Aquatic Ecosystems

Planning for restoration is well advanced in many parts of the state. Increasingly engineering solutions are being used to manage flow and manipulate water regime. In the River Murray water can be delivered to meet ecological objectives such as wetland inundation or to stimulate fish spawning. Environmental regulators are being constructed to artificially inundate Pike and Katarapko floodplains and complement the Chowilla environmental regulator, which can inundate 10,000 hectares of floodplain ecosystems.

Engineering solutions are returning low flows to streams in agricultural land that have been impounded. A total of 74 catchments will have low flows returned to the hydrology to support invertebrate, macrophyte and fish populations. Similarly water is delivered through the South East drainage network to wetlands that receive less water than they did historically.

Although development has been the major factor affecting water availability for aquatic ecosystems across South Australia, climate change looms as an additional factor affecting future water availability.

Rainfall and runoff are non-linear and a catchment needs steady regular rain to soak the soil and allow surface flow and runoff into streams and rivers. A decrease in rainfall means that catchments are soaked less often and runoff can reduce dramatically. The notable Australian example is the catchment for Perth’s reservoirs where a decrease in rainfall of 10–15% resulted in a 40–50% decrease in stream flow (Water Corporation 2018). A similar scenario is likely to play out in South Australia.

Statistical downscaling models have been calibrated on a seasonal basis for the 8 NRM regions across South Australia. Precipitation is projected to decrease in all regions and seasons (Table AE1) with the largest relative decreases predicted for spring (Charles and Fu 2014, Beecham 2015).

Extreme El Niño events, which are associated with drought conditions in southern Australia are likely to increase in frequency from 1 every 20 years to 1 every 10 years. The models also predict that the frequency of extreme positive Indian Ocean Dipole events, associated with drier spring, is also likely to increase to a frequency of 1 every 7 years (Cai et al 2014).

The predicted decrease in rainfall and increase in number of consecutive dry days is likely to result in a reduction in stream flow, and a decrease in the duration and extent of inundation in wetlands. Aquifer recharge will also be affected. Temperature is predicted to increase in all regions (Table AE2) but the inland areas will show greater increase as the oceans will moderate temperature increases towards the coasts.

Table AE1: Projected decline in rainfall in South Australian NRM regions The intermediate emissions scenario is RCP4.5 which would stabilise carbon dioxide concentration by 2100. The high emissions scenario (RCP8.5) arises from little effort to reduce emissions and represents a failure to curb warming by 2100 at which point CO₂ concentrations would be 900 parts per million by volume (ppmv). Data drawn from Charles and Fu (2014).

Table AE2: Mean projected change in average annual maximum temperature (°C) compared to the baseline period (1986–2005). The high emissions scenario (RCP8.5) arises from little effort to reduce emissions and represents a failure to curb warming by 2100 at which point CO₂ concentrations would be 900 ppmv. Data drawn from Charles and Fu (2014).

Setting goals for the condition of surface water ecosystems across South Australia needs to consider International agreements such as the Ramsar Convention, water quality guidelines and an understanding of the local ecosystem drivers, functions and processes.

The Australian and New Zealand Environment Conservation Council (ANZECC) published guidelines for fresh and marine water quality in 2000. The default trigger values for nutrients are twice the concentrations in southeast Australia. Consequently, every effort should be made to achieve the water quality guideline concentrations as the degraded conditions have already been taken into account. Within the South East of South Australia, most of the wetlands meet the total phosphorus target of 0.1 mg/L but total nitrogen tends to be in excess of 1 mg/L (Figure AE1).

While water quality guidelines for permissible nutrient concentrations are broadly applicable across aquatic habitats other ecological targets need to be site specific. Critical to all aquatic habitats is the availability of water. Water allocation planning across South Australia has worked to balance the water demands of a range of stakeholders while maintaining water availability for the environmental assets.

Water allocation planning considers minimum permissible stream flow and appropriate water regime for wetlands, including timing duration and extent of inundation. Setting water allocations and flow targets in an uncertain future relies upon sound modelling of future water availability but is informed by numerous site-specific ecological targets.

State of the ecosystem

The approximately 5,000 mound springs in far north South Australia are considered ‘desert jewels’ for their natural and cultural significance. Spring formation occurs in regions of artesian groundwater pressure breaching through minor faults and shales. Groundwater pressure maintains spring flow and supports connectivity and biodiversity.

It is estimated that natural spring flow has declined by 40% due to water extraction from the Great Artesian Basin. The isolation and low spring connectivity has led to a high degree of endemism within springs. For example, Dalhousie Springs host 16 endemic species including 5 endemic fish species

Threats

Aquifer drawdown activities that reduce the pressure head supporting the springs (such as pastoral bores, mining and town supplies) can lead to a reduction in spring flow, and a reduction and fragmentation of habitat. For example in the Lake Cadibarrawirracanna Springs Complex the number of vents has fallen from 21 to 6 as reduced pressure in the aquifer led to extinction of springs (Gotch et al 2016).

