring eutrophic symptom was high, widespread concentrations of elevated chlorophyll-a levels (i.e., phytoplankton), although most estuaries also exhibited at least one other moderate to high symptom (e.g., dissolved oxygen).

- Bricker, S., Ferreira, J., Simas, T. (2003) An integrated methodology for assessment of estuarine trophic status. *Ecological Modelling* 169: 39-60. [view online]
  • Elliott, A.H., Semadeni-Davies, A.F., Shankar, U., Zeldis, J.R., Wheeler, D.M., Plew, D.R., Rys, G.J., Harris, S.R. (2016) A national-scale GIS-based system for modelling impacts of land use on water quality. Environmental Modelling & Software 86: 131-144. [view online]

  • Hicks, M., Semadeni-Davies, A.F., Haddadchi, A., Shankar, U., Plew, D.R. (2019). Updated sediment load estimator for New Zealand. NIWA Client Report, Prepared for Ministry for the Environment, 190: Prepared for Ministry for the Environment [view online]

  • Hume, T. (2018) The fit of the ETI trophic state susceptibility typology to the NZ coastal hydrosystems typology. NIWA Client Report 2017007CH: 34. [view online]

  • Hume, T., Gerbeaux,P., Hart, D., Kettles, H., Neale, D. (2016) A classification of New Zealand’s coastal hydrosystems. NIWA Client Report HAM2016-062: 112.

  • Hume, T., Snelder, T., Weatherhead, M., Liefting, R. (2007) A controlling factor approach to estuary classification. Journal of Ocean and Coastal Management 50: 905-929. [view online]

  • Plew, D.R., Zeldis, J.R., Dudley, B.D., Whitehead, A.L., Stevens, L.M., Robertson, B.M., Robertson, B.P. (2020) Assessing the eutrophic susceptibility of New Zealand Estuaries. Estuaries and Coasts 43: 2015-2033. [view online]

  • Plew, D.R., Zeldis, J.R., Shankar, U., Elliott, A.H. (2018). Using simple dilution models to predict New Zealand estuarine water quality. Estuaries and Coasts 41: 1643-1659. [view online]

  • Robertson, B., Stevens, L., Robertson, B., Zeldis, J., Green, M., Madarasz-Smith, A., Plew, D., Storey, R., Hume, T., Oliver, M. (2016) NZ Estuary Trophic Index Screening Tool 1. Determining eutrophication susceptibility using physical and nutrient load data. Prepared for Envirolink Tools Project: Estuarine Trophic Index, MBIE/NIWA Contract No: C01X1420: 47. [view online]

  • Semadeni-Davies, A., Elliott, S., Shankar, U., Huirama, M. (2021a) Memorandum on modelling nutrient, sediment and E. coli loads to estuaries, Memorandum to Ministry for the Environment. NIWA client report 2021366HN prepared for the Ministry for the Environment. NIWA, Auckland.

  • Whitehead, A.L., Depree, C., Quinn, J.M. (2019) Seasonal and temporal variation in water quality in New Zealand rivers and lakes. NIWA Client Report 2019024CH prepared for the Ministry for the Environment. NIWA, Christchurch.[view online]