Hauraki Auckland Benthic Habitat Map bookmarks

Used in Storymap https://storymaps.arcgis.com/stories/7951d5b1fdfe4c3dae8498aaa6a9a687 and/or InstantApp https://niwa.maps.arcgis.com/apps/instant/media/index.html?appid=29edc889a3b340cab4d382f5a9155c06OverviewNIWA was tasked by Te Kaunihera o Tāmaki Makaurau / Auckland Council to undertake a series of benthic terrain analyses of datasets collected across the wider Hauraki Gulf to produce a desktop habitat study. With nearly 70% of the area managed by Te Kaunihera o Tāmaki Makaurau / Auckland Council located offshore, results from a desktop habitat study provide further insights into Hauraki Gulf benthic habitats and can assist in prioritising areas for management and future research. DataData for these analyses come from two different sources, NIWA’s own internal archive (NIWA voyage TAN1211) and LINZ’s archive of hydrographic surveys: HS52 – Approaches to Auckland (LINZ) HS53 – Kawau Island (LINZ) HS54 – Tamaki Strait (LINZ) TAN1211 – NIWA multibeam mapping voyage Although all the data analyzed were collected for different purposes, reusing legacy multibeam echosounder (MBES) data shows the strength of the ‘map once – use multiple times’ approach that is now standard practice. To achieve the best outcome for scientific interpretation, NIWA has partially reprocessed the bathymetry and seafloor backscatter information from data provided to LINZ. This has improved the data for the purpose of scientific interpretation.

Our analysis of these data focused on: building derivatives and classifying the bathymetry data,  processing the seafloor backscatter data in a qualified way for feature detection and classification,  creating scientifically justified interpretations of the seafloor bathymetry and backscatter data. Results from these studies provide a comprehensive overview of seabed morphology and activity across a large region of the Hauraki Gulf, covering a broad range of marine environments.Bathymetry

Bathymetry (shape and depth of seafloor) is illustrated as a digital elevation model (DEM). This is the processed layer that represents thousands of individual depth measurements collated to form a single continuous DEM. The resolution of each DEM depends on a range of factors including data quality, water depth, and echo sounder settings. The higher the resolution (grid cell size) of the DEM, the greater detail we are able to see on the seabed. From bathymetry data, other layers can be produced to assist in understanding key features of interest within the survey region. These layers are often referred to as derivatives and they are described in detail below.

To aid in depth perception, hillshades have been generated and visualised in conjunction with the bathymetry layer to allow for seafloor structures and depth to be seen. Hillshade layers are derivatives of bathymetry and can be generated with a range of illumination and altitude angles to reveal different components of the dataset. In this dataset, a multidirectional Hillshade was produced 3 times vertical exaggeration. Multidirectional Hillshade is generated by producing hillshades from six different directions (rather than just one direction used in a default Hillshade) and enables complex and subtle features to be visualised without obscuring details in otherwise non-illuminated regions. 

Slope

The slope derivative is a measure of how steep the seafloor is. Slope is measured from the horizontal (i.e., 0°, or flat) and increases to 90° (i.e., vertical). To calculate a slope derivative, the depth values within each cell are compared to adjacent cells (within a user-defined window) to determine the seafloor gradient across the whole dataset. 

Rugosity

Rugosity is a measure of roughness and terrain complexity and is captured as bathymetric variation in three dimensions. Ecological diversity can generally be correlated with the complexity of the physical environment and as such, rugosity can help identify areas where high biodiversity may exist on the seafloor.

Aspect

Aspect is the direction of down-slope dip and can also be thought of as the slope direction. The Aspect derivative produces a layer with cell values that correspond to compass directions presented in degrees from 0/360° (due north) clockwise to 90° (east), 180° (south) and 270° (west). Areas with zero slope will also show no aspect. 

Curvature

The curvature derivative describes the shape of slope, and is sometimes referred to as the second derivative of bathymetry. Positive curvature at a location indicates that the surface is upwardly convex, e.g., a mound. Negative curvature indicates that the surface is upwardly concave, e.g., a depression. A neutral value of 0 indicates that the surface is flat. The colour gradient is symmetrical about zero curvature to emphasise curved versus flat seafloor.

