2021 State of the Environment Report Marine Chapter – Expert Assessment – State and Trend – Seabed, deep/outer shelf (30-200 m)

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 Seabed, deep/outer shelf (30-200 m)". 
<strong><em>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 - State and Trend – Seabed, deep/outer shelf (30-200 m)"</em></strong>



DESCRIPTION OF HABITAT/OMMUNITY FOR EXPERT ASSESSMENT
The outer shelf seabed consists primarily of unconsolidated sediments but also includes a mix of other habitats that range from stable gravels and rocky reef through mesophotic and relict coral reefs. However, the condition of several of these habitats are assessed via other SoE metrics, including deepwater corals and sponges, bryozoan reef, seagrasses, algae and coral reefs, so this assessment focusses on the condition of sedimentary habitats on the mid to outer shelf (30-200 m depth, or otherwise outer limit of shelf). Indeed, depths of 30 m to 50 m are commonly found on the inner shelf as well, so despite the “Outer shelf” in the title, this assessment is based on all waters on the shelf deeper than 30m. Note that this assessment differs slightly from the 2011 assessment that focussed on the 50-200 m depth range and the 2016 assessment that focussed on the 25-250 m depth range, and it isn’t clear from the 2011 assessment notes whether that assessment was confined to soft sediment habitat; it is assumed that this was the case, and this assumption was made for the 2016 assessment.  

Soft sediment habitats in the 30-200 m depth range are usually dominated by coarse to fine sand in exposed coast locations at shallower depths that tends to grade to finer sand/silts with increasing depth, but with a strong overlay on this gradient whereby increasing seabed stress in response to swells and currents sweeps away fine fractions thus leaving coarser sands and gravels. Muddy and unstable sediments tend to have little epibenthic cover, but such biogenic cover can be higher in stable coarse sediments where currents are stronger but not excessive. In addition, at shallower depths (30-50m) the seabed may have little epibenthic cover due to disturbance by swells, but this cover may increase as wave energy declines below 50m and the fauna may include a moderate cover of sponges, bryozoans, octocorals and hydroids. Upwellings and productivity may also be an important driver of shelf biogenic habitats. Recent IMOS AUV surveys using high resolution imagery have indicated that a turfing layer of bryozoans, hydroids and other epibenthic fauna may be an extensive feature in many offshore locations. 

While cover of habitat-forming epifauna is lower on fine sediments, it is often still a significant component on coarser sediments that enhances benthic biodiversity in deep shelf waters by providing complex structural living spaces for a large number of species from a variety of taxa (Pitcher et al. 2007a, Buhl-Mortensen et al. 2010, Fromont et al. 2012). Such biological assemblages are generally fragile and readily disturbed by activities such as benthic trawling and large storm events. Nevertheless, the primarily unconsolidated sediments of the outer shelf seabed tend to be dominated by mobile invertebrates such as crustaceans, echinoderms and molluscs, with deposit/detritus feeding types typifying finer sediments and suspension feeding types typifying coarser sediments. 

Our knowledge of these taxa in Australian waters at shelf depths stems mainly from a few broad-scale biodiversity surveys covering the Gulf of Carpentaria (Long et al. 1995; Bustamante et al. 2011), Great Barrier Reef shelf (Pitcher et al. 2007a), Torres Strait (Pitcher et al. 2007b), Pilbara (Pitcher et al. 2016b, Keesing 2019), southern south-east, north-west (Keesing et al. 1018a/b, Pitcher et al. 2016b), and western south –west regions, and the Lord Howe/ Norfolk ridge area (McEnnulty et al. 2011, Williams et al. 2011, Dunstan et al. 2012) (Table 1). Fine/muddy sediments tend to be less diverse than coarse sediments and hard ground but are still diverse — and belie their often barren appearance — with many undescribed species, as well as many ‘unknown’ (sensu Hooper et al. 2013) species (Pitcher et al. 2007ab; McEnnulty et al. 2011, Fromont et al. 2012, Alderslade et al. 2014). In addition, species turn-over between samples in fine/muddy sediments tends to be lower than on coarser sediments and in sessile epifaunal communities (Poiner et al. 1998; Pitcher et al. 2007a). Recent studies have started to focus in infaunal assemblages (e.g. Przeslawski et al. 2013) but, as yet, have not provided a temporal component to assess change or status. 

While there have been other concurrent cross shelf studies by a range of other agencies over this time (e.g. Jordan et al. 2010, and the IMOS AUV program) the majority of these have focussed on hard rather than soft substrates, with few examples of time-series data from soft sediments. 

DATA STREAM(S) USED IN EXPERT ASSESSMENT
This is outlined in the reports and papers included in the reference section.



2021 SOE ASSESSMENT SUMMARY [see attached Expert Assessment for full details]

• 2021 •
Assessment grade: Good
Assessment trend:  Stable
Confidence grade: Limited evidence or limited consensus
Confidence trend: Limited evidence or limited consensus
Comparability: Grade and trend are comparable to the 2016 assessment
• 2016 •
Assessment grade: Good
Assessment trend: Improving
Confidence grade: Limited evidence or limited consensus
Confidence trend: Limited evidence or limited consensus
Comparability: Grade and trend are comparable to the 2011 assessment
• 2011 •
Assessment grade: Good
Assessment trend: Stable
Confidence grade: Limited evidence or limited consensus
Confidence trend: Limited evidence or limited consensus



CHANGES SINCE 2016 SOE ASSESSMENT
No change.

Tags: deep seabed, expert assessment, state and trend
Organization: Australian Ocean Data Network