Biological effects of secondary salinisation on freshwater microinvertebrates in Tasmania:The acute salinity toxicity testing of seven microinvertebrates
thesisposted on 2023-05-26, 02:10 authored by Allen, KS
Secondary salinisation is a major environmental threat to Australian freshwater rivers, streams and wetlands. In Tasmania there has been limited assessment of the extent and impacts of secondary salinisation, and there has been no research into the biological effects of increasing salinity on freshwater taxa or communities. However expansion of land clearing, cropping and irrigation pose major threats to water balance. High surfacewater and groundwater salinities have been recorded within the Tasmanian NAP region, and high water tables and large sub-surface salt stores have been reported in irrigation areas. The salinity tolerances of seven macroinvertebrates (Nousia sp. AV7, Dinotoperla serricauda, Eusthenia spp., Austrochiltonia australis, Paratya australiensis, Physa acuta and Glytophysa sp.) to artificial seawater were determined by measuring the 72 h lethal concentrations required to kill 50% of individuals (LC50). Nousia sp. AV7 was also slowly acclimated to increasing salinities over 4 days to determine whether acclimation resulted in a higher acute salinity tolerance. The 72 h LC50 values for the seven taxa ranged from between 12.6 to 44.9 mS cm-1, with a mean of 26.4 mS cm-1. The mayfly (Nousia. sp. AV7, 12.6 mS cm-1) and the gastropods (Glytophysa sp., 14.5 mS cm-1 and P. acuta, 16.7 mS cm-1) were the most salt-sensitive, followed by the stoneflies (D. serricauda, 18.3 mS cm-1 and Eusthenia spp., 36.7 mS cm-1), and then the macrocrustaceans (A. australis, 41.3 mS cm-1 and P. australiensis, 44.9 mS cm-1). Assessment of the LC50 values for each taxon in combination with their known field distribution in south-eastern Australia indicates that small increases in salinity are likely to have adverse affects on the viability of populations of Nousia sp. AV7, Eusthenia spp., D. serricauda, P. acuta and Glytophysa sp. Populations of these taxa in small headwater streams within the Tasmanian NAP region are particularly at risk. The acute lethal tolerances of the individual taxa were broadly consistent with that of members of the same family or genus tested in the Barwon River catchment, south-west Victoria. It is likely that differences in genetic structure between the 'same species' in the two studies account for discrepancies in acute tolerance at the species level (i.e. P. australiensis and P. acuta). However, variation in salinity tolerance between populations may be reflected in long term and sublethal responses.The acclimation regime implemented in this study did not result in an increase in acute salinity tolerance for Nousia sp. AV7. However an alternative regime incorporating higher conductivities, at least equivalent to the LC25 for Nousia sp. AV7, may result in an enhanced acute lethal response (LC50). It is vital that future field-based and laboratory-based research be undertaken to quantify the capacity of macroinvertebrates, both at the individual and population level, to adapt to changes in salinity regime, including increases over time, and fluctuations in salinity of different frequencies, magnitudes and duration. Acute salinity toxicity testing provides useful information on the osmoregulatory, behavioural and acclimatisation capacity of individual taxa to increasing salt concentrations, and as such provides a crude framework for predicting responses of taxa in the field. However, acute toxicity tests in isolation do not represent the complex interrelationships between natural communities and ecosystems. Thus they are often not relevant to, or useful, in predicting the field distributions of individual taxa, or salinities at which populations are sustainable (e.g. Eusthenia spp.). A combination of laboratory-based (including sensitive life stages, sublethal and long term effects) and field-based research (assessing patterns of salinity and water quality in association with community structure) is required. In addition the most sensitive of aquatic biota (e.g. microbes, algae and microinvertebrates) and a broad range of taxonomic and functional groups must be tested. It is recommended that future research also assess the spatial variation in salinity tolerance between populations of the 'same species' and consider taxonomic differences and/or population genetics. This is essential for the development of appropriate regional salinity guidelines. Finally, long term and intensive monitoring of rivers and wetlands within the NAP region, particularly those aquatic systems rated as high risk is required, so that future planning and land management decisions can be appropriately informed.
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