2021 State of the Environment Report Marine Chapter – Expert Assessment – State and Trend – Canyons

Researchers: Emma Flukes (Point of contact) ,  Huang, Zhi (Author) ,  Nichol, Scott (Author) ,  Post, Alix (Author) ,  Williams, Alan (Author)

Brief description The Marine chapter of the 2021 State of the Environment (SoE) report incorporates multiple expert templates developed from streams of marine data. This metadata record describes the Expert Assessment "State and Trend of canyons". ***A PDF of the full Expert Assessment, including figures and tables (where provided) is downloadable in the "On-line Resources" section of this record as "EXPERT ASSESSMENT 2021 - Canyons"*** ---------------------------------------- DESCRIPTION OF HABITAT/COMMUNITY/PROCESS FOR EXPERT ASSESSMENT Submarine canyons are a class of geomorphic seabed feature used as a physical surrogate for biodiversity distribution during Australia’s marine planning process and in designing the network of Australian Marine Parks. They are one of two ‘special’ feature types (together with seamounts) (Harris et al., 2008) considered to have potentially high influence on and value for marine biodiversity distribution and protection. It is these characteristics that have been used to define some canyons as Marine Key Ecological Features (Department of the Environment, 2012). Submarine canyons are located on all sides of the Australian continental margin. They form deeply incised networks of valleys and channels that extend from the continental shelf edge (<200 m water depth) to the foot of the continental slope (>4000 m water depth). Based on a 2014 compilation of all available bathymetric data for the Australian EEZ, 713 submarine canyons are identified on the continental margin, with an additional 40 canyons that fringe islands within Australia’s external territories (Table 1; Huang et al., 2014). The majority (618) of the identified canyons are classified as ‘blind canyons’, meaning that they are located only on the continental slope and typically with their heads in water depths of ~500 to 1000 m. The other 95 canyons are classified as ‘shelf-incising’ canyons that extend from the slope and incise the continental shelf edge, with canyon heads that define an abrupt depth transition between the slope and shelf of up to 400 m. Between Marine Planning Regions canyons vary in number and spatial distribution. Thus, canyons are most abundant in the South-East (206 canyons) and South-West (204) Marine Regions, with the latter having the higher proportion (51) of shelf-incising canyons. The Temperate East has 124 canyons. In contrast, the North-West and North Marine Regions incorporate 90 and 6 canyons, respectively. Within canyons, the seabed is characteristically irregular with underlying substrate exposed along steep canyon walls, in places forming undersea cliffs hundreds of metres high. These localised areas of hard rock outcrop provide a stable surface for benthic biological communities, including sponges, corals and associated sessile flora and feeding grounds for demersal fishes (Schlacher et al., 2007; Fromont and Pisera, 2011; Currie and Sorokin, 2014; Kloser et al., 2014; Huang et al., 2018). Canyons also provide a pathway for the transport of sediments and nutrients (and pollutants), not only laterally from the shelf to the deep sea, but also vertically via upwelling of cold, nutrient-rich waters from the deep ocean toward the shelf (Kampf, 2010; Currie et al. 2012). An assessment of the habitat potential of submarine canyons on the Australian margin found that shelf-incising canyons have higher potential for supporting benthic communities than those confined to the slope (Huang et al., 2018). Canyons with particularly high habitat potential are located mainly offshore of the Great Barrier Reef and the NSW coast, on the eastern margin of Tasmania and Bass Strait, and on the southern Australian margin. Many of these canyons have complex bottom topography, are likely to have high primary and secondary production, and experience less disturbance to seabed sediment by near-bed currents; this latter factor favoring infaunal communities. DATA STREAM(S) USED IN EXPERT ASSESSMENT National Submarine Canyons of Australia [Geoscience Australia data product] ---------------------------------------- 2021 SOE ASSESSMENT SUMMARY [see attached Expert Assessment for full details] • 2021 • Assessment grade: Good to Poor Assessment trend: Stable or unclear Confidence grade: Limited evidence or limited consensus Confidence trend: Limited evidence or limited consensus Comparability: Grade and trend somewhat comparable to 2016 assessment • 2016 • Assessment grade: Good but poor in the South-East Assessment trend: Unclear Confidence grade: Limited evidence and limited consensus Confidence trend: Limited evidence and limited consensus Comparability: Grade and trend are somewhat comparable to the 2011 assessment • 2011 • Assessment grade: Very good Assessment trend: Stable Confidence grade: Limited evidence or limited consensus Confidence trend: Limited evidence or limited consensus ---------------------------------------- CHANGES SINCE 2016 SOE ASSESSMENT not supplied

Lineage Statement: QUALITY OF DATA USED IN THE ASSESSMENT Very good.

Lineage

Notes Credit
Peer reviews of this assessment were provided by: Peter Harris (GRID-Arendal, Norway)

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