Marine habitats and communities

Australia’s marine ecosystems span from nearshore reefs to the soft-sediment communities of the abyssal plains at more than 5,000 metres (m) depth, including the vast volumes of open ocean that lie between the surface and the sea floor (Figure 1). The ecosystems can be characterised by groupings of types of habitats and communities based on where they are found and the dominant species (Figure 2). These habitats and communities support a great diversity of species, many of which are of substantial economic and cultural importance.

Marine habitats are of varied significance to the diverse range of Traditional Owners around Australia’s coastline. Many of these habitats have provided protection, food and cultural resources for more than 60,000 years and continue to be closely tied to the wellbeing of Indigenous Australians.

Because of the long occupation by Aboriginal and Torres Strait Islander peoples of what is now known as Australia, there are many ancestral connections to seabeds that were previously above sea level; it is estimated that an extra 2 million hectares in landmass would have been exposed and been included in Country for Aboriginal people before the area was inundated during the Meltwater Pulse 1a between 35,000 and 8,000 years ago (Williams et al. 2018c, Benjamin et al. 2020). Coastal communities still have connections to the inundated sites, with sacred stories attached to these spaces (Przeslawski et al. 2020). For many communities, the connection continues, as is the case with kelp forests around Tasmania, and the close cultural, spiritual and ancestral connections women have to these habitats.

For this report, we consider the state and trends of 16 habitats and communities that together broadly encompass the diversity of Australia’s marine ecosystems. In addition to the habitats and communities described here, other habitats that are often considered ‘marine’ but are primarily restricted to coastal environments (such as mangroves and seagrasses) are considered in the Coasts chapter (see the Coastal ecosystems and habitats section in the Coasts chapter).

Figure 1 Australia’s marine regions

Figure 2 Where marine habitats and communities occur

Assessment State and trend of marine habitats and communities
2021
2021 Assessment graphic showing the environment is in good condition, resulting in stable environmental values, and the situation is stable.
Limited confidence
Indigenous assessment
2021 Assessment graphic for an assessment conducted by Indigenous community members, showing the environment is in poor condition, resulting in diminished environmental values, but the trend is unclear.

Most marine habitats and communities are in a good and stable condition. However, reef ecosystems are in poor condition and are declining, and seamounts remain in poor condition because they are recovering extremely slowly from the historical impacts of bottom fishing. It is notable that Traditional Owners assessed habitats and communities as in worse condition than reflected by the ‘western science’ assessments – poor and deteriorating were the most common grades provided by Traditional Owners. Note that the spatial scale of Indigenous and western science assessments may be different. Related to United Nations Sustainable Development Goal targets 14.1, 14.2, 14.4, 14.5

Assessment Seabed habitats and communities – shallow/inner shelf (0–30 metres)
2021
2021 Assessment graphic showing the environment is in good condition, resulting in stable environmental values, but the trend is unclear.
Limited confidence
2016
Assessment graphic from 2011 or 2016 showing the environment was in good condition, resulting in stable environmental values, but the trend was unclear.
2011
Assessment graphic from 2011 or 2016 showing the environment was in good condition, resulting in stable environmental values, and the situation was stable.

Inshore non-reef habitats at 0–30 m are in good condition, except for the south-east and east, based on expert knowledge. The south-east and east are likely in locally poor condition as a result of historical trawling and nutrient inputs (Barrett et al. 2021b). The Indigenous assessment locally was poor, with an unclear trend.

Assessment Seabed habitats and communities – deep/outer shelf (30 metres to shelf break at approximately 200 metres)
2021
2021 Assessment graphic showing the environment is in good condition, resulting in stable environmental values, and the situation is stable.
Limited confidence
2016
Assessment graphic from 2011 or 2016 showing the environment was in good condition, resulting in stable environmental values, and the situation was improving.
2011
Assessment graphic from 2011 or 2016 showing the environment was in good condition, resulting in stable environmental values, and the situation was stable.

Good condition generally, with poorer condition in areas subject to frequent (current or historical) bottom fishing (Barrett et al. 2021c).

Assessment Seabed habitats and communities – upper slope (200–700 metres)
2021
2021 Assessment graphic showing the environment is in good condition, resulting in stable environmental values, and the situation is stable.
Limited confidence
2016
Assessment graphic from 2011 or 2016 showing the environment was in poor condition, resulting in diminished environmental values, but the situation was improving.
2011
Assessment graphic from 2011 or 2016 showing the environment was in very good condition, resulting in enhanced environmental values, and the situation was stable.

