Current Ecological Knowledge of the Flora of Coastal
Lagoon Estuary Systems in South-Eastern Australia
A Literature Review
30 May 2007
SARDI Publication Number F2007/000252-1
SARDI Research Report Series No: 210
Current Ecological Knowledge of the Flora of Coastal
Lagoon Estuary Systems in South-Eastern Australia
A Literature Review
30 May 2007
SARDI Publication Number F2007/000252-1
SARDI Research Report Series No: 210
South Australian Research and Development Institute
SARDI Aquatic Sciences
2 Hamra Avenue
West Beach SA 5024
Telephone: (08) 8207 2400
Facsimile: (08) 8207 5481
The authors warrant that they have taken all reasonable care in producing this report. The report has been through the SARDI Aquatic Sciences internal review process, and has been formally approved for release by the Chief Scientist. Although all reasonable efforts have been made to ensure quality, SARDI Aquatic Sciences does not warrant that the information in this report is free from errors or omissions. SARDI Aquatic Sciences does not accept any liability for the contents of this report or for any consequences arising from its use or any reliance placed upon it.
© 2007 SARDI Aquatic Sciences
This work is copyright. Apart from any use as permitted under the Copyright Act 1968, no part may be reproduced by any process without prior written permission from the author.
Printed in Adelaide May 2007
SARDI Aquatic Sciences Publication Number F2007/000252-1 SARDI Research Report Series Number 210
ISBN Number 0730853659
Author: Jason Nicol
Reviewers: Greg Collings and Kelly Marsland Approved by: Q. Ye
Date: 30 May 2007
Distribution: E-Water Cooperative Research Centre and SARDI Aquatic Sciences Library Circulation: Public Domain
Table of ContentsTable of Contents... i Acknowledgements ... 1 Executive Summary ... 2 1. Introduction ... 3 2. Submergent Plants ... 4 2.1. Ruppia spp... 4 2.2. Charophytes (Lamprothamnium spp.)... 4
3. Samphire and Saltmarsh Plants... 5
3.1. Native Species... 6
3.2. Exotic Species... 7
4. Trees... 8
5. Conclusions, Knowledge Gaps and Further Research ... 9
5.1. Submergent Macrophytes... 9
5.2. Samphire and Saltmarsh Plants... 10
5.3. Trees... 10
The author thanks the E-Water Cooperative Research Centre for funding this review and Greg Collings and Kelly Marsland for comments on an early draft of this review
This review examines and synthesises the current information regarding the flora of coastal lagoon estuary systems in southeastern Australia and the potential impacts of river regulation and freshwater abstraction for domestic, industrial and agricultural uses. Coastal lagoon estuaries are characterised by large salinity ranges (fresh to hypersaline conditions may occur in the same year) and for species to persist in these systems they must be able to tolerate these ranges. The reduction in freshwater inflows due to river regulation and abstraction has probably resulted in increased salinity ranges and hypermarine conditions now occur for longer periods than before regulation. This has probably had significant implications for estuarine flora; many species that require salinities lower than seawater for part of their life cycle may have become locally extinct and the flora now resembles that of marine systems or hypersaline inland water bodies.
The only submergent species that are able to tolerate the large salinity ranges that occur in lagoon estuaries are from the genera Ruppia and Lamprothamnium. Similarly, the emergent, riparian and tidal species also must be able to tolerate large salinity ranges and in southeastern Australian systems species from the genera Halosarcia, Juncus, Sarcocornia, Suaeda, Sporobolus, Samolus, Melaleuca
and Triglochin are the dominant species.
The absolute salinity thresholds for Ruppia spp., Juncus kraussii and Sporobolus virginicus are known, the impact of salinity on the germination of Melaleuca ericifolia, J. kraussii and S. virginicus have been investigated and the interaction between salinity and flooding is well understood for M. ericifolia. In addition the physiological mechanisms of salinity tolerance in Melaleuca halmaturorum, M. ericifolia and Ruppia spp. have been investigated.
Information on the salinity thresholds and impacts of sub-lethal salinities of key estuarine species is important when designing environmental flow provisions for estuaries. Freshwater inflows need to be of sufficient volume to reduce the salinity below thresholds for critical life history stages (i.e. germination, flowering, seed set) to ensure species persistence.
