Robyn Watts

Estuaries, lagoons and enclosed embayments: habitats that enhance population subdivision of inshore fishes

Several studies have suggested that estuaries, lagoons and enclosed embayments may offer special opportunities for local subdivision in marine species. We used data from published papers and unpublished theses to examine the effect of such water bodies on allozyme differentiation of seven species of inshore fishes in Western Australia. We included species that differ in their dispersal, and hence their intrinsic potential for gene flow. Over large distances, subdivision was generally greater among estuarine populations than among conspecific marine populations collected over similar distances. Over small distances, paired marine and estuarine samples were generally more divergent than pairs of marine samples separated by similar distances. For species with a low capability for dispersal, estuaries appear to add to the high levels of genetic subdivision that commonly result from other factors. Under special circumstances estuaries may also provide opportunities for genetic divergence in species with a high capability of dispersal that are rarely subdivided at a large geographical scale. These observations indicate that estuaries can increase the genetic subdivision of populations of inshore fishes, and that species that use both marine and estuarine habitats are likely to have greater genetic subdivision than those that are restricted to marine habitats.

Dam reoperation in an era of climate change

Climate change is predicted to affect the future supply and demand for water resources. Current water-management practices may not adequately cope with the impacts of climate change on the reliability of water supply, flood risk, health, agriculture, energy generation and aquatic ecosystems. Water managers can adapt to climate variability by structural change, such as increasing the size or number of dams, building desalination plants and transferring water between catchments; however, a broader set of alternatives with multiple beneficial outcomes for society and the environment should be explored. We discuss how modifying dam operations, ‘dam reoperation’, can assist with adaptation to climate change and help restore ecosystems. The main operating purpose of a dam (e.g. flood management, hydropower or water supply) will influence dam reoperation strategies. Reoperation may require integration across sectors or involve multiple dams, enhancing benefits such as water supply or hydropower while simultaneously achieving ecosystem restoration. We provide examples of lessons learned during extreme scenarios (e.g. floods and droughts), where operational flexibility has been demonstrated. We contrast structural climate-change adaptation strategies (e.g. building new dams) and their resulting detrimental environmental outcomes with dam reoperation, which can maximise benefits for ecosystems and society.

When trends intersect: The challenge of protecting freshwater ecosystems under multiple land use and hydrological intensification scenarios

Intensification of the use of natural resources is a world-wide trend driven by the increasing demand for water, food, fibre, minerals and energy. These demands are the result of a rising world population, increasing wealth and greater global focus on economic growth. Land use intensification, together with climate change, is also driving intensification of the global hydrological cycle. Both processes will have major socio-economic and ecological implications for global water availability. In this paper we focus on the implications of land use intensification for the conservation and management of freshwater ecosystems using Australia as an example. We consider this in the light of intensification of the hydrologic cycle due to climate change, and associated hydrological scenarios that include the occurrence of more intense hydrological events (extreme storms, larger floods and longer droughts). We highlight the importance of managing water quality, the value of providing environmental flows within a watershed framework and the critical role that innovative science and adaptive management must play in developing proactive and robust responses to intensification. We also suggest research priorities to support improved systemic governance, including adaptation planning and management to maximise freshwater biodiversity outcomes while supporting the socio-economic objectives driving land use intensification. Further research priorities include: i) determining the relative contributions of surface water and groundwater in supporting freshwater ecosystems; ii) identifying and protecting freshwater biodiversity hotspots and refugia; iii) improving our capacity to model hydro-ecological relationships and predict ecological outcomes from land use intensification and climate change; iv) developing an understanding of long term ecosystem behaviour; and v) exploring systemic approaches to enhancing governance systems, including planning and management systems affecting freshwater outcomes. A major policy challenge will be the integration of land and water management, which increasingly are being considered within different policy frameworks.

Effects of different management regimes on aquatic macroinvertebrate diversity in Australian rice fields

The maintenance of invertebrate diversity within agricultural environments can enhance a number of agronomically important processes, such as nutrient cycling and biological pest control. However, few Australian studies have been undertaken which specifically address the effects of commercial management regimes on rice field biodiversity. In this study, we compared aquatic macroinvertebrate communities within Australian rice fields cultivated under three commercial management regimes: conventional-aerial (agrochemicals applied, aerially sown), conventional-drill (agrochemicals applied, directly drill-sown) and organic-drill (agrochemical-free, directly drill-sown). These comparisons were undertaken using a combination of community assessment approaches, including morphospecies richness, abundance, diversity and community composition. In general, greater biodiversity existed within macroinvertebrate communities that developed under organic management regimes than under conventional regimes (i.e., higher morphospecies richness and Shannon diversity). Although there were significant differences in several parameters across management regimes early in the rice-growing season, as the growing season progressed the invertebrate communities that developed in the different management regimes became more similar. Only community composition analyses showed significant differences late in the growing season, with functional differences across aquatic faunal assemblages suggested by increased predator abundance in communities sampled from the organic management regime. In order to improve biodiversity within these aquatic environments, management techniques need to be examined individually and the most disruptive processes identified. Alternative management procedures can then be developed that minimise biodiversity loss whilst still delivering required agronomic outcomes.

