Paul C. E. Bailey

A review of the salt sensitivity of the Australian freshwater biota

In Victoria, Australia, both dryland salinity and salinity in irrigation regions are serious agricultural problems. One option to control the latter is to pump groundwater to maintain it below the surface. However, this leaves a saline wastewater for disposal, probably into local streams or wetlands. This review of the salt sensitivity of the biota of Australian streams and wetlands gives information of interest to those responsible for developing controls on these discharges. The review addresses the lethal and sub-lethal effects of salinity on microbes (mainly bacteria), macrophytes and micro-algae, riparian vegetation, invertebrates, fish, amphibians, reptiles, mammals, and birds. Data suggest that direct adverse biological effects are likely to occur in Australian river, stream and wetland ecosystems if salinity is increased to around 1 000 mg L−1. The review highlights a general lack of data on the sensitivity of freshwater plants and animals to salinity increases.

Effects of salinity on river, stream and wetland ecosystems in Victoria, Australia.

Salinity is a growing problem in many parts of the world. In Australia both dryland salinity and salinity in irrigation regions are serious problems. Options being considered to control the latter involve pumping the groundwater to lower the watertable; however, this leaves a saline wastewater to be disposed, probably into local streams or wetlands. A previous review by the authors covered the lethal and sub-lethal effects of salinity on individual species within the following groups of aquatic organisms: microbes (mainly bacteria), macrophytes and micro-algae, riparian vegetation, invertebrates, fish. amphibians, reptiles and mammals and water birds. That review concentrated on Australian information, information it is argued that will be more typical of the effects occurring over a large part of the world than data from the better watered areas of North America and Europe. This paper uses these data on individual species to determine the possible adverse effects of saline wastewater discharges on aquatic ecosystems, in particular lowland rivers and streams and wetlands. In these freshwater systems, the macroinvertebrates and plants (riparian vegetation, macrophytes and micro-algae) were assessed to be the most salt sensitive biological communities, with direct adverse biological effects likely to occur when salinity is increased to around 1000 mg 1-t. More subtle sublethal and indirect effects possibly occur at salinities below this, however the scientific data are not presently available to assess the extent that this might occur. A set of guidelines for assessing the possible biological effects in particular salt-affected rivers, streams or wetlands is developed.

Effects of salinity on the survival, growth and development of tadpoles of the brown tree frog, Litoria ewingii

We investigated the effects of 4% seawater (sw), 8% sw, 12% sw and 16% sw (1.4 g NaCl L–1, 2.8 g NaCl L–1, 4.2 g NaCl L–1 and 5.6 g NaCl L–1, respectively) on survival, mass and development of larvae of the brown tree frog, Litoria ewingii. Salinity of 16% sw significantly decreased survival of tadpoles such that 39% of tadpoles in 16% sw treatment survived to metamorphosis compared with 92% in the control group (freshwater). Growth (mass) of 16% sw tadpoles (0.048 g ± 0.005 g) slowed significantly during early development compared with control tadpoles (0.105 g ± 0.004 g); however, there was no significant difference in final metamorphosis mass between 16% sw (0.192 g ± 0.008 g) and control tadpoles (0.226 ± 0.006 g). Time taken to reach metamorphosis was greater for 16% sw tadpoles (84 ± 1.8 days) than for control tadpoles (55 ± 0.84 days). Tadpoles at salinity concentrations of 4% sw, 8% sw and 12% sw were significantly heavier than control tadpoles at metamorphosis. Our results show that moderate levels of salinity (16% sw) are sufficient to significantly reduce survival and retard development of tadpoles of L. ewingii.

Impact of secondary salinisation on freshwater ecosystems: effects of contrasting, experimental, short-term releases of saline wastewater on macroinvertebrates in a lowland stream

In Australia, secondary salinisation of land and water resources is a serious environmental problem. We conducted two field experiments to examine the effects on macroinvertebrates of increased salt concentration (approximately 1000 and 2000 mg L–1 total dissolved solids (TDS)) and mode of salt water release (continuous press release approximately 1500 mg L–1 and four, separate pulses of approximately 3400 mg L–1 TDS). The abundance of the gastropod Ferrissia tasmanica, the mayfly Baetis sp. 5 and scraper and predator functional feeding groups were significantly reduced at 1500 mg L–1, with the effect exacerbated in pulse release channels. Moreover, the pooled abundance of Oligochaeta, Copepoda and Acarina was significantly reduced at 2000 mg L–1 and the number of drifting animals in the pulse treatment channels significantly increased following commencement of salt water release during daylight hours. In contrast, the abundance of 49 other macroinvertebrate taxa, collector–gatherer functional group and species diversity were unaffected by salinity or release schedule. We conclude that much of the existing macroinvertebrate fauna appears to be halotolerant. However, the abundance of halosensitive species will be reduced at 1500 mg L–1. Delivering short pulses of high salt concentration is more detrimental than delivering the same salt load at a low concentration over a longer period of time.

