State and trends of marine biodiversity: Quality of habitats and communities

2016

Quality of habitats and communities

Eighteen habitats and communities, ranging from the nearshore to the abyss, and from the seabed to the water column, were assessed for their current state and recent trends. Habitats and communities were assessed across 4 spatial regions associated with defined depths of the total water column recognised as representing distinct faunal compositions:

  • the inner shelf environment (0–25 metres in depth)
  • the outer shelf environment (25–250 metres)
  • the slope environment (250–700 metres)
  • the abyss environment (more than 700 metres).

It should be noted that not all regions around Australia include all depth ranges. For example, the North Marine Region does not have any areas with a sea floor deeper than 700 metres.

Generally, habitats and communities in the Temperate East and South-east marine regions were reported to be subject to higher historical impacts than in other regions, such as those associated with fishing by bottom trawlers on shelf and slope communities (Pitcher et al. 2016b). Condition of habitats and communities in the Great Barrier Reef ranges from poor and deteriorating (coral) to good and stable (macroalgae, offshore banks and shoals), with high spatial variability in the state and trends of habitats and communities. Overall, average coral cover across reefs within the Great Barrier Reef system declined from 28 per cent in 1986 to 13.8 per cent in 2012 (Figure MAR25), mainly because of consecutive cyclones and outbreaks of crown-of-thorns starfish (see State and trends of indicators of marine ecosystem health), with those in the south declining most severely. More recent surveys of the Great Barrier Reef have reported both increases and decreases in coral cover, with trends highly variable across sites monitored. High rainfall in 2010, 2011 and 2012 was observed to result in localised increases in nutrient inputs and phytoplankton community biomass in the Great Barrier Reef lagoon (Thompson et al. 2015a, 2016). An extensive survey of the Great Barrier Reef and the reefs of the north-west was carried out in the first half of 2016. This recorded widespread and extensive bleaching of coral reefs, particularly in the north-east, as a result of high water temperatures associated with overall increasing water temperatures (see Ocean temperature) and an extreme El Niño event (Figure MAR26). Consequently, the overall condition of habitats and communities may differ from that reported in GBRMPA (2014a). Further detail on the state and trends of habitats and communities in the Great Barrier Reef is provided in GBRMPA (2014a).

North Marine Region

The relatively lower use of the marine environment and the remoteness of much of the North Marine Region mean that habitats and communities across this area are assumed to be in very good condition, although very few data exist from the region, and there are few long-term datasets that can be used to determine trends. Knowledge of the state of seabed habitats in the region is largely limited to the outer shelf regions of the Gulf of Carpentaria (e.g. Bustamante et al. 2011), Torres Strait (e.g. Pitcher et al. 2007b), and the carbonate banks and terraces of Van Diemen Rise (Przeslawski et al. 2014).

A recent voyage of discovery in the western parts of the Oceanic Shoals Commonwealth Marine Reserve established baselines for midwater species assemblages and sea-floor communities of carbonate banks, terraces and pinnacles within the reserve. The reserve has been identified as a hotspot for sponges, with more species and communities found within the reserve than in surrounding sea-floor areas. A total of 57 new species for the region, 7 new species for Australia and 13 new species for the Indo–Pacific region were recorded during the survey, including several species that are identified by the IUCN as vulnerable, near threatened and endangered.

Mapping of prawn trawl operations across the region (Pitcher et al. 2007b) suggests that seabed communities of sponges and corals appear to be mostly unaffected by domestic fishing operations, largely because such mapping analyses place most sponge and coral communities in areas that do not overlap with trawl fisheries. However, this is yet to be verified by in situ studies, including more recent assessments of trawling operations and consideration of historical trawling by foreign vessels through the region. The trawling footprint across the marine region is estimated to be less than 15 per cent of the total area of the marine region, and the swept area within the footprint (as a measure of the intensity of trawling) is estimated to be less than 20 per cent (see Box MAR2).