Spatially restricted habitat is impacted by concentrated commercial bottom fishing in some areas; biota is slow to recover. Available information is generally poor. State is poorest in the South-east and Temperate East marine regions (Althaus et al. 2021a).

Assessment Seabed habitats and communities – deep (>700 metres)
2021
2021 Assessment graphic showing the environment is in good condition, resulting in stable environmental values, and the situation is stable.
Limited confidence
2016
Assessment graphic from 2011 or 2016 showing the environment was in good condition, resulting in stable environmental values, and the situation was stable.
2011
Assessment graphic from 2011 or 2016 showing the environment was in very good condition, resulting in enhanced environmental values, and the situation was stable.

State is highly variable for 700–1,500 m, dependent on historical fishing impacts; fishing footprint has decreased since 2016. Habitats beyond 1,500 m are overall in a very good, stable condition in all regions, due to minimal human interactions at these depths to date (O’Hara et al. 2021).

Assessment Water column habitats and communities – on-shelf (neritic; 0–200 metres)
2021
2021 Assessment graphic showing the environment is in good condition, resulting in stable environmental values, and the situation is stable.
Limited confidence
2016
Assessment graphic from 2011 or 2016 showing the environment was in good condition, resulting in stable environmental values, but the trend was unclear.
2011
Assessment graphic from 2011 or 2016 showing the environment was in very good condition, resulting in enhanced environmental values, but the situation was deteriorating.

No consistent trend among locations based on an assessment of phytoplankton, zooplankton and fish larvae (Richardson et al. 2021a). The Indigenous assessment locally was poor, with a deteriorating trend.

Assessment Water column habitats and communities – off-shelf (oceanic) epipelagic (0–200 metres)
2021
2021 Assessment graphic showing the environment is in good condition, resulting in stable environmental values, and the situation is stable.
Limited confidence
2016
Assessment graphic from 2011 or 2016 showing the environment was in good condition, resulting in stable environmental values, but the trend was unclear.
2011
Assessment graphic from 2011 or 2016 showing the environment was in very good condition, resulting in enhanced environmental values, and the situation was stable.

No consistent trend among locations based on an assessment of phytoplankton and zooplankton (Richardson et al. 2021c).

Assessment Water column habitats and communities – off-shelf (oceanic) deep (>200 metres)
2021
2021 Assessment graphic showing the environment is in good condition, resulting in stable environmental values, and the situation is stable.
Limited confidence
2016
Assessment graphic from 2011 or 2016 showing the environment was in good condition, resulting in stable environmental values, but the trend was unclear.
2011
Assessment graphic from 2011 or 2016 showing the environment was in very good condition, resulting in enhanced environmental values, and the situation was stable.

State and trends of biological components of the deep parts of the water column are largely unknown, but available evidence suggests a good status with likely improvements in some areas, but potential declines elsewhere (Trebilco 2021b).

Assessment Canyons
2021
2021 Assessment graphic showing the environment is in good condition, resulting in stable environmental values, and the situation is stable.
Limited confidence
2016
Assessment graphic from 2011 or 2016 showing the environment was in good condition, resulting in stable environmental values, but the trend was unclear.
2011
Assessment graphic from 2011 or 2016 showing the environment was in very good condition, resulting in enhanced environmental values, and the situation was stable.

Bottom-fishing impacts determine the condition of individual submarine canyons; variation is from highly impacted to largely pristine and functionally intact. State is poorest in the south-east region (Nichol et al. 2021). The Indigenous assessment locally was good, with an unclear trend.

Assessment Seamounts
2021
2021 Assessment graphic showing the environment is in poor condition, resulting in diminished environmental values, and the situation is stable.
Adequate confidence
2016
Assessment graphic from 2011 or 2016 showing the environment was in poor condition, resulting in diminished environmental values, and the situation was stable.
2011
Assessment graphic from 2011 or 2016 showing the environment was in good condition, resulting in stable environmental values, and the situation was stable.

Habitats range from very good to very poor, with trends stable to improving. Recovery from historical fishing pressure is expected to be extremely slow and potentially impeded by ocean acidification (Althaus & Williams 2021). The Indigenous assessment locally was good, with an unclear trend.

Assessment Oceanic reefs
2021
2021 Assessment graphic showing the environment is in very good condition, resulting in enhanced environmental values, but the situation is deteriorating.
Adequate confidence
2016
Assessment graphic from 2011 or 2016 showing the environment was in good condition, resulting in stable environmental values, and the situation was stable.

Warmer seas and associated coral bleaching may be starting to erode what has been a very good state, but not at all reefs (Stuart-Smith & Edgar 2021b).

