2016 SoE Marine Chapter - State and Trends - Seabed, outer shelf (25 m - 250 m)

The Marine chapter of the 2016 State of the Environment (SoE) report incorporates multiple expert templates developed from streams of marine data. This metadata record describes the Expert Assessment "The state and trends of quality of habitats and communities – seabed, outer shelf (25 m - 250 m)". The full Expert Assessment, including figures and tables (where provided), is attached to this record. Where available, the Data Stream(s) used to generate this Expert Assessment are accessible through the "On-line Resources" section of this record.

DESCRIPTION OF HABITAT/COMMUNITY 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 (25-250 m depth). Indeed, depths of 25 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 25m. Note that this assessment differs slightly from the 2011 assessment that focussed on the 50-200 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. The 2011 assessment indicated the overall condition was good at a national scale but was poor in the SE and East, and that these conditions were likely to be stable- based on limited evidence and trends based on the previous assessment. Given that bottom trawling is likely to be the main pressure on soft-sediment habitat in this depth range, it is assumed that the 2011 assessment was based on perceptions of regional patterns in expected trawl impact. 
Soft sediment habitats in the 25-250 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 bio-genic cover can be higher in stable coarse sediments where currents are stronger but not excessive. In addition, at shallower depths (25-50m) the seabed may have little epibenthic cover due to disturbance by swells, but this cover may increase as wave energy declines below 50 m 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 bio-genic 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), southern south-east, north-west, and western south –west regions, and the Lord Howe/ Norfolk ridge area (McEnnulty et al. 2011, Williams et al. 2011, Dunstan et al. 2012). 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’ (sensuHooper 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).
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. 
DATA STREAM(S) USED IN EXPERT ASSESSMENT
This assessment is based on data derived from Marine National Facility Surveys described in MarLIN (http://www.marine.csiro.au/marlin/search.html) and accessible through the CSIRO Data trawler (http://www.cmar.csiro.au/data/trawler/). Links to specific data sets are provided in the "On-line resources" section of this record.
• Voyage of Discovery north-west (SS05/2007)
• Voyage of Discovery south-west (SS07/2005 &amp; SS10/2005)
• Tasmanian seamounts surveys (SS01/1999, SS02/2006 &amp; SS02/2007, SS01/2008 &amp; TT01/2008)
• Habitat and population assessment of giant crabs (2003 - 2005) 
• Gulf of Carpentaria survey : beam trawl megabenthos (SS03/1990)
• Mapping &amp; Characterisation of Biotic &amp; Physical Attributes of the Torres Strait (Epibenthic Sled)
• Great Barrier Reef Seabed Biodiversity Project  (Epibenthic Sled)

2016 SOE ASSESSMENT SUMMARY [see attached Expert Assessment for full details]
• 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 2011 SOE ASSESSMENT
It is unclear why the South-west Region was rated poor in 2011 and if the assessment was confined to soft-sediment habitats or also included hard substrates. It is assumed that the 2011 assessment was based on perceptions of regional patterns in expected trawl impact rather than actual data on impacts. This assessment incorporates preliminary results of the current national sedimentary habitats status assessment and new information on trawl footprints in the five marine regions.

Tags: benthic habitat, expert assessment, outer shelf, seabed
Organization: Australian Ocean Data Network