The estimated depth surfaces for the inner shelves around Lord Howe Island and Balls Pyramid have substantially enhanced the detail of the features that are very difficult to access using vessel-based platforms. The methods presented here provide a more accurate and detailed seabed model derived from satellite imagery and provide data that are more amenable for integration with multibeam echosounder (MBES) data. The application of multiple filters and the use of larger filter windows (10 cell size) provide a smoothing level similar to that produced with 4–5 m cell size MBES grids. Standard deviation filters were shown to remove outliers and artefacts which may occur from tiling edge effects or surface disturbance, and median 10 (10 cell size) filters were shown to produce a level of smoothing comparable with MBES data.
The selection of filters for individual studies depends on the level of pixel-to-pixel variation within the satellite image and the resolution of bathymetry datasets used for integration. The combination of the standard deviation and median 10 filters were selected for this study due to the surface disturbance observed on the eastern inner shelf and improved RMSE performance of the filtered surface. Image partitioning further improved surface accuracy through tailoring the regression to the east–west variation observed across the image. Quantitative (e.g., RMSE calculations) and qualitative (e.g., plotting residuals) error assessments indicate the reliability of the surface and its fitness for purpose.
The empirical, band ratio method for depth estimation from satellite imagery provides a relatively simple method of shallow depth estimation [10
]. Importantly, in this study the WV2 image for Lord Howe Island reveals high water clarity (Figure 1
), and this high clarity together with the high-resolution of the WV2 image enables a reasonably accurate product to be generated for the inner shelf. For the Balls Pyramid shelf, severe sun glint reduced the suitability of the method. Physics-based approaches may offer more accurate surfaces, however such approaches would also be compromised by sun glint and require increased model parameterisation and complex data processing [10
]. Therefore, the empirical, ratio-based approach employed here provides an efficient and relatively accurate method suitable for seabed geomorphic analysis.
4.1. Comparison of Shelf Morphology
The creation of a seamless bathymetry model for the entire shelf region of the two island shelves enabled geomorphic features to be mapped from the shoreline to shelf break at the same resolution and scale. The high-resolution shelf (5 m cell size) and regional (land, and shelf and slope, 8 m cell size) DEMs provide detailed information on shelf morphology, which allow for comparisons of the extent and distribution of fossil reefs. This, in turn, informs interpretations on the formation and driving processes of shelf features. The two oceanic shelves possess a diverse range of accretionary and erosional geomorphic features which have been defined and described. Submerged fossil coral reefs, basins, channels, pavements and terraces are identified on both shelves, with the expression and extent of features typically more pronounced on the larger shelf surrounding Lord Howe Island.
Mid-shelf fossil reefs dominate both shelves in 25–50 m, comprising a similar proportion of shelf area (approximately one third). The reefs form concentric patterns encircling large basins, which are inferred to be paleolagoons. The morphological similarities and comparable depth distributions suggest the mid-shelf reefs developed concurrently, with the mid-shelf reef around Balls Pyramid appearing to have drowned with postglacial sea-level rise [30
] while the Lord Howe Island shelf backstepped to form the modern reef [28
]. The Lord Howe Island mid-shelf fossil reef accreted several metres during the Holocene (9–2 ka) [29
], and it is presumed the Last Interglacial (125 ka, Marine Isotope Stage, MIS 5) reef material forms a significant component of the reef foundations, and possibly deposits from preceding interglacials (MIS 7, 9, 11).
The lateral and vertical extent of reef development is greatest on the southwestern shelves, interpreted as the more exposed, windward setting. Forereef buttresses border the eastern, western, and southern rims of the mid-shelf reefs and outer-shelf pavements, indicating variable exposure gradients which are typical of the mid-ocean setting [26
]. The development of larger buttresses (5–6 m height) on the southern rim of the Lord Howe Island fossil reef suggest the southern reef was exposed to significantly higher prevailing energy conditions from due south than occurred around the surrounding shelf where buttresses were reduced in size (1–2 m height).