Many springs along the western Great Artesian Basin contain high concentrations of sulfides, which remain locked in wet anoxic sediments (Shand 2013). A risk with springs drying is that these sediments are exposed to air and sulfide is oxidised to sulfate which upon rewetting forms sulfuric acid. The sulfuric acid can dissolve the carbonate structure of the mounds and affect local flora and fauna. Sulfates precipitating at the edge of the vent area can also reduce pH and affect spring vegetation and fauna (Gotch et al 2016). The only means of avoiding acid sulfate issues is maintaining water over sulfidic sediments.

Damage by domestic and feral animals including grazing and trampling decreases vegetation cover and erosion. Travertine mounds, which form by accumulation of carbonates over hundreds of thousands of years, are particularly vulnerable as they are easily eroded, not only by cattle but also by vehicles (Gotch et al 2016).

Future considerations

Water extracting activities have decreased the volume of water discharged through springs and it is prudent to continue investigating and monitoring any activity that may alter hydrogeology. Coal seam gas and coal mining in the Arckaringa Basin are not expected to impact on groundwater supply to springs east of Peake and Denison Inlier (Keppel et al 2015). Springs in the Toondina, Peake Creek and Mt Dutton complexes are most vulnerable to coal developments in the Arckaringa Basin. However, as groundwater recharge zones are not well known, and aquifer connectivity not well defined, these should be rigorously assessed for all future developments.

There is some evidence that some springs have a contribution from modern groundwater which may emanate from fractured rock aquifers (Keppel et al 2015). While the local contribution to the aquifer may be small it suggests some local connectivity with surface water resources, which may need to be afforded additional protection.

A number of spring complexes remain under-explored because of their remote nature. However, given the high degree of endemism and high conservation value a more complete inventory of the flora, fauna and hydrochemistry is required to understand these environments and appreciate the scale of the conservation task (Keppel et al 2016).

Conservation needs to be tailored for each spring complex and this is not as simple as fencing to exclude grazing animals. There are instances where fencing of springs has led to an expansion of Phragmites sp significantly increasing the evapo-transpiration which exceeds the upward flows and ultimately drying out of the spring. Managing grazing and water extracting activities is required for conservation of these desert jewels. An investment in monitoring and investigation is also required to ensure these unique springs are adequately conserved and do not decline further.

State of the ecosystem

The hydrology of the South East has been extensively modified by a combination of drainage schemes, land clearance and water extraction.

Many wetlands in the South East are recognised as having a high dependency on the regional unconfined aquifer and are at risk from increasing competition for water resources and groundwater level decline. Wetland inventories have mapped over 16,000 extant wetlands and identified 45 ecologically significant groundwater dependent wetland complexes including the RAMSAR listed Piccaninnie Ponds, Bool and Hacks Lagoons (Harding 2012).

A 2,515-km drainage network been established to remove water and saline groundwater enabling agricultural productivity in the region but reducing the wetland extent by 93%. Less than 10% of remaining wetland is considered intact (Harding 2007). Water regime of wetlands is fundamental to the maintenance of wetland habitat to sustain food foraging and breeding.

The Coorong and Lower Lakes is a Ramsar listed wetland that supports migratory waders and domestic aquatic birds. The Coorong does not have the numbers of birds that were supported historically. This may in part be due to conditions in other wetlands globally declining. However, Australia has obligations to ensure domestic habitat is protected.

The importance of the Lower Lakes and Coorong for water birds cannot be overstated. In November 2007 these sites hosted 249,146 water birds, which is 91% of all waterbirds surveyed at the Living Murray Icon sites across the Murray–Darling Basin (Kingsford and Porter 2008).

One target in the Lower Lakes, Coorong and Murray Mouth Icon Site Condition monitoring plan (Maunsell 2009) is ‘maintain or improve water bird populations’. It was judged by Paton et al (2018) that this target was not achieved as bird numbers did not meet the long-term ecological threshold for 25 species in the Lower Lakes and 40 species in the Coorong.

Although the site is considered to be in decline it still hosted 92,000 waterbirds from 51 species in the Lower lake and 187,500 waterbirds from 60 species in the Coorong in January 2018. Indicative of poor food resources, approximately 80% of shorebirds in the Coorong were observed to be foraging. This was attributed to low numbers of Chironomid larvae, and low abundance of Ruppia tuberosa seeds and turions which comprise the diet of shorebirds in the region.