Seafloor Backscatter

Seafloor backscatter data is collected at the same time as bathymetry data but measures the energy of the returning acoustic signal. The acoustic energy can be correlated to seafloor substrate and can effectively delineate substrate boundaries that may not be observable in the bathymetry data or derivatives. 

Benthic Terrain Model

Based on NIWA’s National Benthic Terrain Classification, the Benthic Terrain Model uses a range of bathymetric derivatives to classify the physical structure of the seafloor into geomorphic zones, e.g., flat plains, ridges, depressions etc. These classifications are in accordance with a national standard and can be used to compare with other regions.HS52 Features

Collected in 2017, the hydrographic survey HS52 – Approaches to Auckland covers an area of 280 km2 between Auckland’s Northshore, Rangitoto and the Whangaparāoa Peninsula. The water depth within the HS52 survey ranges from less than 1 m along the coastline, to approximately 40 m in the northeastern portion, east of Tiritiri Matangi Island. Bathymetry data reveal the variability of the seafloor from the near coastal rocky reefs to the broad flat seabed within the Hauraki Gulf channels.

<!-Nearshore Strata Faulting

Nearshore rocky outcrop showing tilted parallel rock units dipping to the west and offset by faults. These outcrops are likely the offshore extension of Late Oligocene Early Miocene-age deep water mud and sandstones of the Waitemata Group.

Backscatter imagery highlights the boundary between the hard rocky substrate (lighter) compared to the deeper and likely soft sediment substrate (darker)

<!Tessellated Reef

Tessellated rocky reef possibly representing a more textured, highly fractured, unit of the Waitemata Group. 

Backscatter imagery highlights the more textured hard rocky reef substrate (lighter) compared to the deeper and smoother soft sediment substrate (darker).

<!-Channels

Channels between the mainland and Hauraki Gulf Islands display a unique seabed backscatter signature compared to adjacent seafloor. Varying backscatter is indicative of a substrate boundary. For example, lighter regions near the coast and within the channel may be indicative of larger sediment grains such as sand, compared to the flat plains with darker backscatter which could be dominated by muddy sediment.

Linear Sediment Waves

Linear sediment waves on the eastern side of the channel between Whangaparāoa Peninsula and Tiritiri Matangi Island. Sediment wave crests are oriented approximately east-west and may be related to currents between the coast and the Hauraki Gulf Islands.

<!-Rounded Depressions

A cluster of rounded depressions may be indicative of relict or active seabed seepage and potential sites of sensitive ecosystems. Seafloor backscatter reveals the sloping rim of these features may have a harder substrate compared to their base.

<!-Pockmarks

Pockmarks following distinctive linear arrangement, oriented NW-SE. This provides evidence of seafloor fluid seepage, possibly along faults. The pockmarks are well expressed in seafloor backscatter as light circular dots on the darker seabed background.

<!--Rocky Reef at ~30 m

Rocky reef patch located in ~30 m water depth. Possible important habitat for benthos that require hard/rocky substrate for community building. This feature is a distinctive high backscatter region, surrounded by lower backscatter seafloor.

<!-Rugose and Textured Seafloor

Rugose and textured seafloor characterised by higher backscatter return compared to surrounding seafloor, possibly formed via current winnowing and/or may represent important habitats for hard substrate benthos.

<!--Imprint of Pipeline

Bathymetric imprint of pipeline from Mairangi Bay. Expression extends over 400 m in length and 60 m width. Seafloor backscatter highlights pipeline well, showing it as a high-intensity feature (indicative of a harder substrate) relative to surrounding seafloor.

<!- Anchoring Footprint

Anchoring footprint on the seabed. Linear scours caused by anchor deployment and feathering marks caused by gauging of the anchor chain scope whilst vessel swings. Regions with anchoring impacts have a lighter backscatter return compared to the surrounding seabed.