Estuaries are highly productive and variable systems that usually have extensive benthic, pelagic and fringing biota (Deeley and Paling 1999). The habitats of these systems in southeastern Australia are diverse but can be constrained by chemical conditions such as highly variable salinity induced by poor water exchange with the marine environment and the seasonal flow patterns of the rivers that flow into these systems (Scheltinga et al. 2006). Flooding can be uncommon but even when these estuaries receive small, intermittent freshwater inflows they have the potential to be major drivers of the system as they may be the sole source of freshwater and other materials (Scheltinga et al. 2006).
Many estuaries in southeastern Australia are highly modified, being subjected to both urban and rural pressures (Scheltinga et al. 2006). There is a common perception that freshwater is wasted once it reaches the marine environment (Gillanders and Kingsford 2002) and river regulation and abstraction for domestic, industrial, and agricultural uses has resulted in a reduction in the volume of freshwater inflows into many estuaries (Bornman et al. 2002; Gillanders and Kingsford 2002; Scheltinga et al. 2006). The reduction in freshwater inflows has probably resulted in even larger salinity ranges and hypermarine conditions occurring for longer periods than would have occurred before regulation, which can have significant consequences on the biota of the system (Scheltinga et al. 2006). However, little is known about the relationship between freshwater flows and aquatic and riparian vegetation of estuaries (Gillanders and Kingsford 2002; Scheltinga et al.
2006). It is likely that the flora of these systems now closely resembles that of marine systems (Wortmann et al. 1997) or hypersaline inland water bodies with many of the species that require salinities lower than seawater, for part or all of their life cycle, being extirpated.
Environmental flow research on Australian estuaries has focussed on systems in tropical and sub-tropical climates (Hamilton and Gehrke 2005), with the majority of work concentrated on species that are commercially fished (Scheltinga et al. 2006). Environmental flow provisions for estuaries need to be able to provide conditions suitable for estuarine biota to survive and be able to complete their life cycles. Estuarine environmental flow research needs to be able to inform managers about the environmental conditions that key species or communities require to persist and how much fresh water is required to provide those conditions. This review will focus on the flora (aquatic and riparian) of coastal lagoon estuary systems with inflows from large river systems in southeastern Australia (e.g. The Coorong, Gippsland Lakes) and the relationship between reduced flows and higher salinity and the consequences for estuarine flora.
Coastal lagoon estuary systems are separated from the sea by a coastal barrier (generally a series of dunes); the entrance is usually constricted or periodically closed to the sea and characterised by a narrow entrance channel within broad tidal and back barrier sand flats (Digby et al. 1998). Tidal movement is strongly attenuated by constriction at the mouth and exchange between the estuary and ocean in southeastern Australian systems is much less than systems in other parts of Australia due to the small tidal range (Digby et al. 1998; Scheltinga et al. 2006). During years without high summer rainfall larges areas of the estuary may characterised by hypersaline conditions in summer and early autumn (Deeley and Paling 1999).
Little is known about the flora of coastal lagoon estuaries in southeastern Australia. The large spatial and temporal salinity ranges (fresh to hypersaline) that can occur in these systems dictates that only a few highly specialised species are able to persist in the permanently inundated, tidal and riparian areas.
2. Submergent Plants
Submergent plants are major primary producers (e.g. Congdon and McComb 1979; Verhoeven 1980; Brock 1982a), provide important structural habitat (e.g. Lloyd and Walker 1986; Kantrud 1991; West and King 1996; Gehrke and Harris 2000; Morgan and Gill 2000; Balcombe and Closs 2004; Katende 2004; Clavero et al. 2005) and stabilise sediments (e.g. Sainty and Jacobs 1994) in freshwater, marine and estuarine systems. The large temporal salinity changes, probably prevent the establishment of most freshwater or marine species. The only submergent species capable of persisting in such conditions are from the genera Ruppia (Brock 1981b) or Lamprothamnium
A comprehensive review of the ecology, ecophysiology and taxonomy of Ruppia megacarpa, R. polycarpa and R. tuberosa was undertaken by Nicol (2005). This review; however, neglected to identify that the impact of sub-lethal salinities on the germination, growth and reproductive output of the aforementioned species was a significant knowledge gap that needs to be addressed.