Assessment Shallow rocky reefs (<30 metres)
2021
2021 Assessment graphic showing the environment is in poor condition, resulting in diminished environmental values, and the situation is deteriorating.
Somewhat adequate confidence
2016
Assessment graphic from 2011 or 2016 showing the environment was in poor condition, resulting in diminished environmental values, and the situation was deteriorating.

Trend data are lacking for large parts of the coast. Warm-water events and overgrazing by sea urchins negatively affect some temperate reef habitat, particularly in the east and south-east (Stuart-Smith et al. 2021b). The Indigenous assessment regionally was poor, with an unclear trend.

Assessment Algal beds
2021
2021 Assessment graphic showing the environment is in good condition, resulting in stable environmental values, but the situation is deteriorating.
Adequate confidence
2016
Assessment graphic from 2011 or 2016 showing the environment was in good condition, resulting in stable environmental values, but the situation was deteriorating.
2011
Assessment graphic from 2011 or 2016 showing the environment was in very good condition, resulting in enhanced environmental values, and the situation was stable.

Algal beds are in good condition nationally, but poorer and continuing to decline in the east and south-east as a result of warming and cascading effects of fishing. Stable elsewhere but vulnerable to heatwaves (Barrett et al. 2021e).

Assessment Coral reefs (<30 metres)
2021
2021 Assessment graphic showing the environment is in poor condition, resulting in diminished environmental values, and the situation is deteriorating.
Adequate confidence
2016
Assessment graphic from 2011 or 2016 showing the environment was in poor condition, resulting in diminished environmental values, and the situation was deteriorating.
2011
Assessment graphic from 2011 or 2016 showing the environment was in good condition, resulting in stable environmental values, and the situation was stable.

Overall, Australian coral reefs are in a poor state and declining. However, their condition varies considerably within and among regions (Richards et al. 2021) The Indigenous assessment regionally was poor, with a deteriorating trend.

Assessment Deepwater corals and sponges (mesophotic; 30–150 metres)
2021
2021 Assessment graphic showing the environment is in good condition, resulting in stable environmental values, but the trend is unclear.
Limited confidence
2016
Assessment graphic from 2011 or 2016 showing the environment was in good condition, resulting in stable environmental values, and the situation was improving.
2011
Assessment graphic from 2011 or 2016 showing the environment was in good condition, resulting in stable environmental values, and the situation was stable.

Good condition nationally, but with south-east and east under some ongoing trawl pressure on low vertical relief systems. Ocean warming is an increasing threat (Barrett et al. 2021a).

Assessment Deepwater corals and sponges (dark; >150 metres)
2021
2021 Assessment graphic showing the environment is in good condition, resulting in stable environmental values, but the trend is unclear.
Limited confidence
2016
Assessment graphic from 2011 or 2016 showing the environment was in good condition, resulting in stable environmental values, and the situation was stable.

Slow-growing fauna are exposed to direct fishing impacts, resulting in highly variable condition across depths and between regions. The south-east and east regions have the highest historical impacts from bottom fishing, especially trawling (Althaus et al. 2021b).

Assessment Bryozoan reefs
2021
2021 Assessment graphic showing the environment is in good condition, resulting in stable environmental values, and the situation is stable.
Limited confidence
2016
Assessment graphic from 2011 or 2016 showing the environment was in poor condition, resulting in diminished environmental values, and the situation was stable.
2011
Assessment graphic from 2011 or 2016 showing the environment was in poor condition, resulting in diminished environmental values, and the situation was stable.

Nationally, the overall grade for bryozoan reefs is good condition, with stable to slightly improving trend, and poor for the south-east and east (Barrett et al. 2021d).

Water column

The water column (pelagic environment) provides habitat for microbes, phytoplankton, zooplankton, fish and animals at higher levels in the food web (higher trophic levels), including seabirds, marine reptiles and marine mammals. Many of the species found in water column habitats have substantial economic and social value (see Marine species).

Overall, water column habitats can be considered to be in a good state with a stable trend, based on chlorophyll-a (representing an index of phytoplankton biomass), zooplankton biomass and fish larval abundance (Richardson et al. 2021a, Richardson et al. 2021c, Trebilco 2021b; Figure 3).

The main pressures on all water column habitats are climate change, causing acidification and declines in dissolved oxygen, fishing and pollution (Richardson & Schoeman 2004, Burgess et al. 2018).