On the outer shelf, where there was ample substrate available for colonisation on the outer-shelf pavement, the reefs cover similar relative areas on both shelves. These reefs formed as ridges and patch reefs, with ridges most developed on the southern outer shelves. Similar paleoshoreline features have been described around the Australian continental shelf [51
] and the linear, sub-parallel configuration of these features suggest beach barrier or coral reef origins. Occurring at 40–80 m depth, these features may have formed during postglacial sea-level rise of the Early Holocene or during earlier interstadials (e.g., MIS 3).
The dense network of patch reefs on the mid shelf and the linear reef systems on the eastern and northern inner shelves of Lord Howe Island are interpreted as transitional fossil patch and fringing reefs that developed as the reef retreated landward with postglacial sea-level rise. As the linear reefs have a maximum relief of 4 m, the associated lagoons are likely to be shallow and therefore more typical of fringing reef than barrier systems [52
]. While the less-exposed west coast is dominated by reef accretion, the more exposed eastern, northern and southern coasts have limited coral accretion and the substratum comprises volcanic and calcarenite outcrops [23
]. Along the southern coast, the nearshore waters adjoining the steep basalt cliffs are characterised by boulder stacks and plunging cliffs, which likely extend to form the contiguous reef mapped along the southern inner shelf.
Unlike the mosaic of different reef morphologies observed on the Lord Howe Island inner shelf, the Balls Pyramid inner shelf possesses a more limited inner shelf fossil reef. The Balls Pyramid pinnacle comprises steep cliffs which plunge into shallow waters, and the contiguous inner shelf reef surrounding the island is likely dominated by volcanic bedrock. The concentric formation of the outer edge of the inner shelf reefs, intersected with narrow channels, suggests constructional fossil reef origins which may have accreted, in part, during the postglacial rise in sea-level.
In addition to accretionary geomorphic features, the shelves exhibit diverse erosional features and morphologies. Complex networks of basins and channel systems characterise the northeast shelves, interpreted as the more sheltered, leeward setting. Basin features are interconnected to the channels, which are interpreted as inter-reef passages which would have functioned to transport water from the paleolagoons when the sea level was at or near the fossil reef surface. Three prominent channels dissect the Lord Howe Island mid-shelf reef, whereas distinct channels are not apparent on the mid shelf around Balls Pyramid. However, the leeward setting is apparent on the Balls Pyramid shelf through developed channels on the northeast outer shelf and the extension of a large northern mid-shelf basin.
During periods of lower sea level when the shelf was exposed, the mid- and outer-shelf channels appear to have fed sediment off the shelf edge, as suggested by the sub-bottom profiles presented for the Balls Pyramid northeast shelf by [30
]. These processes are similarly inferred for the Lord Howe Island shelf, where distinct channels are evident on the northeast middle shelf of Lord Howe Island, with a complex network of channels extending across the northeast shelf. Sediment samples collected from the slope areas by [53
] indicate the transport and deposition of sediments off the shelf into slope areas during periods of lower sea level.
The karstification of limestone shelf features likely occurred during times of lower sea level, as suggested by onshore deposits of calcarenites around Lord Howe Island [27
]. Karst features including dolines, caves and subaerially exposed speleothems were documented within calcarenite sequences around the island, which experienced dissolution and weathering following deposition during the MIS 7 [27
]. Morphological characteristics of the mid-shelf basins, including steep basin rims and a sand-inundated low-profile reef, suggest the basin morphology may reflect karstification processes (e.g., [54
]). The steeper basin rims and greater extent of the mid-shelf basins around Lord Howe Island (60 km2
around Lord Howe Island; 24 km2
around Balls Pyramid) likely reflect the greater volume of water drainage from the larger shelf system during periods when the sea-level was at or near the fossil reef surface and during lowstands when the shelves were exposed.