2.2. Charophytes (
There is little information regarding the distribution of charophytes in estuarine systems in southeastern Australia. Garcia (1999) reported that Lamprothamnium macropogon was the only charophyte present in lakes of southeastern South Australia and southwestern Victoria where the
salinity was greater than 5‰ TDS (Chara spp. and Nitella spp. were only present in lakes where the salinity was lower than 5‰ TDS). This species was recorded in lakes in southeastern South Australia and southwestern Victoria with salinities ranging from 2-58‰ TDS (Garcia 1999) and in Western Australia in water bodies with salinities ranging from 2-78‰ TDS (Burne et al. 1980). Brock and Lane (1983) recorded Lamprothamnium papulosum in salinised wetlands in the Western Australian wheat belt with salinities ranging from 9-125‰ TDS and that the upper salinity threshold for this species was reported to be 210‰ TDS (Brock 1985). In more recent investigations in the Western Australian wheat belt, Lamprothamnium sp. (possibly several species) was recorded in wetlands with salinities of 2-90‰ TDS (Strehlow et al. 2005). The aforementioned studies did not sample estuarine systems (Burne et al. 1980; Brock and Lane 1983; Brock 1985; Garcia 1999; Strehlow et al. 2005); however, Geddes (1987) reported that L. macropogon was sometimes common in the South Lagoon of the Coorong. The salinity at the time of sampling ranged from 40-140‰ TDS but no abundance estimates or specific locations were reported (Geddes 1987). In addition, Ye et al. (2002) observed Chara spp. in Lake George (an artificial lagoon estuary in the southeast of South Australia). The charophyte observed in this system was probably misidentified and was L. macropogon because the salinity of the system was greater than 5‰ TDS when sampled. Therefore, it is likely that Lamprothamnium spp. are the only charophytes that will have persistent populations in coastal lagoon estuaries.
There is no information available on the absolute salinity thresholds or effects of sub-lethal salinities on the performance of L. macropogon. Brock (1985) reported that the salinity thresholds for L. papulosum was 2-210‰ TDS and a project being funded by the national Action Plan for Salinity (NAP) through the Centre for Natural Resource Management (CNRM) and being undertaken by SARDI Aquatic Sciences in conjunction with the University of Adelaide, the South East Catchment Natural Resources Management Board and the South Australian Environment Protection Authority investigating the ecology of Lake George has a component that will investigate the salinity thresholds of the dominant benthic macrophytes (one of which is
3. Samphire and Saltmarsh Plants
Samphire and saltmarsh ecosystems flourish in coastal areas wherever the accumulation of sediments is equal to or greater than the rate of land subsidence and where there is adequate protection from destructive waves and storms. They are among the most productive ecosystems in the world and are outwelling systems that export energy into adjacent estuarine and marine waters. The outwelling of energy into the adjacent waters primarily drives detrital food webs with grazing food webs being of less significance. In addition these systems support large
numbers of wading birds and, at times, migratory waterfowl. The vegetation is dominated by grasses, rushes and sub-shrubs that are tolerant of high soil and water salinities and waterlogging or periodic inundation (Mitsch and Gosselink 1993).
3.1. Native Species
In southeastern Australian coastal lagoon systems the commonest species are Halosarcia pergranulata ssp. pergranulata (Short and Colmer 1999; Nicol et al. 2006), Sarcocornia quinqueflora
(Holt et al. 2005; Nicol et al. 2006), Suaeda australis (Clarke and Hannon 1967; Holt et al. 2005; Nicol et al. 2006), Juncus kraussii, Triglochin striatum, Sporobolus virginicus and Samolus repens (Clarke and Hannon 1967).