Condition of depth zones

The water column of the ocean is often subdivided into depth zones (Figure 2):

  • neritic zone – waters over the continental shelf in depths of 0–200 m
  • epipelagic zone – waters offshore in depths of 0–200 m
  • mesopelagic zone – waters offshore in depths of 200–1,000 m
  • bathypelagic zone – waters offshore in depths of 1,000–4,000 m
  • abyssal zone waters offshore in depths more than 4,000 m.

In neritic waters, phytoplankton biomass, zooplankton biomass and larval fish abundance appear to be stable. Excess nutrients (eutrophication), sedimentation and run-off are important pressures in some regions (Uye 1994, Lin et al. 2020).

In epipelagic waters, phytoplankton and zooplankton biomass are increasing in the South-east and South-west marine regions (see Marine ecosystem processes), but trends in all other regions are less clear. Pelagic fisheries within Australian waters are generally in good state and well managed (Molony et al. 2021) (see Other fishes and Commercial fishing). Offshore oil and gas exploration can act as a pressure on these habitats and communities (see Marine petrochemical and mineral industries), with several impacts, including from noise (see Anthropogenic marine noise).

The habitats and communities in waters below 200 m are currently thought to be relatively pristine (Kloser & Kunnath 2020, Martin et al. 2020, Kloser & Kunnath 2021, Trebilco 2021b), although large-scale oceanographic changes are likely to be changing the distribution and characteristics of these habitats and communities in some areas (see Climate and climate change). Measurements of mesopelagic communities in the south-east over recent decades may indicate an increase in overall biomass, but could also indicate changes in the mix of species (community composition). Trends in other regions are unclear.

Close to the sea floor, bottom fishing is an important additional pressure (Hidalgo & Browman 2019, Drazen et al. 2020, Martin et al. 2020); however, impacts on deepwater habitats and communities are poorly understood. Deepwater species may be less resilient to changes than those from shallower waters, because these species are relatively long-lived and slow to reproduce.

Figure 3 Water column productivity: (left) concentration of chlorophyll-a; (middle) zooplankton biomass; (right) larval fish abundance from IMOS National Reference Stations

C = carbon; mg/m3 = milligram per cubic metre

Note: Black dots represent data points, and the lines (and shading) represent the linear regression (and confidence intervals) of the data after the seasonal cycle has been removed. Colours show the direction and statistical significance of the trend (blue = decreasing; red = increasing; black = no statistically significant trend).

Source: Data from the Integrated Marine Observing System

The Integrated Marine Observing System (IMOS) measures various aspects of the water column, with some measurements (e.g. chlorophyll) taken over the long term (approaching 20 years). Continued monitoring will enable future identification of both abrupt and long-term changes. It will also provide the ability to distinguish long-term trends from short-term variability (Poloczanska et al. 2013, Hoegh-Guldberg et al. 2014).

Coral reefs

Coral reefs provide immense value to oceanic ecosystems, and the Australian community and economy. They act as spawning and nursery grounds for many fish species; as tourism and recreation areas; and as buffer zones against high tides, rising sea levels and storms for coastal areas and communities.

Overall, Australian coral reefs are in a poor and declining condition (Richards et al. 2021, Stuart-Smith & Edgar 2021b). Climate change is the most significant pressure on Australian coral reefs, and the outlook for our coral reefs is poor.

High water temperatures result in coral bleaching, which occurs when corals expel the algae (zooxanthellae) living in their tissues, often causing the coral to turn completely white. If high temperatures persist, or if other pressures further stress already bleached corals, the coral dies.

Coral die-off affects other species that depend on coral reefs for food and habitat (GBRMPA 2019) (see case study: Australia’s changing reefs). For example, the impacts of mass coral bleaching events in 2016 and 2017 extended to other reef-dwelling animals in the North-east, North-west and Temperate East marine regions (Stuart-Smith et al. 2018, Stuart-Smith et al. 2019, Edgar et al. 2020).

Regional reef condition

Although overall assessments are poor, reef condition varies within and among regions, with reefs in some localities in good condition (Richards et al. 2021, Stuart-Smith & Edgar 2021b):

Figure 4 Changes in coral reef cover at various sites in Western Australia in relation to site-specific exposure to damaging waves (blue dots) and heat stress (red areas), since 1990

DHW = degree heating weeks; m =metre; SE =standard error

Notes:

  1. For Cocos (Keeling) Islands and Ningaloo Reef, open and closed circles represent 2 separate monitoring programs.
  2. SE bars give an indication of the variability of the underlying measurements.
  3. See Figure 5 for a national summary.