HS53 Features

Surveyed in 2016-17 for hydrographic charting, survey HS53 – Kawau Island covers an area of 184 km2 between Tāwharanui Peninsula, Mahurangi Peninsula and the region surrounding Kawau Island. The water depth within the HS53 – Kawau Bay survey ranges from the coast to ~61 m water depth, with an average depth of 28 m. The seafloor is shallowest within Kawau Bay and deepens eastward into the central Hauraki Gulf/Tīkapa Moana through the North and South Channels. Within Kawau Bay, away from the coast, the seabed morphology is generally gentle and undulating.

R oRocky Platform of Tilted Units

Extensive rocky reef platform to the east of Mahurangi East Peninsula and Big Bay. The rocky outcrop shows tilted parallel rock units offset by faults. These outcrops are likely the offshore extension of Pakiri Formation of Warkworth Subgroup (Waitemata Group).

Units show variable dip direction and angles, consistent with onshore observations. The rocky unit extends offshore, becoming less obvious in bathymetry data, but observable in seafloor backscatter as relatively high intensity substrate compared to surrounding seabed. 

<!--Shipwreck – Sailing Ship REWA

One of many nearshore wrecks in these surveys. This wreck lies close to the beach at Martins Bay on Moturekareka Island. The Rewa’s three masts are clearly visible in bathymetric data as ~20 m straight poles lying perpendicular to the main hull of the ship.

<!-Bathymetric Step

An isolated ~5 m high, 400 m long bathymetric step located in the channel between Kawau Island and Tāwharanui. The step faces to the SSW and is parallel to the orientation of form lines of the onshore Waipapa Group sandstone and siltstone (Waipapa Composite Terrane). The step is characterised as lower seafloor backscatter compared to surrounding seabed. 

<!RRocky Reef / Geological Boundary – Offshore Omaha

This rocky reef offshore Omaha hosts two distinctive morphologies. The northern unit has less relief and shows subtle linear structures that may correspond to stratigraphic layers. The southern unit is highly textured with fractured and jointed surface texture. The two outcrops could represent the geological contact between Waipapa composite terrain (north) and Waitemata group (south), as mapped onshore. The nearshore region around Omaha, and Goat Island to the north has been mapped to a corresponding degree (1 m resolution) and overlaps with this survey. Seafloor backscatter shows variable intensity, across the textured, rugose regions, and lower backscatter over the surrounding flat featureless region. 

<!--Geological Boundary – Eastern Kawau Island

The eastern margin of Kawau Island hosts an extensive rocky reef. The onshore geological units comprise the Waipapa composite terrain and early Miocene Waitemata group. The offshore rocky reef shows two morphologically distinct rocky reef structures, to the north the bedded and fractured Waipapa terrain and to the couth, the tessellated and highly rugose Waitemata group. The seafloor backscatter in this region shows a wide band of variable seafloor backscatter. Higher seafloor backscatter is characteristic of the rocky nearshore reefs, with adjacent seafloor backscatter showing complex high and low backscatter patterns, indicative of variable substrates/habitats.

<!- Anchoring Footprints

Anchoring footprint on the seabed west of Kawau Bay and Mayne Islands (~13 m water). Linear scours are caused by anchor deployment and feathering marks caused by gauging of the anchor chain scope whilst vessel swings. No seafloor backscatter is available in this region, zoomed in image shows distinctive featuring mark caused by ships swinging on anchor. 

<!HHuman Infrastructure – Cable or Pipeline

Very faint lineation representing a pipeline or cable that runs from Goldsworthy Bay to Takangaroa Island and on to Kawau Island near Mansion house. The pipeline/cable is visible in satellite imagery and extends over 4.5 km within the surveyed region. The cable footprint is ~4 m wide.  

<!--Sediment Ripples

Sediment ripples in channel opening between Kawau Island and Tāwharanui Peninsula. Sediment wave crests are oriented approximately southeast-northwest and may be related to currents between the coast and the Hauraki Gulf Islands. The seafloor backscatter across the sediment waves shows lower backscatter return over the sediment waves, indicating finer grainsize substrate over the bedforms compared to surrounding area. 