There is little information on the autecology of native saltmarsh species in Australia. Clarke and Hannon (1970) investigated the impact of salinity and waterlogging on the growth and survival of Suaeda australis, Sporobolus virginicus, Juncus kraussii and Triglochin striatum. The aforementioned species were subjected to salinities the equivalent of 0, 20%, 40%, 60% 80% and 100% of seawater (35‰ TDS) (Clarke and Hannon 1970). Suaeda australis and T. striatum survived in all treatments and J. kraussii and S. virginicus survived in salinities up to 80% of seawater salinity (28‰ TDS) (Clarke and Hannon 1970). However, all species tested showed reduced growth rates at salinities greater than 20% (7‰ TDS) of seawater (Clarke and Hannon 1970). Naidoo and Kift (2006) reported that J. kraussii would grow under waterlogged conditions in salinities up to 70% of seawater (24.5‰ TDS) but germination of seeds was greatly reduced in salinities above 20% of seawater (7‰ TDS). In addition, the aforementioned researchers showed that seeds subjected to high salinities (38.5‰ TDS) germinated when transferred to freshwater (Naidoo and Kift 2006). Short and Colmer (1999) investigated the salt tolerance of Halosarcia pergranulata ssp. pergranulata by subjecting plants to seven different salinity treatments ranging from 10-800 molm-3 NaCl (0.58-46.75‰ TDS) for 83 days. Plants survived in all salinities but
growth was significantly lower at salinities higher than 200 molm-3 NaCl (11.69‰ TDS) (Short
and Colmer 1999).
The aforementioned results demonstrated that high salinities significantly reduced growth rates in the aforementioned halophytes. However, all but two species could tolerate salinities of at least 35‰ TDS. The expansion of these species into areas with lower salinities is probably prevented by competition with faster growing species with lower salinity thresholds, which restrict the distribution of these species to areas where there is very little competition (i.e. areas with high salinity) (sensu Harty 2004; Rogers et al. 2006). The results also showed that J. kraussii
and S. virginicus require salinities lower than seawater to survive (Clarke and Hannon 1970); and therefore, require inflows of freshwater sufficient to reduce salinities to a maximum of
approximately 80% of seawater (28‰ TDS) all of the time. Results also showed that J. kraussii is adapted to persist through periods of high salinity in the seed bank and requires freshwater inflows sufficient to reduce salinity to below 7‰ TDS to germinate (Naidoo and Kift 2006). Samphire and saltmarsh loss and the corresponding increase in mangrove communities in drowned river valley and coastal inlet habitats have been well documented in eastern Australia over the past 50 years (e.g. Harty 2004; Rogers et al. 2006). This phenomenon has been attributed to urban and agricultural development, which has resulted in increased freshwater inflows and eutrophication (e.g. Harty 2004; Rogers et al. 2006). In coastal lagoon systems, where freshwater inflows have decreased, it is expected that this phenomenon has not occurred and there has probably been an increase in the abundance of samphire and saltmarsh species (especially species that can tolerate salinities greater than seawater). No pre-regulation data is available for these habitats, but palaeolimnological studies using diatoms and pollen as indicators of historical condition (sensu Gell et al. 2005) may give an insight into the historical distribution of salt marsh species.
In recent studies Nicol et al. (2006) observed H. pergranulata ssp. pergranulata, J. kraussii, Sarcocornia quinqueflora and S. australis in the estuarine section of Hunters Creek (a small creek on Hindmarsh Island that connects Lake Alexandrina and the Coorong, which bypasses Mundoo Barrage). In addition, H. pergranulata spp. pergranulata is widely distributed on saline areas on the lower River Murray floodplain (Holt et al. 2005; Nicol et al. 2006) and around the edges of Lake George (J. Nicol pers. obs.). Samolus repensand S. virginicus, S. quinqueflora, S australis, J. kraussii and T. striatum
are common in brackish to saline wetlands in the southeast of South Australia (Nicol 2006; J. Nicol pers. obs.).
3.2. Exotic Species
There is little information available regarding the distribution and abundance of exotic species in the tidal or riparian zone of estuaries in southeastern Australia with the exception of Spartina angelica and S. x townsendii (a hybrid between S. alterniflora and S. maritima). Spartina spp. are native to Europe and were introduced to stabilise and reclaim tidal mudflats (Muyt 2001).
Spartina spp. are extremely competitive plants that are able to transform estuarine beaches, mudflats, salt marshes and mangrove ecosystems into Spartina dominated meadows (Muyt 2001). These changes to the intertidal ecosystem can have serious consequences by eliminating fish, crustacean and shorebird habitat and accumulating sediment that increases terrace widths, changes channel widths and depths, creates marsh islands and alters water flow (Muyt 2001).
