2019/20 Results and comparisons with previous years

Usage guide: toggle between tabs to show maps of key indicators of ecosystem health in relation to the main issues facing Te Awarua-o-Porirua harbour, click and drag the year slider underneath each map to see change over time, and hover over map data to see specific values.

Sedimentation & bathymetry

Sediment (particularly muddy sediments) discharged into rivers, streams and harbours can negatively impact a range of values, including ecosystem health and the way people use water for recreational, cultural and spiritual purposes.

Sedimentation rate

The depth of sediment overlying concrete pavers buried at discrete sites provides an indicator of estuary sedimentation. The map shows monitoring site (circles) annual sedimentation over a rolling five year period and the whole harbour average of these values (shaded region). Positive values indicate where there has been net sediment deposition (accumulation) and negative values indicate net erosion.

See Stevens LM, Forrest BM 2020. Te Awarua-o-Porirua Harbour Sediment Plate Monitoring 2019/2020 for technical methods and further information.

Bathymetry

Contours displayed at 0.25 m intervals derived from depth databases gridded at 10 m resolution. Bathymetry depths (m) are referenced to Porirua Sounding Datum (SD) which is linked to Porirua Chart Datum (CD), defined as 2.55 m below Land Information New Zealand (LINZ) steel pin C1K1 at the Mana Cruising Clubrooms.

See Waller & Stubbing 2019. Porirua Harbour bathymetric survey 2019 for technical methods and further information.

Bathymetry change

Contours derived from differencing the 10 m resolution depth databases from each combination of surveys. Red areas indicate sedimentation, while blue areas indicate general harbour erosion. Bathymetry depths (m) are referenced to Porirua Sounding Datum (SD) which is linked to Porirua Chart Datum (CD), defined as 2.55 m below LINZ steel pin C1K1 at the Mana Cruising Clubrooms.

See Waller & Stubbing 2019. Porirua Harbour bathymetric survey 2019 for technical methods and further information.

Habitat

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.

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 2020. Broad Scale Intertidal Habitat Mapping of Te Awarua-o-Porirua Harbour for technical methods 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 Forrest, Stevens & Rabel 2020. Fine Scale Intertidal Monitoring of Te Awarua-o-Porirua Harbour, Stevens & Forrest 2020. Te Awarua-o-Porirua Harbour Sediment Plate Monitoring 2019/2020 and Oliver & Conwell 2014. Te Awarua-o-Porirua Harbour subtidal sediment survey: Results from the 2010 survey for technical methods 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 2020. Broad Scale Intertidal Habitat Mapping of Te Awarua-o-Porirua Harbour for technical methods 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 Stevens & Forrest 2020. Broad Scale Intertidal Habitat Mapping of Te Awarua-o-Porirua Harbour and Matheson & Wadhwa 2012. Seagrass in Porirua Harbour for technical methods and further information.

Macroinvertebrates

Macroinvertebrates, also known as macrofauna, are the animals living on top of or within the sediment matrix. 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 Forrest, Stevens & Rabel 2020. Fine Scale Intertidal Monitoring of Te Awarua-o-Porirua Harbour and Oliver & Conwell 2014. Te Awarua-o-Porirua Harbour subtidal sediment survey: Results from the 2010 survey for technical methods and further information.

Nutrient enrichment

When an estuary or coastal environment becomes over-enriched with nutrients excessive growths of algae can occur (eutrophication). These include growths of ‘opportunistic’ macroalgae such as the red seaweed Gracilaria chilensis, and blooms of potential harmful phytoplankton (microscopic algae that drift in water currents), which can include species that release biotoxins.

The apparent Redox Potential Discontinuity (aRPD) depth is a subjective measure of the enrichment state of sediments according to the depth of visible transition between brown oxygenated surface sediments and deeper grey-black less oxygenated sediments.

Macroalgae

Opportunistic macroalgal blooms are a primary consequence of estuary eutrophication (nutrient enrichment). Macroalgal blooms can deprive seagrass beds of light, causing their decline, while decaying macroalgae can accumulate subtidally and on shorelines causing oxygen depletion and associated nuisance odours in the sediments beneath. The main problem species in New Zealand are the red seaweed Gracilaria chilensis and the bright green Ulva. In Porirua Harbour over recent years, there has also been an increased prevalence of the filamentous green mat-forming species Chaetomorpha ligustica.

See Stevens & Forrest 2020. Broad Scale Intertidal Habitat Mapping of Te Awarua-o-Porirua Harbour for technical methods and further information.

Sediment oxygenation

The apparent Redox Potential Discontinuity (aRPD) is a time-integrated measure of the enrichment state of sediments according to the visual transition between brown oxygenated surface sediments and deeper less oxygenated grey/black sediments. The aRPD usually occurs closer to the sediment surface as organic matter loading increases. Values are rated against thresholds modified from those presented in the New Zealand Estuary Trophic Index.

See Forrest, Stevens & Rabel 2020. Fine Scale Intertidal Monitoring of Te Awarua-o-Porirua Harbour and Stevens & Forrest 2020. Te Awarua-o-Porirua Harbour Sediment Plate Monitoring 2019/2020 for technical methods and further information.

Total Organic Carbon

Total Organic Carbon (TOC) is a measure of the organic content of sediments, which is associated with their enrichment status. Sediments with a high TOC (>1-2%) often display symptoms that indicate excessive enrichment, including reduced oxygenation. Values are rated against thresholds derived from the New Zealand Estuary Trophic Index (ETI).

Total Nitrogen

Nitrogen is a key nutrient for plant and algae growth in estuarine and marine environments. Total nitrogen in sediments is an indicator of their trophic status and potential for algal blooms or other symptoms of excessive enrichment. Values are rated against thresholds derived from the New Zealand Estuarine Trophic Index (ETI).

Metal contaminants

Trace metals, when in high concentrations, can have toxicant effects on aquatic life in both a dissolved state and when attached to sediment particles. Zinc and copper in particular are often used as proxies for the suite of other potential urban contaminants (e.g. polycyclic aromatic hydrocarbons, plasticisers) or legacy contaminants such as the historic pesticide DDT. These types of contaminants often end up in estuaries via stormwater runoff. Copper is approximately 5 to 10 times more toxic to aquatic life than zinc, but generally occurs in lower concentrations.

Values are rated against thresholds derived from the New Zealand Estuary Trophic Index (ETI), which in turn are scaled relative to 2018 Australia and New Zealand Guidelines (ANZG) for sediment quality. Ratings of “good” and “very good” correspond to ‘safe’ values that are less than ANZG (2018) default guidelines, while “moderate” corresponds to values between the default and high-guideline values, reflecting “possible” ecological effects. “Poor” sites exceed the high-guideline value.

See Forrest, Stevens & Rabel 2020. Fine Scale Intertidal Monitoring of Te Awarua-o-Porirua Harbour, Sorenson & Milne 2009. Porirua Harbour targeted intertidal sediment quality assessment and Oliver & Conwell 2014. Te Awarua-o-Porirua Harbour subtidal sediment survey: Results from the 2010 survey for technical methods and further information.

Zinc

Copper

Additional results

Te Awarua-o-Porirua Harbour Catchment Sediment Monitoring Programme

Sea Level Rise Mapping (scroll/zoom the map to Te Awarua-o-Porirua Harbour)

Recreational Water Quality Monitoring Programme (results for the wider region including Te Awarua-o-Porirua Harbour)

Guardians of Pāuatahanui Inlet Cockle Survey