Observations of reefs collected to date by the Reef Life Survey suggest that offshore reefs are in good condition (see Box MAR5), although trends are unclear. Above average rainfall across much of northern Australia in 2011, 2014 and 2015 introduced higher nutrient loads into the water column, with mixed responses by plankton communities, as measured by the IMOS National Reference Station at Darwin. Monitoring from this station suggests that communities are stable, but the highly variable nature of community biomass results in unclear trends in populations (see also State and trends of indicators of marine ecosystem health).

Coral Sea Marine Region

The habitats and communities of the Coral Sea Marine Region are generally assumed to be in very good condition, largely because of the offshore nature of the marine region. Information on habitats and communities throughout the region is largely lacking, with most information available relating to offshore coral reefs and shoals. Less is known about slope, abyss, canyon and seamount habitats and communities (see Ceccarelli et al. 2013). Surveys of offshore reefs have observed that damage from cyclones and coral bleaching characterises most of the central and northern Coral Sea reefs, but those in the south of the Coral Sea appear in very good state (albeit with some evidence of fishing impacts; see Box MAR5). Reefs in the Coral Sea protected from fishing in the former Coringa–Herald and Lihou Reef national nature reserves are showing signs of recovering fish populations (Edgar et al. 2015), and Elizabeth and Middleton reefs have fish communities that are considered to be in good condition by global standards (Edgar et al. 2014). Above average rainfall has occurred in the region in several years since 2011, although consequences for habitats and communities are unknown.

Temperate East Marine Region

The outer shelf and upper slope of the Temperate East Marine Region are important areas for commercial fishing—operations that have occurred throughout the marine region for more than 100 years. Seabed communities are particularly affected by bottom trawling, which has been estimated to have a footprint of 30–40 per cent of the shelf area and less than 20 per cent across the slope (see Box MAR2). Within the footprint area on the shelf, the swept area is estimated to range from approximately 80 per cent in the north to 20–60 per cent in the central and south parts of the region. On the slope, the swept area is estimated to be less than 30 per cent.

Modelling suggests that gorgonians, bryozoans, Solenosmilia spp., sponges, soft corals and some other cnidarians have been reduced by approximately 10–20 per cent from their untrawled status, and several other seabed species groups have been reduced by 5–10 per cent (Pitcher et al. 2015). Decreasing fishing effort in the past decade has led to a declining trawling footprint, which has reduced the impacts on these communities. Fishery closures and marine reserves offer additional protection for seabed communities. The lack of observational data on the distributions of sponges and corals throughout the Temperate East Marine Region restricts any assessment of ongoing impacts and recovery from past impacts.

The large, brown, canopy-forming algal species Phyllospora comosa has disappeared from around 70 kilometres of coastline around Sydney (Coleman et al. 2008). This is likely to be from the effects of water pollution. Although the lack of early baseline data precludes a definitive understanding of the causes of sea urchin Centrostephanus rodgersii barrens (where increasing urchin populations have overgrazed kelp beds) throughout the marine region, it is likely that historical reductions in the abundance of urchin predators such as blue groper and eastern rock lobsters may be partly responsible. Urchin barrens have been estimated to extend through more than 50 per cent of algal bed habitat along the central and southern coasts of New South Wales, but with considerable site variability (Andrew & O’Neill 2000).

Seabed habitats and communities are also affected by anchoring of container and bulk-carrier vessels in the marine region, which scours the sea floor. Vessel activity across Australia is currently increasing, with associated impacts also likely to be increasing (Davis et al. 2014; see also Box MAR3 and Marine vessel activity).

Knowledge of taxa contributing to upper slope benthic communities is largely limited to the Lord Howe and Norfolk Ridge area (see Williams et al. 2011, Harris et al. 2012). Information on the outer shelf and abyss is lacking, although a survey of abyssal depths off Australia is scheduled for 2017. Above average rainfall and river run-off during 2011 supplied much of the east coast of Australia with above average sediment and nutrient loads, but the consequences for pelagic habitats and their communities are largely unquantified.

South-east Marine Region

Similar to the Temperate East Marine Region, the outer shelf and upper slope of the South-east Marine Region are important areas for commercial fishing. Bass Strait also contains relatively high concentrations of activities associated with oil exploration and production (see Marine oil and gas exploration and production). Both activities have affected seabed communities across the region, with impacts around oil platforms considered to be largely localised around the platforms themselves, although no coordinated ongoing monitoring of benthic communities is currently occurring. Commercial fishing activity has been identified as a key human activity affecting bryozoan reefs, and deep shelf coral and sponge communities (see Box MAR2).