Spartina angelica produces large quantities of fertile seed and poses a greater invasion risk than S. x
townsendii, which only reproduces asexually (Muyt 2001; Jessop et al. 2006). Seeds of S. angelica are dispersed by hydrochory and rhizomes of both species are dispersed by water and mud and during physical removal (Muyt 2001).
Major infestations of Spartina spp. are found in the Tamar, Rubicon (Muyt 2001) and Little Swanport (Hedge and Kriwoken 2000) estuaries in Tasmania and South Gippsland in Victoria with a small infestation present at the mouth of the Gawler River, north of Adelaide in South Australia (Muyt 2001). In Victoria there is evidence to suggest that S. angelica is replacing S. x
townsendii in various locations due to its more vigorous form and ability to spread by seed (Muyt 2001). Due to environmental limitations it is unlikely that Spartina spp. will spread into New South Wales or further north in Gulf St Vincent in South Australia (Muyt 2001).
Mangroves are not generally found in coastal lagoon estuaries associated with large rivers in southeastern Australia (mangroves are generally restricted to tropical or sub-tropical regions) (Scheltinga et al. 2006). Species from the genus Melaleuca are probably the most common trees in the riparian zone of lagoon estuaries in southeastern Australia.
Melaleuca halmaturorum is the dominant tree in the Murray estuary and Coorong (Cooke 1987; Nicol et al. 2006) and widely distributed around the edges of fresh to saline wetlands (inland and coastal) throughout South Australia (Foulkes and Heard 2003; Holliday 2004). Marcar et al.
(1995) reported that M. halmaturorum was highly salt tolerant and could survive in areas where the soil salinity was in excess of 44‰ TDS. The physiological mechanism of salt tolerance in M halmaturorum is well understood, the amino acids: proline, N-methyl-L-proline and trans-4-hydroxy-N-methyl-L-proline accumulate in the leaves of this species and play an important role in osmoregulation (Naidu et al. 2000). Melaleuca halmaturorum seeds require exposed sediment to germinate, will remain viable when submerged in freshwater for between 30 and 84 days (Nicol and Ganf 2000) and survivorship of seedlings and saplings subjected to flooding was dependent on age and size (Denton and Ganf 1994). Four-month-old seedlings will die within three weeks of being top flooded (i.e. no parts of the plant extend above the water surface) but 1 and 2-year-old saplings will tolerate partial inundation for at least 14 weeks (Denton and Ganf 1994). The impact of salinity on germination and the interaction between salinity and flooding (sensu Salter
et al. 2007) on survival of M. halmaturorum has not been investigated.
Melaleuca ericifolia is a small clonal tree, which is common around the edges of the Gippsland Lakes and other fresh to brackish wetlands in southeastern Australia (Bowkett and Kirkpatrick
2003; Robinson et al. 2006; Salter et al. 2007). The upper salinity threshold for M. ericifolia was reported as 30‰ TDS (Bird 1978); however, Salter et al. (2007) reported that there was a significant interaction between flooding and salinity tolerance. The aforementioned researchers demonstrated that this species will tolerate soil salinity of at least 41.8‰ TDS and water salinity of 33‰ TDS when the growing medium is exposed to the atmosphere but the salinity tolerance is much lower when plants are grown under waterlogged or flooded conditions (all plants died after 9 weeks when subjected to flooding or waterlogging at 26.95 and 33‰ TDS but 90% of plants survived at 1.1‰ TDS) (Salter et al. 2007). The leaves of M. ericifolia accumulate the amino acids proline and trans-4-hydroxy-N-methyl-L-proline, which play an important role in osmoregulation (Naidu et al. 2000).
Seed germination is also influenced by salinity; M. ericifolia seeds will germinate when subjected to 16‰ TDS but failed to germinate when subjected to 32‰ TDS (Robinson et al. 2006). However, a significantly higher proportion of seeds germinated when the salinity was <2‰ TDS (Robinson et al. 2006). Robinson et al. (2006) also demonstrated that short exposure to high salinity (32‰ TDS) for at least 16 days did not affect seed viability. Seeds subjected to 33‰ TDS germinated shortly after they were transferred to fresh conditions (Robinson et al. 2006).