Alternative stable states in the aquatic vegetation of shallow urban lakes. II. Catastrophic loss of aquatic plants consequent to nutrient enrichment

The theory of alternative stable states predicts that high nutrient concentrations increase the probability of shallow lakes switching from a state dominated by vascular macrophytes to one dominated by phytoplankton and/or other algae. In the first paper of this series it was demonstrated that chronic, low-level nutrient loading did not affect a switch across vegetation states. To test the possibility that higher nutrient loadings result in vegetation changes, replicated mesocosms (~3000 L) were placed in an urban lake densely colonized by Vallisneria americana Michaux, a submerged angiosperm, and were subjected to higher levels of chronic nutrient enrichment. Moderate and high nutrient loadings significantly increased phytoplankton biomass and produced extensive, dense mats of floating algae. Many mesocosms became covered by the floating fern Azolla pinnata R.Br. This reduced light penetration and concentrations of dissolved oxygen in the water column profoundly and resulted in the complete loss of V. americana from almost all nutrient-enriched mesocosms within 4 months. A catastrophic loss of submerged aquatic plants so rapidly after nutrient enrichment is a relatively novel experimental finding, particularly in terms of the likely mechanism; that is, shading and subsequent anoxia caused by dense mats of floating plants other than algae.

Alternative stable states in the aquatic vegetation of shallow urban lakes. I. Effects of plant harvesting and low-level nutrient enrichment

Shallow urban lakes are often subject to chronic nutrient enrichment and their submerged plants are sometimes harvested to facilitate recreational use. The theory of alternative stable states predicts that: (i) low levels of nutrient enrichment should have little effect on the existing communities of submerged macrophytes in such lakes; but (ii) harvesting the plants should facilitate a shift to phytoplankton dominance. These two predictions were tested with large (3000 L), replicated mesocosms in a shallow urban lake densely colonized by the submerged angiosperm, Vallisneria americana Michaux. Harvesting V. americana substantially increased light penetration through the water column, but did not significantly increase phytoplankton biomass. Vallisneria americana regrew rapidly after harvesting and Chara species, which were previously absent, appeared in the harvested mesocosms. Chronic low-level nutrient enrichment significantly increased phytoplankton biomass (>100 μg chlorophyll a L–1) but not epiphyte biomass on the leaves of V. americana or on plastic leaf surrogates. The aboveground biomass and leaf area index of V. americana were not affected significantly by nutrient enrichment. The theory of alternative stable states successfully predicted the resilience of the submerged angiosperm community to low-level nutrient enrichment, but did not accurately predict the response to plant harvesting. The response of the lake vegetation to higher levels of nutrient enrichment is reported.

Environmental deterioration increases tadpole vulnerability to predation

Human-induced environmental change is occurring at an unprecedented rate and scale. Many freshwater habitats, in particular, have been degraded as a result of increased salinity. Little is known about the effects of anthropogenic salinization on freshwater organisms, especially at sublethal concentrations, where subtle behavioural changes can have potentially drastic fitness consequences. Using a species of Australian frog (Litoria ewingii), we experimentally examined the effects of salinization on tadpole behaviour and their vulnerability to a predatory dragonfly nymph (Hemianax papuensis). We found that tadpoles exposed to an ecologically relevant concentration of salt (15% seawater, SW) were less active than those in our freshwater control (0.4% SW). Tadpoles in elevated salinity also experienced a higher risk of predation, even though the strike rate of the predator did not differ between salt and freshwater treatments. In a separate experiment testing the burst-speed performance of tadpoles, we found that tadpoles in saltwater were slower than those in freshwater. Thus, it would appear that salt compromised the anti-predator response of tadpoles and made them more susceptible to being captured. Our results demonstrate that environmentally relevant concentrations of aquatic contaminants can, even at sublethal levels, severely undermine the fitness of exposed organisms.