HS54 Features

Collected in 2016-17, hydrographic survey HS54 - Tāmaki Strait covers 310 km2 between Waiheke, Motutapu and Maotuihe Islands, the Noises, Mahurangi Peninsula and the southern eastern shore of Tāmaki Makaurau/Auckland. Water depths within the HS54 – Tāmaki Strait survey range from the coast to ~46 m water depth, with much of the enclosed waters between eastern Tāmaki Makaurau/Auckland and Waiheke Island less than 10 m depth. The HS54 – Tāmaki Strait survey lies adjacent to the HS52 - Approaches to Auckland survey, with many seabed features observed across both datasets.

1.     Pockmarks

Pockmarks following distinctive linear arrangement, oriented NW-SE over ~3 km within surveyed region. This linear array of pockmarks aligns with those identified in HS52 (Approaches to Auckland), to form over 9 km of pockmarks that extend from Tiritiri Maitangi in the northwest, to the Noises in the southeast. This lineation is parallel to known fault in the area, mapped onshore Tiritiri Maitangi. Pockmarks provide evidence of fluid seepage, possibly along faults. The pockmarks are well expressed in seafloor backscatter as light circular dots on the darker seabed background.

2.      Mound Field

Hundreds of mounds at ~ 5 -10 m depth scattered within the channel between Waiheke Island and the mainland, between Duder Regional Park in the east to Motukaraka Island in the west. The mounds are circular, and densely spaced, with some joining to form sublinear aggregations of high local relief towards the west. The eastern limit of the mounds is characterised by low surrounding seafloor backscatter (likely fine sediment) and an abrupt increase high backscatter mounds to the west, likely representing hard, rocky substrate. Mounds are less than 1 m in vertical relief, and individually ~ 50 m across.

<! <Round Depressions

A cluster of rounded depressions is concentrated south of Waiheke Island in the Waiheke Channel. These features may represent relict or active seabed seepage and potential sites of sensitive ecosystems. Seafloor backscatter reveals the sloping rim of these features may have a harder substrate compared to their base.

4.      Anomalous High Backscatter Features

Several anomalous positive relief and high backscatter structures lie near Cable Bay offshore the northern Rotoroa Island. These features are rounded and may represent the physical footprint of a human activity, due to their highly localised nature and relatively small scale. 

5.     Submarine Channel

A prominent linear bathymetric channel between Waiheke and mainland. The channel is filled with dark seafloor backscatter (finer sediment) compared to surrounding seabed. 

6.      Vessel Mooring Blocks

The seabed footprint of vessel moorings manifests as irregular mounds in shallow bays within Putaki/Putiki bays, Waiheke Island. Origin of footprint verified using satellite imagery.

<!7.Scoured Channel

Strongly scoured and rugose bathymetry channel between Waiheke, Crusoe Islands and Motuihe Island/Te Motu-a-Ihenga. Deep scours are symmetrical about the narrowest section of the channel, likely forming due to increased current activity within the confined channel. Seafloor backscatter suggests the substrate is relatively hard, or coarser grained compared to open flat seafloor. The base of channel scours host complex sediment bedforms and rocky reef outcrops. Rocky reefs show distinctive tilted parallel rock units dipping to the southwest. Adjacent onshore rock units include East Coast Bays Formation of Warkworth Subgroup (Waitemata Group) and Waipapa Group sandstone and siltstone (Waipapa Composite Terrane). 

->DDredged Channel & Circular Mounds

A series of small circular mounds lead into the dredged channel leading into Pine Harbour Marina. Two dominant clusters of mounds lie to the south of the dredged channel, and likely represent the bathymetric footprint of a human activity. 

<!->Geological Folding

Highly distinctive curvilinear folded rocky reef unit runs along the eastern margin of Motukorea/ Browns Island. The adjacent onshore unit is the Auckland Basalts tuff (Kerikeri Volcanic Group) of Auckland Volcanic Field. Backscatter imagery highlights the more textured hard rocky reef substrate (lighter) compared to the deeper and smoother soft sediment substrate (darker).