5. Conclusions, Knowledge Gaps and Further Research
There is little information regarding either the relationship between estuarine vegetation and freshwater flows (Scheltinga et al. 2006) or how river regulation and abstraction have changed the salinity regimes in estuaries (Gillanders and Kingsford 2002). Information on the salinity thresholds, effects of sub-lethal salinities on growth, survival and reproductive output, the interaction between salinity and flooding (for emergent and riparian species), the interaction between salinity and light compensation points (for submergent species) and the impact of salinity on germination and recruitment (regeneration niche sensu Grubb 1977) is required when determining environmental water provisions for estuaries.
5.1. Submergent Macrophytes
The absolute salinity thresholds of Ruppia spp. and the physiological mechanisms for salt tolerance are well understood (Brock 1979; Brock 1981a; Brock 1981b; Brock 1982b; Brock 1982a; Brock 1983). However, the salinity required for germination, flowering and seed set (the effects of sub-lethal salinities) are not known and an understanding these processes is needed to determine the environmental water requirements for these species. For example, if a period of low salinity that allows germination to occur is not followed by salinities appropriate for flowering and seed set, the seed bank may become depleted or exhaused.
Brock (1982a) reported that when R. megacarpa and R. tuberosa were subjected to salinities close to the upper thresholds for survival, flowering did not occur and reproduction was entirely asexual. In addition there is no information on the salinities required for germination. Therefore, the regeneration niches (sensu Grubb 1977) of these species are unknown.
The absolute salinity thresholds as well as the effects of sub-lethal salinities are not well understood for L. macropogon. The CNRM project should address most of the these knowledge gaps. This information can be used to estimate environmental flow requirements for estuaries where this species is important and where it has colonised areas that may have been occupied by other species in the past (e.g. R. megacarpa).
5.2. Samphire and Saltmarsh Plants
The absolute salinity thresholds for the common native samphire and saltmarsh species (except J. kraussii and S. virginicus (Clarke and Hannon 1970) and the effects of sublethal salinities (except J. kraussii (Naidoo and Kift 2006) are not well understood, especially the effect on reproductive output and seed germination. The interaction between salinity and flooding (sensu Salter et al.
2007) is also poorly understood for the majority of Australian species.
Bornman et al. (2002) suggested that freshwater inflows (particularly overbank flows) were important in South African estuaries because it covers the supra-tidal area with freshwater, reduces the depth of the floodplain water table and inturn makes the groundwater more accessible for halophytes growing on the floodplain. This is probably the case for estuaries in southeastern Australia but the relationship between freshwater flows and saltmarsh and samphire plants has not been investigated. Excessive freshwater flows (especially with water of high nutrient concentrations) have been shown to facilitate mangrove expansion into saltmarsh areas and reduce the area of saltmarsh habitat (Harty 2004; Rogers et al. 2006). However, these species may require occasional or periodic freshwater flows to successfully recruit or produce seed. The production and fate of detritus from saltmarsh ecosystems and the impact on estuarine trophodynamics is also not well understood in systems of southeastern Australia. The impact of
Spartina spp. on trophodynamics is also not well understood.
The absolute salinity thresholds for M. halmaturorum and M. ericifolia have not been determined but the interaction between flooding and salinity identified by Salter et al. (2007) for M. ericifolia has implications for planning environmental water provisions.
Long-term inundation is detrimental to both species (Denton and Ganf 1994; Salter et al. 2007) and water level fluctuations are required for the germination and recruitment of M. halmaturorum
(Nicol and Ganf 2000). The effects of flooding with freshwater on the germination (Nicol and Ganf 2000), growth and survival (Denton and Ganf 1994) of M. halmaturorum have been investigated. However, the interaction between salinity and flooding on adult and juvenile plants and the effect of salinity on seed germination have not been investigated for this species. The effect of salinity on seed germination may be important for M. halmaturorum; seedlings and saplings are generally absent in areas where soil and water salinities have been high in recent years (e.g. The Coorong (Nicol et al. 2006), Lake George (J. Nicol pers. obs.) but are common where salinities are lower (e.g. Bucks Lake (Nicol 2006) despite healthy adult trees being present at all sites.
The impact of sub-lethal salinities on growth, survival and reproductive output are not well understood for either species as a result of the difficulty involved in investigating these factors in long-lived species.
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