Rehabilitation of Swamp Paperbark (Melaleuca ericifolia) wetlands in south-eastern Australia: effects of hydrology, microtopography, plant age and planting technique on the success of community-based revegetation trials

Wetlands dominated by Swamp Paperbarks (Melaleuca spp., Myrtaceae) are common in coastal regions across Australia. Many of these wetlands have been filled in for coastal development or otherwise degraded as a consequence of altered water regimes and increased salinity. Substantial resources, often involving community groups, are now being allocated to revegetating the remaining wetland sites, yet only rarely is the effectiveness of the rehabilitation strategies or on-ground procedures robustly assessed. As part of a larger project investigating the condition and rehabilitation of brackish-water wetlands of the Gippsland Lakes, we overlaid a scientifically informed experimental design on a set of community-based planting trials to test the effects of water depth, microtopography, plant age and planting method on the survival and growth of seedlings of Melaleuca ericifolia Sm. in Dowd Morass, a degraded, Ramsar-listed wetland in south-eastern Australia. Although previous laboratory and greenhouse studies have shown M. ericifolia seedlings to be salt tolerant, the strongly interactive effects of waterlogging and salinity resulted in high seedling mortality (>90%) in the field-based revegetation trials. Seedlings survived best if planted on naturally raised hummocks vegetated with Paspalum distichum L. (Gramineae), but their height was reduced compared with seedlings planted in shallowly flooded environments. Age of plants and depth of water were important factors in the survival and growth of M. ericifolia seedlings, whereas planting method seemed to have little effect on survival. Improved testing of revegetation methods and reporting of success or otherwise of revegetation trials will improve the effectiveness and accountability of projects aiming to rehabilitate degraded coastal wetlands.

Germination characteristics of Melaleuca ericifolia Sm. (swamp paperbark) and their implications for the rehabilitation of coastal wetlands

Swamp paperbark, Melaleuca ericifolia Sm., is a small, clonal tree that occupies fresh- and brackish-water wetlands across south-eastern Australia. Seeds collected from Dowd Morass, a secondary-salinised Ramsar-listed wetland of the Gippsland Lakes region in eastern Victoria, showed very low viability (< 6%), with less than 50% of the seeds germinating even under ideal laboratory conditions. Greatest germination occurred with surface-sown seeds, germinated in darkness at a mean temperature of 20°C and salinity < 2 g L–1. At 20°C, maximum germination occurred at a salinity of 1 g L–1; germination fell rapidly at a near constant rate with increasing salinity. Lower temperatures, while moderating the inhibitory effects of salinity, markedly reduced germination; higher temperatures increased the inhibitory effects of salinity and light and reduced overall germination rates. Seeds subjected to brief inundation with saline water germinated rapidly if flushed by, and subsequently grown under, freshwater conditions. Specific timing of management interventions, particularly manipulations of water regime to control salinity regimes, are required if germination of M. ericifolia on the landscape scale is to be successful. Even so, the low overall viability of the seeds would present difficulties to large-scale, seed-based rehabilitation efforts.

Domain shifts in the aquatic vegetation of shallow urban lakes: the relative roles of low light and anoxia in the catastrophic loss of the submerged angiosperm Vallisneria americana

The catastrophic loss of Vallisneria americana from a shallow urban lake in south-eastern Australia following nutrient enrichment has been reported previously. Two experiments are reported here to determine whether light attenuation or shifts in dissolved oxygen were more important in mediating this loss of submerged plants. The first experiment tested the response of dense beds of Vallisneria to different levels of shade in the field. The second tested the effect of (i) shading and (ii) anoxia on the performance of Vallisneria grown in the glasshouse. In the field, plants persisted after 3 months beneath 100% shade cloth, although with significantly reduced biomass (63%). In contrast, plant biomass beneath 70% shade cloth was reduced by only 9%. The field trials indicate that Vallisneria is highly tolerant to severe light attenuation. In the glasshouse, microcosms that were not artificially aerated become anoxic, and all plants died within 24 days in two of the three replicate microcosms. In shaded microcosms that were artificially aerated, plant biomass was reduced by 48% but no plants died. Severe reductions in dissolved oxygen associated with shading were primarily responsible for the rapid loss of Vallisneria, rather than light attenuation alone was concluded from the current study.