Coastal biology

Monitoring sites

In 2019, we monitored sediment-dwelling animals at two sites in the Firth of Thames, three sites in Whāingaroa (Raglan) Harbour, five sites in Tairua Harbour and four sites in Coromandel Harbour. We have previously monitored three other sites in the Firth of Thames, and three other sites in Whāingaroa Harbour (see map below). Monitoring sites are distributed throughout the main part of each estuary and are located at the mid-intertidal level of sand and mud flats (in areas without seagrass or mangroves).

Monitoring frequency and history

Monitoring is currently undertaken once per year in spring (i.e. between September and November). Monitoring began at the Firth of Thames and Whāingaroa Harbour in 2001, at Tairua Harbour in 2012, and Coromandel Harbour in 2019.

Monitoring technique

We currently collect 10 cores (13 cm in diameter and 15 cm deep) from a permanent plot located at each monitoring site. Each plot is approximately 10,000 m² (100 m x 100 m), and is divided into 10 equal-sized sectors. Within each sector a random location is selected and a core sample taken. Cores taken in adjacent sectors must be at least five metres apart.

We separate animals from the sediment in each sample using a sieve with 500 µm mesh. We then preserve samples in 70 per cent isopropyl alcohol (diluted with tap water) and add a stain (Rose Bengal) that turns all the animals red/pink.

In the laboratory we separate the stained animals from the sediment and other debris, identify and count the animals to the lowest practicable taxonomic level (generally species or species group), and then store the samples in 50 per cent isopropyl alcohol.

This indicator presents a Traits Based Index (TBI) score for each monitoring site in the Firth of Thames, Whāingaroa (Raglan) Harbour, Tairua Harbour and Coromandel Harbour. The information is derived from the sediment-dwelling animals identified in the core samples collected from each monitoring site from 2012 to 2020.

The Traits Based Index (TBI), developed by NIWA and Auckland Council, is based on the numbers of types of animals exhibiting particular traits. It has been applied to Waikato region monitoring data following a series of trials and efforts to improve taxonomic resolution to be consistent with Auckland.

All animals identified in samples are first categorised according to traits (e.g., their feeding mode, degree of mobility, body size, etc.) that are reflective of their ability to perform certain ecosystem functions.  The index value is then calculated based on the numbers of taxa in seven particular trait groups (groups that have previously been shown to respond to mud and heavy metal contaminants).

Index values range between zero and one (with values near zero indicating highly degraded sites, and values near one indicating more pristine environments). Areas with high numbers of taxa per functional group (and high index values) are thought to have a greater capacity to cope with species losses. This is a component of resilience that contributes to maintenance of ecosystem functions in the face of stress and environmental disturbance. This is what the TBI gauges and, as such, is a meaningful indicator of estuarine health.

There are no formal guidelines or standards available for assessing this data in New Zealand at present. Our approach is based on similar programmes that have been developed by the National Institute of Water and Atmospheric Research (NIWA). Auckland Council uses similar estuary monitoring programmes.

With a maximum of eight years of data suitable for TBI calculations, trends in estuarine health over time have not yet been analysed statistically. Thresholds for TBI values that constitute good, intermediate, and poor functional redundancy (or good, moderate, and poor health) are based on Auckland estuarine monitoring data.

Qualified and experienced personnel carry out all sorting and taxonomic identification in line with a processing, identification and quality assurance protocol. On each sampling occasion a different person re-sorts, re-identifies and re-counts at least 10 percent of the samples from each site for quality control. The minimum acceptable sorting efficiency is 95 percent, and the minimum acceptable identification and counting efficiency is 90 percent.

At each site we collect information on the type and number of sediment-dwelling animals present. Our monitoring results from 2001 to 2011 have been reported on in a previous indicator, and further detail can be found on the REMP webpage, and in technical publications. 

In this indicator we report on monitoring results from 2012 onwards using an estuarine health index called the Traits Based Index (TBI). The TBI summarises animal community characteristics based on the animals’ biological traits. Biological traits are the physical and behavioural characteristics that define a particular species (e.g. body size, mobility, feeding behaviour). The TBI tells us about the health of our estuaries. It ranges between zero and one, with one being most healthy and zero being least healthy. We can use the TBI to track the health of our estuary sites over time, and to compare the health of different estuaries.

The graphs below show the TBI score for each monitoring site in the three currently monitored estuaries for 2012 to 2020. You can click on the image to view it full-size.)

Comparison of the four estuaries shows:

• Many of the sites are moderately healthy (having a TBI score between 0.3 and 0.4), some are in poor health (having a TBI score less than 0.3), and some are in relatively good health (having a TBI score greater than 0.4).
• Only Kuranui Bay and Miranda were sampled in 2020. These were both similar to previous TBI results and indicate that sites in the Firth of Thames are broadly similar to one another in terms of estuarine health.
• There was much greater difference between sites in Whāingaroa (Raglan) Harbour, when compared to the differences between sites in other estuaries. Haroto Bay and Kaitoke Bay were the least healthy, whereas Ponganui Creek was the healthiest. Whatitirinui Island TBI score increased from the previous year, returning to the level observed in 2016.  
• There were differences between sites in Tairua Harbour, with Pauanui having slightly higher TBI scores (indicating a healthier environment) than those at Pepe Inlet, Gumdigger Gully and Oturu Stream.
• Four sites were sampled for the first time in Coromandel Harbour in 2019; sites in the northern part of the harbour (McGregor Bay and Coromandel Town) were moderately healthy, whereas sites in the southern part of the harbour (Brickfield Bay and Awakanae Stream) were in relatively good health. Results from 2020 were similar to the previous year.
• At most sites, there were only slight changes in the TBI, and therefore in estuarine health, between 2012 and 2020. We expect the TBI score to vary slightly from year-to-year due to natural variability. There is not yet enough data to analyse trends statistically.

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