Terrace and step features are associated with lowstand sea levels during the last glacial period (Last Glacial Maximum ~21 ka) and the preceding interstadial and glacial periods of lower sea level. The depth range of these features are distributed across a wide spread of depths (45–217 m), corresponding to a range of lower sea levels. Similar mean depths of step features occur on both shelves (79–80 m), which may be associated with MIS 3. Morphologically, terrace step feature patterns are remarkably similar for the two shelves, particularly in the northwest where several distinct terraces form with rimmed margins (Figure 8
f,g). The shelf break similarly varies around the island shelves (83–217 m, mean depth 130 m). Shelf planation is proposed to have occurred rapidly after the formation of the shield volcanoes (6–7 million years ago), with marine abrasion accounting for the majority (90%) of erosion [26
]. Following shelf planation, carbonate sequences were deposited over the basalt platform [29
], and accretionary and erosional processes during sea level lowstands shaped the variable nature of the shelf break.
The availability of substrate for coral colonisation, leading to reef formation, is a key factor differentiating the morphology of the two shelves. The larger size of the shelf and thus the original formative volcano of Lord Howe Island, translates to larger island remnants that remained after shelf planation. The greater extent of reefs around the Lord Howe Island inner shelf was likely facilitated through the availability of shallow substrates and the larger island size which presumably provided greater shelter from exposure. Possibly, slightly warmer sea temperatures and/or more favourable currents (e.g., upwelling) or levels of exposure may have enabled coral to grow faster on the Lord Howe Shelf. In contrast to Lord Howe Island, the Balls Pyramid shelf possesses minimal shallow inner-shelf substrates and the steep pinnacle provides little shelter from high wind and wave energies. Although the areal extent of shelf features are reduced in comparison to Lord Howe Island, substantial past reef development is evident on the Balls Pyramid.
4.2. Applications for Management
Previous studies of the distribution of benthic assemblages around the island shelves and broader Lord Howe region have shown strong correlations to geomorphic features and shelf regions [31
]. Abundant hard corals were recently discovered growing on the mid-shelf reef of Balls Pyramid, showing increased abundance associated with the mid- and outer-shelf reef features [37
]. It is likely that similar distributions of hard corals occur around the Lord Howe Island mid-shelf reef, particularly given the development of a modern fringing reef and the more extensive fossil reef. The outer shelf pavement has been characterised as an area of sand veneers and rhodolith beds [32
], with gorgonian whips and fans observed on the outer shelf and shelf break [37
]. Investigations of benthic invertebrates around the Lord Howe Island shelf have shown that the infaunal benthic community structure was significantly different between geomorphic zones (fossil reef, basins and outer shelf, [56
While geomorphology appears to be a useful surrogate for benthic assemblages for the mid- to outer-shelf features, benthic communities around the inner shelf appear to be strongly structured by the hydrodynamic regime [23
]. Geomorphology is considered to be less useful as a surrogate in this zone, however terrain variables derived from the DEM, such as seafloor ruggedness, may provide useful proxies for explaining distribution of benthic assemblages.
Seafloor habitat mapping is an important component for marine spatial planning and fisheries management [7
]. The high-resolution bathymetry model and geomorphic characterisation produced in this study feed directly into the management needs identified by marine park managers [34
]. These macro-scale classifications of geomorphic features fit within the hierarchical framework of biome and provincial characterisations of the seafloor and biogeography for the broader Lord Howe region [3
]. The datasets produced by this study reveal detailed bathymetric information and characterise the geodiversity of the shelf landscape. The continuous depth information and stratification of the shelf into distinct features can be utilised in the ongoing planning and management of the shelf environment. An understanding of geodiversity around the shelf can assist in the experimental design of future data collection, and can identify areas of potential biodiversity, which can be targeted for further exploration.
Future research will focus on resolving the timing of reef accretion around the Balls Pyramid shelf and the evolution of the shelf features. The distribution of benthic assemblages around the Lord Howe Island shelf will also be explored to further examine relationships between biota to underlying geomorphology and terrain variables.