Source: Reproduced from Gilmour et al. (2019). Reprinted by permission from Springer Nature Coral Reefs The state of Western Australia’s coral reefs, Gilmour JP, Cook KL, Ryan NM, Puotinen ML, Green RH, Shedrawi G et al. (2019), advance online publication, 4 April 2019 (doi: 10.1038/sj. CORAL REEFS.)

There continues to be a lack of systematic monitoring in some marine regions (e.g. the North-west), for reefs below the safe working depths for scuba diving (generally more than 15 m) and for offshore reefs. Also lacking is the collection of metrics other than coral cover, which prevents assessment of the state and trends of overall biodiversity (Richards & Day 2018) and important ecosystem change (e.g. flow-on effects of fishing; (Nash & Graham 2016)). A coordinated approach is needed to integrate monitoring data across studies.

Reef recovery and management

Many Australian reefs appear able to recover from individual disturbance events, given enough time. However, the complex, global and pervasive nature of climate change makes the outlook for coral reefs very poor. The frequency, severity and spatial extent of disturbances to reefs are expected to continue to increase with climate change, reducing the opportunity for recovery. In addition, recovery can be slowed by local pressures, such as poor water quality (Osborne et al. 2017, Ortiz et al. 2018, Evans et al. 2020b, Thompson et al. 2021).

Management interventions and restoration strategies are increasingly being investigated to mitigate declines (National Academies of Sciences 2019) (see also case study: Marine restoration in a changing climate). Some of these initiatives have had positive outcomes on small scales, such as through implementation and enforcement of protected areas, and management of land-based and coastal activities that affect reefs (GBRMPA 2019). For example, long-term enforcement of Ashmore Reef Marine Park has resulted in apparent increases in the biomass of large fish populations from previously depressed levels. Oceanic reefs may be more resilient than coastal reefs; fish, in particular, seem to be less affected by habitat change and recover when there is adequate protection from fishing.

Upscaled application of safe and effective interventions may provide opportunities for building reef resilience. However, unless climate change is addressed, no intervention strategy will slow the deterioration of coral reefs (Condie et al. 2021b).

Case Study Australia’s changing reefs

Australian reefs are in a state of considerable change (Stuart-Smith et al. 2021a). In addition to the direct impacts of human activities, climate change is resulting in extremely dynamic responses in reef communities, through alterations to habitats and the effects of heat stress on reef species. Since the 2016 state of the environment report, 3 major heatwaves (2016, 2017 and 2020) have resulted in widespread coral bleaching, and annual maximum sea temperatures along much of the Australian coastline continue to increase (see the Climate and Extreme events chapters).


This case study provides a synopsis for shallow (<30 metres) temperate and tropical reefs around Australia, including their macroalgal and coral habitats, fishes and motile invertebrates, on the basis of a nationally standardised dataset (Stuart-Smith et al. (2021c)).


The 2016 and 2017 bleaching events reduced coral cover in the northern Great Barrier Reef (Stuart-Smith et al. 2017, AIMS 2020), at some locations in the North-west Marine Region, and in the Coral Sea (Harrison et al. 2019b). Severe tropical cyclone Olwyn had substantial, but highly localised, impacts at Ningaloo Reef, as did severe tropical cyclone Debbie in the Whitsunday Islands. In contrast, most southern parts of the Great Barrier Reef and Coral Sea have experienced increases in coral cover following previous disturbances (Figure 5) (see also AIMS 2020, Souter et al. 2021).



Figure 5 Changes in the cover of live coral and large canopy-forming seaweeds (labelled collectively in the figure as ‘kelp’) at national monitoring locations surveyed as part of the Reef Life Survey, Australian Institute of Marine Science and Australian Temperate Reef Collaboration programs

GBR = Great Barrier Reef

Note: Percentages of coral and kelp represent the sum of all live hard corals and canopy-forming seaweeds (including laminarian kelps and fucoid seaweeds), respectively. Differences between the 2011–15 average and the 2016–20 average (periods represented by the 2 grey-shaded blocks in the panels) are expressed as change per year on the map. (See Stuart-Smith et al. (2021a) for additional detail and description of methods.)


Temperate regions have experienced variable changes in total cover of canopy-forming seaweed (including true ‘kelps’ and fucoid algae that form canopies on rocky reefs). Some significant kelp losses (e.g. Vergés et al. 2016) and replacement of species have been reported in some places. A small rebound in kelp cover has occurred in the South-west Marine Region following the 2011 marine heatwave (see Rocky reefs and kelp beds).


Warmer temperatures have caused changes in the species composition and local abundances of reef fishes, not only directly but also in some cases through habitat degradation (Stuart-Smith et al. 2021c; Figure 6). Reef fish community composition, which changed dramatically as a result of the 2011 heatwave in south-western Australia (Stuart-Smith et al. 2017, Day et al. 2018), has still not returned to pre-2011 states.