Bool Lagoon is another example of a site that hosts high waterbird diversity and high numbers of breeding species but has undergone considerable hydrological and vegetation change. Bird census data is collected to set quotas in the game reserve. However, in general there is a dearth of systematically collected bird census data for other sites in the South East, with notable exceptions (Wainwright and Christie 2008). Knowledge is required on how birds use habitat and what is required for restoration.

Threats to wetlands

The major threats to wetlands in the South East are lack of water and salinisation. Vegetation surveys conducted pre-2000 and post-2000 in a number of wetland complexes, identified a change in species composition from species requiring fresh conditions to species preferring more saline conditions (Goodman 2012).

In some regions hardwood plantations are drawing heavily on the groundwater with groundwater declining by up to 0.7 m per year (Brookes et al 2017). Within the hard wood forested area in the Hundreds of Coles and Short, there are 4,050 wetlands, with combined areas of 212.15 m², impacted by groundwater decline (Brookes et al 2017).

Climate change is expected to have a tremendous impact on wetlands in the South East. A case study on Middlepoint Swamp indicated that 47–70% of the swamp was at risk of terrestrialisation by 2030 (Harding et al 2015). The vegetation communities with the highest water requirements are predicted to be lost from the wetland by 2030 and replaced by brackish herbland and exotic pasture grasses. There is concern that the impacts could be even more severe in shallower groundwater dependent ecosystems (Deane et al 2018).

Threats to Ewens Ponds

Ewens Ponds are clear groundwater-fed sinkhole lakes in the South East of South Australia. The ponds host endemic and threatened species such as Glenelg Spiny Crayfish Euastacus bispinosus, and Ewens Pygmy Perch Nannoperca variegata.

EPA water quality records (EPA 2007) identified elevated nitrogen leading to poor water quality and stated that this could lead to excessive algal growth. Total nitrogen concentrations in Ewens Ponds in 2014 were 5.8 ± 0.5 mg/L. Total phosphorus concentrations during the same period were 0.22 ± 0.007 mg/L. The dissolved orthophosphate remains reasonably low, which presumably is due to the formation of insoluble P-containing materials such as calcium-fluoro-apatites, in the high-calcium karst landscape.

Benthic algae inhabit the sediments in Ewens Ponds, displacing macrophytes with Rhizoclonium sp dominating sediments from 2–5m depth and Lyngbya sp dominating from 5–8 m depth (Lui 2017). Pelagic phytoplankton would typically thrive in the nutrient concentrations found at Ewens Ponds but the residence time is 0.48 days and so phytoplankton are flushed from the system before populations can establish (Rigosi et al 2015). Epiphytic algae cover many of the submerged macrophytes. At current nutrient concentrations the biomass of epiphytes reduces light availability by 20%.

Threats to waterbird habitat

There are numerous threats to waterbird habitat. Within the Coorong recent infestations of algal mats is constricting the flowering heads of Ruppia tuberosa, its seeds are a major food source for small migratory wading birds. The algae has probably arisen from loss of macrophye beds and high concentrations of nutrients entering the lagoons and concentrating within the sediments, although the exact causes remain unknown.

Ruppia tuberosa also requires elevated water levels through early to mid-summer so it can complete its life cycle. Water level in the South Lagoon can only be maintained by ensuring a head of water in the North Lagoon with flow through the barrages. Threats to other bird habitat in the South East are groundwater decline, feral predators, grazing and a lack of broad-scale restoration.

Future considerations

Declining groundwater level and groundwater flow is a major threat to wetlands ecosystems in the South East. Shallow groundwater dependent wetlands are most at risk. Harding et al (2015) suggest that identification, prioritisation and protection of resilient ecosystems, as well as potential surface water augmentation from existing regional drainage infrastructure and restoration of water levels using weirs could prove to be an important adaptation strategy for protecting important wetlands in the future.

The unique clear water ponds, Ewens and Piccaninnie Ponds, require high groundwater flow to maintain flushing and not allow pelagic phytoplankton communities to establish. Any increase in nutrients could increase the growth of benthic and epiphytic algae. Relieving nutrient limitation of epiphytic algae would mean that epiphyte biomass could reach levels that would reduce the amount of light reaching macrophyte leaves by 80% (Rigosi et al 2015).

A systematic inventory of how aquatic birds use wetlands in the South East is required. This will provide an additional layer of knowledge to inform the provision of water to those habitats through the management of the south east drainage network. River regulation and draining of the once expansive tracts of wetland landscapes has led to fragmentation of habitat and a heavy reliance on a few remaining wetland complexes to support migratory and domestic bird populations.

Management of aquatic ecosystems is often considered at the individual wetland level. However, these are connected systems and particularly in the case of waterbirds, there is a need to manage the Lower Lakes, Coorong and South East wetlands holistically. Goals and targets need to be defined for breeding, feeding and wading habitat and this goals translated to appropriate salinity and water regime for individual sites in the region.