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:
- Tool 1: Determining eutrophication susceptibility using physical and nutrient load data
- Tool 2: Assessing Estuary Trophic State using measured trophic indicators
- Tool 3: Assessing Estuary Trophic State using a Bayesian Belief Network
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, 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.
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