Larger fishes that are the focus of reef fisheries have remained stable or have slightly increased in the South-east Marine Region and in some areas where they have been protected (Figure 7). They remain depleted around Sydney, but show signs of recovery at Ashmore Reef (which had depressed large fish biomass in the 2016 state of the environment report; also see Stuart-Smith et al. 2017, Speed et al. 2019).



Figure 6 Trends in the RFTI, an indicator of biodiversity responses to ocean warming at national monitoring locations surveyed as part of the Reef Life Survey, Australian Institute of Marine Science and Australian Temperate Reef Collaboration programs

GBR = Great Barrier Reef; RFTI = Reef Fish Thermal Index

Note: Increases in this index reflect changing community composition with increasing local abundance of reef fish that prefer warmer temperatures, whereas decreases reflect increasing species that prefer cooler temperatures. The values can be interpreted as the typical temperature preference for fish surveyed (measured in °C). Differences between the 2011–15 average and the 2016–20 average (the periods represented by the 2 grey-shaded blocks in the individual trend plots) are expressed as change per year on the map. (See Stuart-Smith et al. (2021c) for additional detail and description of methods.)



Figure 7 Trends in the biomass of large fishes (20+ cm) at long-term reef biodiversity monitoring locations around Australia surveyed as part of the Reef Life Survey, Australian Institute of Marine Science and Australian Temperate Reef Collaboration programs

cm = centimetre; GBR = Great Barrier Reef; kg/m2 = kilogram per square metre

Note: Values are log-transformed kilograms per 50 m × 10 m patch of reef. Separate trends are shown for sites monitored inside marine protected areas with no-take regulations (red lines) versus areas with some or all fishing allowed (orange lines). Differences between the 2011–15 average and the 2016–20 average (the periods represented by the 2 grey-shaded blocks in the individual trend plots) are expressed as change per year on the map. (See Stuart-Smith et al. (2021c) for additional detail and description of methods.)


The Living Planet Index (Loh et al. 2005) shows that, on average, populations of temperate species in Australian waters are declining (Figure 8); this indicator is heavily influenced by common species. Additional knowledge gaps remain around trends for many smaller invertebrates, seaweeds and corals, including rare species that are not well recorded by reef monitoring programs.



Figure 8 LPI, relative to 2008, for reef fishes and invertebrates in temperate and tropical Australia

LPI = Living Planet Index

Note: Coloured lines show the average trend; shading around the trend lines show 95% confidence intervals. Methods for calculating trends are provided in McRae et al. (2017); see Stuart-Smith et al. (2021c) for additional detail.


Management measures and their effectiveness


Management plans for 44 Australian marine parks, many of which contain shallow reef habitat, were enacted in 2018 and are yet to be evaluated. Recreational and commercial fisheries managers have a poor information base to effectively respond to, or account for, the climate and habitat impacts on reef fisheries, which could potentially impact fisheries sustainability in some regions (e.g. Brown et al. 2021).


Outlook


The rapid responses of marine life to shifting and variable ocean climates observed over the past 5–10 years are likely to continue. Management of fisheries and activities that rely on shallow reef life need to adapt to the dynamic nature of reef life in the short term; a reduction in carbon emissions is the only longer-term solution to change. Habitat loss is likely to continue to be an issue for coral reefs (Wilson et al. 2008, Stuart-Smith et al. 2021c); temperate species that have no shallow habitat further south of Australia to retreat to appear to be at greatest risk.

Rocky reefs and kelp beds

Rocky reefs and kelp (algal) beds are iconic of Australia’s temperate waters, and have immense economic and cultural importance to Australians (see the Coasts chapter).

Overall, the condition of Australian rocky reefs is poor, particularly in the east and around major cities, but is still generally good in southern regions (Stuart-Smith et al. 2021b). Whereas algal beds are currently in good condition overall nationally, and large canopy-forming seaweeds are still dominant in many locations in south-western Australia, western Victoria and Tasmania, they are deteriorating in the east and south-east (Butler et al. 2020, Glasby & Gibson 2020, Barrett et al. 2021e). The health of rocky reef and algal beds nationally is being affected by:

  • climate change, through rising temperatures and heatwaves, and nutrient and pH variation associated with changing current systems
  • overgrazing by sea urchins and other species, resulting from range extensions and removal of top predators by fishing
  • decreasing water quality from coastal run-off.

