Habitats

The main habitats monitored are unvegetated sediments (e.g. mud and sand areas) and areas vegetated with salt marsh and seagrass. Degraded habitat is a major contributor to reduced aquatic ecosystem health.

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Substrates

In terms of estuarine health, a key broad scale focus is on understanding the spatial extent and temporal change in mud-dominated sediment (>50% mud content) across intertidal areas.

See Stevens & Forrest 2020b - habitat mapping for technical methods, data tables, and further information

Mud content

At discrete fine scale and sedimentation monitoring sites, mud content is determined from laboratory analysis of surface sediment samples, and results are rated against thresholds derived from the New Zealand Estuary Trophic Index. A sample mud content of 25% is considered the threshold above which significant ecological changes in associated macroinvertebrate communities can occur.

See Cummings et al. 2022, Forrest et al. 2022 and Stevens et al. 2022 for technical methods, data tables, and further information

Salt marsh

Salt marsh is upper tidal vegetation able to tolerate saline conditions where terrestrial plants are unable to survive. This habitat type is important in estuaries as it is highly productive, naturally filters and assimilates sediment and nutrients, acts as a buffer that protects against introduced grasses and weeds, and provides an important habitat for a variety of species including fish and birds.

See Stevens & Forrest 2020b - habitat mapping for technical methods, data tables, and further information

Seagrass

Seagrass (Zostera muelleri) beds are important ecologically because they enhance primary production and nutrient cycling, stabilise sediments, elevate biodiversity, and provide nursery and feeding grounds for a range of invertebrates and fish. Although tolerant of a wide range of conditions, seagrass is vulnerable to light reduction due to fine sediments in the water column, burial from sediment inundation, macroalgal overgrow where nutrient enrichment is excessive, and sediment oxygen reduction where macroalgal overgrowth or excessive organic enrichment occurs. Note that surveys prior to 2008 did not cover the whole harbour with areas not surveyed shaded in grey. Seagrass beds refer to areas with 30% coverage or greater.

See Matheson & Wadhwa 2012 and Stevens & Forrest 2020b - habitat mapping for technical methods, data tables, and further information

Macroinvertebrates

Macroinvertebrates, also known as macrofauna, are the animals living on top of or within the sediment. The abundance, composition and diversity of macrofauna are commonly-used indicators of estuarine health. The AZTI’s Marine Biotic Index (AMBI) is one of several marine biotic indices that assesses estuarine health based on the types and numbers of macrofauna and their known tolerances to environmental stress. Lower AMBI values generally indicate better ecological conditions. Values are rated against thresholds derived from the New Zealand Estuary Trophic Index.

See Oliver & Conwell 2014 and Forrest et al. 2022 for technical methods, data tables, and further information