< CCurved Seabed Markings

Rounded mounds and curvilinear high backscatter markings cover ~1 km2 of seabed to the northeast of Motukorea/ Browns Island. Similar to those near Cable Bay offshore the northern Ponui Island/ Chamberlins Island. These features may represent the physical footprint of a human activity, due to their highly localised nature and relatively small scale.

]-->Sediment Wave Field

An isolated oval-shaped region of linear-curvilinear sediment waves lies in channel between Motutapu Island and Motuihe Island/Te Motu-a-Ihenga. Sediment wave crests are oriented approximately east west and may be related to currents between the coast and the Hauraki Gulf Islands.

TAN1211 Features

NIWA undertook voyage TAN1211 in 2012 as a part of the Oceans Survey 2020 mapping program on behalf of the New Zealand Government. The water depth within the TAN1211 survey ranges from ~20 m water depth in the waters around Great Barrier Island/Aotea and Little Barrier Island/Te Hauturu-o-Toi to ~140 m water depth in the northern portion of the surveyed area, near Flax and Mokohinau Islands. The area surveyed during voyage TAN1211 covers an area of 1268 km2.

R>Rocky Reef – Simpson Rock

Located south of the Mokohinau Islands, surrounding Simpson Rock, these outcrops are dominated by high seafloor backscatter, indicative of harder, substrate, particularly on the eastern side of Simpson Rock to depths of 80 m. Rocky reef extends northward to Motukino/Fanal Island. Exposed rocky islands are composed of Whitianga Group Rhyolite of the Coromandel Volcanic Zone. Seafloor backscatter on the western and southern side shows alternating high and low reflectivity bedform structures.

<!  Reef and sediment ripples

Reef and sediment ripples well represented in seafloor backscatter by high and moderate reflectivity, adjacent to lower reflectivity surrounding seabed in the channel west of Te Hauturu o Toi/Little Barrier Island. Linear sediment ripples trend to the northeast-southwest and extend from higher backscatter rocky structures.  

<!  Rocky Reef – Mokohinau & Flax Islands

Complex and irregular network of higher seafloor backscatter sediment ripples north of Te Hauturu o Toi/Little Barrier Island. Ripples are difficult to discern in bathymetric data, due to data quality and noise. Ripples extend at least 4 km and trend generally in a northeast-southwest direction and could reflect dominant current flows. 

<!-  Sediment Ripples – North Little Barrier Island

Complex and irregular network of higher seafloor backscatter sediment ripples north of Te Hauturu o Toi/Little Barrier Island. Ripples are difficult to discern in bathymetric data, due to data quality and noise. Ripples extend at least 4 km and trend generally in a northeast-southwest direction and could reflect dominant current flows.

<!-  Sediment Wave Field

Sediment wave field between Aotea/Great Barrier Island and Coromandel Peninsula. Bedforms comprise a range of seafloor backscatter intensities, indicative of variable substrate grainsize. To the east, bedforms are asymmetrical, with steeper lee slopes facing west, and larger, with heights up to 15 m and wavelengths of ~400 m. To the west, bedforms are more widespread, symmetrical, and smaller, with heights of less than 1 m and wave lengths of ~100 m.

_________________Item Page Created: 2023-10-25 02:10 Item Page Last Modified: 2025-04-05 18:03Owner: davidsons_NIWAFeature of Interest Points ARC23303 Item id: f3c3ec58a9434b5e92109e446697c621Benthic Terrain ModelsItem id: a0fabd907e654d6da0c804d94254d22fHauraki BackscatterItem id: fac151e9667e400287d990475d7bfb16Hauraki BathymetryItem id: 562feb7e4e49483bb1c153681241c9e6Bathymetry Derivatives AspectItem id: 0b1b4a0620fc4ad9902473b5b3564747Bathymetry Derivatives CurvatureItem id: a65fb8208e354d47b7cd9746050cf270Bathymetry Derivatives RugosityItem id: 935cfcd32a944dc68e277620d324ba12Bathymetry Derivatives SlopeItem id: bae7dfc08c5748318a838f9e020f5125