Managing pressures on rocky reefs and kelp beds

Since ocean warming and heatwaves cannot be locally controlled, it is important to manage fishing pressure and coastal inputs of nutrients to build the resilience of rocky reefs and algal beds to future warming stresses and reduce degradation related to excess nutrients.

Implementing marine reserves to aid the recovery of sea urchin predators, such as lobsters and large fishes, will be important to reduce the risk of overgrazing by urchins and increase the chances of improvement in this habitat in the longer term (Johnson et al. 2013). Restoration efforts are emerging as a possible way of reducing the risk of kelp loss (see case study: Marine restoration in a changing climate), including rebuilding predator populations through directed strategies.

Management may also need to consider emerging pressures:

  • At present, introduced species do not seem to have had large effects; however, the invasive algae species Undaria pinnatifida (wakame) and Caulerpa taxifolia remain problematic in some areas. As well, the red alga Grateloupia turutura is increasing in range, cover and seasonal extent in Tasmania and may extend to other states in the future.
  • An emerging pressure also associated with climate change is increasing tropicalisation (change in fish community composition with greater representation of tropical species), which may increase the extent of grazing on algal beds by herbivorous fish species.

In recent years, new monitoring initiatives have narrowed gaps in the understanding of endemic seaweed and invertebrate species, and significantly improved the ability to detect and track changes in rocky reefs and algal beds around temperate Australia. These include the IMOS autonomous underwater vehicle program, ecological monitoring programs associated with the establishment of coastal marine parks, and the Reef Life Survey, a citizen science program that monitors reef systems around Australia. However, data are still very limited for large parts of the coastline, and for most endemic and rare species.

Canyons and seamounts

Marine canyons and seamounts are defined as marine key ecological features because of their influence and value for marine biodiversity (Harris et al. 2008, DAWE 2015):

  • Canyons provide a pathway for the transport of sediments and nutrients (and pollutants), laterally from the shelf to the deep sea, and also vertically via upwelling of cold, nutrient-rich waters from the deep ocean towards the shelf (Kämpf 2010, Currie et al. 2012).
  • Seamounts provide ‘oasis’ habitats in the deep sea that often support increased biomass and productivity, including dense aggregations of corals with high associated biodiversity (Clark et al. 2010, Rowden et al. 2010). These may represent Vulnerable Marine Ecosystems, as defined by the United Nations (Clark et al. 2011, Williams et al. 2020b).

Submarine canyons are located around the Australian continental margin, but are most common in the South-east and South-west marine regions (Figure 9). Australia also has many seamounts (undersea mountains, often with volcanic origin); the best known are the Tasmanian seamounts in the South-East Marine Region and the Tasmantid seamount chain in the Temperate East Marine Region.

Figure 9 Marine canyons and seamounts in Australian waters

Canyon and seamount condition

The condition of Australian marine canyons and seamounts ranges from very good to very poor, depending largely on the level of historical and current bottom fishing (Althaus & Williams 2021, Nichol et al. 2021).

Bottom fishing has had the largest impact on biodiversity in Australian marine canyons and seamounts (Althaus et al. 2009, Clark et al. 2016, Williams et al. 2020a). Canyons and seamounts that are in reserves or areas where bottom fishing is excluded are stable or improving, although baseline and timeseries data are very limited (Althaus & Williams 2021, Nichol et al. 2021).

Climate change is a major emerging pressure; acidification associated with climate change presents an important threat, particularly for corals and other calcifying organisms (Thresher et al. 2015, Trotter et al. 2019) (see Ocean acidification). Shelf-incising canyons are potentially vulnerable to marine heatwave events.

Pollution is another emerging pressure. Significant volumes of microplastics have recently been found to be accumulating in deep-sea sediments in the Great Australian Bight (Barrett et al. 2020), and may be even more concentrated in submarine canyons (e.g. Mordecai et al. 2011, Pham et al. 2014).

Resilience of the biological communities in canyon and seamount habitats is likely to be very low, with recovery from physical disturbance requiring timescales of decades to centuries (Harris 2014, Goode et al. 2020, Williams et al. 2020a).

There is no ongoing monitoring of canyon benthic communities in Australian marine parks to determine their effectiveness in protecting or promoting recovery of regions affected by bottom fishing. Recent surveys in the North-west (Post et al. 2020), South-west (Trotter & Montagna 2020) and Coral Sea (Brooke & Nichol 2020) marine regions have sought to gather baseline information to assess future trends in canyon benthic communities, although within a limited number of canyons.