The footprint of trawling across shelf areas throughout the marine region has been estimated to range from 25 per cent in the east to less than 5 per cent in the west. On the slope, it is estimated to be as high as 40 per cent. This highlights the spatial variability in historical impacts on seabed communities and their current state. Within the footprint area on the shelf, the swept area is estimated to range from approximately 40 per cent in the east to less than 10 per cent throughout parts of Bass Strait. In slope regions, the swept area within the footprint ranges from approximately 50 per cent to approximately 80 per cent in western areas of the region. Although fishery closures and the introduction of marine reserves are reducing ongoing impacts on these communities, recovery of habitat-forming biota on the outer shelf and upper slope is predicted to be very slow (Williams et al. 2012).

Limited data regarding benthic habitats and their condition are available for Australian seamounts outside the Tasmanian seamounts cluster. Within this cluster, all seamounts with peaks less than approximately 1100 metres have been subject to bottom trawling (Koslow et al. 2000, Althaus et al. 2009). Although fishery closures and the implementation of marine reserves through the region have permanently excluded bottom trawling from most seamounts in the region, recovery from past trawling is expected to be slow. Seamounts subject to bottom trawling have shown no change consistent with recovery in faunal assemblages across periods of 10 years (Williams et al. 2010b).

Large canopy-forming seaweeds are still prevalent in many locations throughout the marine region, but range extension by the urchin Centrostephanus rodgersii, particularly in eastern parts of the region, has led to overgrazing of kelp beds in temperate rocky reefs where populations of the urchin occur. This has had substantial impacts on natural habitats (Ling 2008, Ling et al. 2015). Further loss of kelp beds may be avoided and recovery of some grazed areas may be possible if populations of urchin predators such as lobsters and large fish are allowed to increase. In Tasmania, significant warming of ocean waters in the past 30 years has resulted in the loss of more than 90 per cent of cover of Macrocystis spp. on the east coast (Johnson et al. 2011). This loss of extensive beds of a major habitat-forming species has resulted in giant kelp forests of south-eastern Australia being listed as a threatened ecological community under the EPBC Act. Long-term, localised impacts of nutrients from major urban centres throughout the marine region have reduced Ecklonia cover, and changed algal communities from a canopy-dominated state to a turf (Gorman et al. 2009).

Introduced species are preying on native invertebrates, including commercial species (e.g. the Northern Pacific starfish, Asterias amurensis; Ross et al. 2003), and altering seabed habitats and communities (e.g. the New Zealand screw shell, Maoricolpus roseus; Reid 2010). Increased inshore aquaculture can introduce higher nutrient loads into the inner shelf water column, affecting macroalgal assemblages up to 100–400 metres from farms (Oh et al. 2015; see also the Coasts report).

Information on habitats in abyssal regions is largely restricted to geomorphic features in the southern Tasmanian area. Most areas surveyed contain relatively high abundances of habitat-forming species. Seamount peaks have been found to have relatively low abundances; this is thought to be the result of bottom-trawl fishing (Thresher et al. 2014). Warming of the water column has resulted in range extensions of several species in addition to C. rodgersii, affecting habitats and altering plankton communities throughout the region (see Box MAR1).

South-west Marine Region

Overall, the habitats and communities in the South-west Marine Region are in good condition. However, few long-term datasets exist that can be used to determine trends in seabed habitats and communities across the region. Increasing pressures associated with industries (e.g. oil and gas exploration activity in the Great Australian Bight, the coastline’s potential for renewable energy developments, marine vessel activity) and climate change indicates an increasing need for baseline and monitoring data.

Information on outer shelf, slope and abyssal habitats is limited for the South-west Marine Region, with sparse data available for the Great Australian Bight (James et al. 2001, Rogers et al. 2013) and for selected locations along the Western Australian continental margin (Williams et al. 2010a, Fromont et al. 2012). Recent surveys conducted under the Great Australian Bight Research Program and the Great Australian Bight Deepwater Marine Program will contribute substantially to our knowledge of benthic and pelagic habitats and communities on the outer shelf, slope and abyss, adding to our understanding of this area within the marine region.