Other seabed habitats

Other seabed habitats and communities include those found on soft bottoms (i.e. silt, sand and gravel) at all depths, as well as reef habitats and communities found in waters deeper than 30 m in both temperate and tropical waters (Figure 2). These include both ‘twilight’ (‘mesophotic’) reefs (those in waters of approximately 30–150 m, where a small amount of light from the surface is still present) and ‘dark’ reefs (those below approximately 150 m, where no light from the surface reaches). Seabed habitats include reefs formed by deepwater corals (both hard and soft corals), sponges and bryozoans (e.g. Figure 10). Coral species are found throughout Australia’s shelf waters and can make up a significant component of the cold-water assemblage. Both sponges and corals are frequently used as indicators of benthic Vulnerable Marine Ecosystems in conservation planning because they are vulnerable to demersal fishing (Tracey et al. 2008, FAO 2009, Williams et al. 2015a) and climate impacts (ocean warming and acidification; Thresher et al. 2015, Stevenson et al. 2020). They may also be vulnerable to emerging offshore activities that include bottom contact.

Australia’s seabed habitats harbour a highly diverse benthic fauna (Schlacher et al. 2007, Williams et al. 2010a, McEnnulty et al. 2011, Dunstan et al. 2012, McCallum et al. 2013), including commercially targeted fish species. Many valuable and highly regarded fish species occur in these habitats – for example, blue-eye trevalla (Hyperoglyphe antarctica, Schedophilus labyrinthica), pink ling (Genypterus blacodes) and blue grenadier (Macruronus novaezelandiae). Also occurring in these habitats are threatened or management-dependent species, such as many deepwater sharks, including gulper sharks (Centrophorus spp.) (Williams et al. 2012).

Figure 10 Examples of deep-sea corals and sponges from >250 m. First row: (left) stony coral – Solenosmilia variabilis, (middle and right)  black corals. Second row: (left) Alcyonacea octocoral, (middle) octoral, (right) seapen. Third row: (left 3 photos) demosponges, (right 2 photos) glass sponges

Condition of seabed habitats

The state of Australia’s seabed habitats is highly variable among and within regions (Althaus et al. 2021b, Althaus et al. 2021a, Barrett et al. 2021c, Barrett et al. 2021d, Barrett et al. 2021a, Barrett et al. 2021b), and largely linked to historical and current levels and distributions of commercial bottom fishing (e.g. Pitcher 2016, Pitcher et al. 2016, Pitcher et al. 2018). A lack of data on pre-impact conditions has made assessment of the current state difficult (Foster et al. 2015). However, knowledge of very deep-sea habitats around Australia has increased substantially since the 2016 assessment due to recent survey effort in the Great Australian Bight, off the eastern Australian margin and in the Coral Sea (MacIntosh et al. 2018, Williams et al. 2018a, Williams et al. 2018b, O’Hara et al. 2020, Schmidt Ocean Institute 2020).

Deepwater corals and sponges have low resilience to impact. Individuals of many of these species are centuries old, and recovery would not be expected within management timeframes (Williams et al. 2010b, Fallon et al. 2014). Australia’s longest deepwater biological timeseries shows little recovery 20 years after the cessation of fishing (Williams et al. 2020a). Many species have a small range of tolerance to changes in temperature or water chemistry, reducing their resilience to effects of climate change (Stevenson et al. 2020).

Even for habitats where recovery from historical impacts of fishing has been more evident, this improvement is increasingly being offset by impacts from climate change and pollution (Althaus & Williams 2021, Althaus et al. 2021a, Barrett et al. 2021a, O’Hara et al. 2021). The longer-term outlook for Australia’s seabed habitats is therefore for declining condition, or at least significant species turnover, as temperate regions undergo tropicalisation, and the warmest tropical regions warm beyond the tolerance of species, with no possibility of replacement with other species (Marzloff et al. 2016, Marzloff et al. 2018).

There is no ongoing monitoring of deep seafloor habitats in Australia, so biodiversity or oceanographic trends are unknown. The impact of plastics, dissolved pollutants or electric cables on the deep ocean is unknown.

The IMOS autonomous underwater vehicle program, in conjunction with Australian marine park and marine protected area monitoring programs, is likely to be a key source of information for understanding the state and trends of seabed habitats in the future.

Managing pressures on seabed habitats

Deepwater bottom fisheries are considered to be sustainably managed in Australia, so damage to further areas is not anticipated, but caution will be needed as other offshore industries develop. Recovery of habitat-forming biota, as well as of some of the most impacted species such as gulper sharks, is predicted to be very slow (Williams et al. 2012). Thus, long-term commitments to management – through both reserves and fishery closures, and effort management – are necessary to achieve improvements in condition.