The footprint of bottom trawling throughout shelf and slope regions is estimated to be less than 20 per cent, with the total swept area within the footprint estimated to be less than 40 per cent (see Box MAR2). The highest impacts of bottom trawling have been on benthic communities in the Spencer Gulf in South Australia.

Large canopy-forming seaweeds are still prevalent in many locations surveyed in south-western Australia, although there are documented examples of habitat loss from the south-east Indian Ocean marine heatwave (e.g. Wernberg et al. 2012). The heatwave caused the northern extent of the canopy-forming species Ecklonia to contract south by more than 100 kilometres (Wernberg & Smale 2015, Wernberg et al. 2016), and had similar influences on other canopy-forming algae such as Scytothalia dorycarpa (Bennett et al. 2015). Warming of the water column has been associated with declining abundance of phytoplankton communities (Siegel et al. 2013; see also State and trends of indicators of marine ecosystem health).

Ongoing monitoring of the water column is currently facilitated through an IMOS National Reference Station located off Perth; sampling at an additional site off Esperance ceased in 2013. Spatially and temporally variable sampling is facilitated through other IMOS facilities. Long-term trends of warming are correlated with declining phytoplankton biomass (Siegel et al. 2013) as low-biomass tropical communities move further south (Thompson et al. 2015a).

North-west Marine Region

Habitats and communities of the North-west Marine Region are assumed to be in good condition, mostly because of the remoteness of large parts of the marine region, although there are very few data or long-term datasets that can be used to determine trends. Localised areas where habitats and communities are being impacted either directly or indirectly, through increased recreational use of the marine environment, are likely to exist. Habitats where large capital dredging projects and substantial shipping activities associated with oil and gas development are occurring are also likely to be affected, such as those in the Pilbara area.

Offshore habitats are considered to be in good condition based on the distribution of bottom-trawling fishing, although modelling of the recovery of seabed habitats and communities after extensive historical bottom trawling in the 1970s and 1980s suggests that these habitats are likely to remain depleted (Fulton et al. 2006). This is yet to be verified with in situ observations. The bottom-trawling footprint associated with more recent bottom trawling is estimated to be less than 20 per cent across the marine region, with the trawl area swept within the footprint less than 40 per cent (see Box MAR2). The highest impacts of bottom trawling on benthic communities have occurred in Shark Bay.

The state of reef communities varies across the marine region, with a recent sequence of high summer water temperatures causing varying degrees of bleaching and coral mortality in different areas, notably from Exmouth to the Abrolhos Islands in association with the marine heatwave of 2011. Elevated water temperatures continued across coastal and offshore Pilbara waters in 2012 and 2013, resulting in bleaching of much of the western Pilbara and Montebello Islands Marine Park. Bleaching impacts associated with climate extremes driven by increasing baseline water temperatures and the 2015–16 El Niño event are yet to be fully determined; however, surveys conducted to date have shown that large areas of bleaching occurred at Scott and Seringapatam reefs, and parts of the inshore Kimberley coast, but little or no bleaching was observed in more southerly areas such as Ningaloo Reef and the Montebello Islands. Many habitats and communities lack long-term ongoing monitoring, which limits the establishment of trends, although Ningaloo Reef and some offshore reefs have ongoing monitoring programs. Many nearshore reefs in the north-west have been little studied, and recent surveys have found surprisingly diverse communities. Ongoing monitoring of the water column, facilitated through an IMOS National Reference Station located at Ningaloo Reef, ceased in 2013.

Evans K, Bax NJ, Smith DC (2016). Marine environment: State and trends of marine biodiversity: Quality of habitats and communities. In: Australia state of the environment 2016, Australian Government Department of the Environment and Energy, Canberra, https://soe.environment.gov.au/theme/marine-environment/topic/2016/state-and-trends-marine-biodiversity-quality-habitats-and, DOI 10.4226/94/58b657ea7c296