Movement patterns of southern bell frogs (Litoria raniformis) in response to flooding

Within the semiarid regions of New South Wales, Australia, the endangered southern bell frog (Litoria raniformis) occupies a landscape that is characterised by unpredictable rainfall and periodic flooding. Limited knowledge of the movement and habitat-occupancy patterns of this species in response to flood events has hampered conservation efforts. We used radio-tracking to assess changes in movement patterns and habitat occupancy of L. raniformis (n = 40) over three different periods (November, January and April/May) that coincided with the flooding, full capacity and subsequent drying of waterbodies within an irrigation landscape. We assessed (1) the use of permanent and ephemeral habitats in response to flooding and drying and (2) distances moved, turning angles and dispersion of frogs during wetland flooding, full capacity and drying. Individuals remained in permanent waterbodies in November but had abandoned these areas in favour of flooded ephemeral waterbodies by January. As the ephemeral waterbodies dried, radio-tracked individuals moved back into permanent waterbodies. The movement patterns of radio-tracked individuals were significantly different in the three radio-tracking periods, but did not differ significantly between sexes. Individuals moved significantly greater distances over 24 h, in straighter lines and movements were more dispersed while they occupied ephemeral waterbodies during January than when they occupied permanent waterbodies during November and April/May. Local weather conditions did not influence movement patterns when all three tracking periods were modelled together using a single linear stepwise regression. The dynamic distribution of habitat patches over space and time, combined with changing patterns of resource utilisation and movement of L. raniformis, highlights the importance of incorporating both permanent and ephemeral habitat patches into conservation plans. Reductions in flood frequency and extent of ephemeral wetlands due to modified flooding regimes have the capacity to limit dispersal of this species, even when permanent waterbodies remain unchanged.

High levels of genetic subdivision in peripherally isolated populations of the atherinid fish Craterocephalus capreoli in the Houtman Abrolhos Islands, Western Australia

The atherinid fish Craterocephalus capreoli Rendahl is abundant in the Houtman Abrolhos Islands, 70 km off the Western Australia coast and ≃250 km south of the southern limit of the range of the species along the mainland. Electrophoretic examination of 7 allozyme loci at 17 sites in the Houtman Abrolhos revealed a substantially lower level of polymorphism than found in an earlier study of the species in its mainland distribution, with many of the uncommon alleles and some common ones missing. There is a very high degree of genetic subdivision among the populations in the Houtman Abrolhos, measured by a mean FST of 0.437 over a distance of 35 km. This FST (standardized variance in allelic frequencies) is six times that found previously among populations along the mainland coast over distances up to 850 km. The subdivision of populations in the Houtman Abrolhos is similar within one island group on a scale up to 12 km, and between two groups that are separated by 15 km of deep water. Significant differences in allelic frequencies were found between populations from the open shore and enclosed lagoons less than 800 m apart, but the overall levels of subdivision were similar for the two types of environment. Previous work had shown high levels of genetic subdivision in the Houtman Abrolhos for a gastropod with direct development. The results for C. capreoli demonstrate that the archipelago favours subdivision even for a species with potentially much greater mobility and different life history.

Can flow velocity regulate epixylic biofilm structure in a regulated floodplain river

Scour is one of the most important regulators of biofilm structure and function, especially in floodplain rivers where low gradients and flood frequencies limit potential for scouring. In this study, we experimentally examined the effects of flow velocity on the biomass and taxonomic composition of epixylic biofilms from floodplain reaches of the Murrumbidgee River, south-eastern Australia. Six blocks from each combination of colonisation period (30 or 70 days) and condition (wet or dried), were individually exposed to no velocity (control), or velocities of 0.3 m s-1 (low), 0.55 m s-1 (intermediate), or 1 m s-1 (high) in a laboratory flume. Biofilms exposed to all the experimental velocities had significantly lower dry mass (F3,94; P < 0.001), ash-free dry mass (F3,94; P < 0.001) and chlorophyll a (F3,94; P < 0.001) than the control. Losses of ash-free dry mass (F1,94; P < 0.05) and chlorophyll a (F1,94; P < 0.001) were significantly higher from wet biofilms exposed to each velocity than from dried biofilms. All velocities resulted in a substantial reduction in taxonomic richness among all treatments, with filamentous chlorophytes completely removed by velocities of 0.55 m s-1. These results indicate the potential to delineate thresholds for the response of biofilm biomass and algal taxa to flow velocity in floodplain rivers based on knowledge of antecedent conditions regulating biofilm development. This information significantly improves our understanding of the potential for ecological change using environmental flow releases in low-gradient floodplain rivers.

Using river-scale experiments to inform variable releases from large dams: a case study of emergent adaptive management

Case studies of successful adaptive management generally focus on examples that have frameworks for adaptive management embedded from project conception. In contrast, this paper outlines an example of emergent adaptive management. We describe an approach whereby targeted research and collaboration among stakeholders assisted learning, and ultimately the development of interim operational guidelines for increased within-channel flow variability in the highly regulated Mitta Mitta River, which is managed as part of the River Murray System in the Murray–Darling Basin, Australia. Environmental monitoring of four variable flow trials evaluated the response of water column microbial activity, benthic and water column metabolism, the structure and composition of algal biofilms, and benthic macroinvertebrates to increased flow variability created by varying the release from Dartmouth Reservoir. Each trial built upon lessons from previous trials, with collaboration among key stakeholders occurring before, during and after each trial. Institutional conditions encouraged a shift to adaptive management over time that helped to achieve environmental, social and economic objectives downstream of the dam. A key lesson is that adaptive management does not have to be specified a priori, but can emerge within a trusting relationship between stakeholders as long as they are willing and able to change their operational paradigm.

Hypoxia, Blackwater and Fish Kills: Experimental Lethal Oxygen Thresholds in Juvenile Predatory Lowland River Fishes

Hypoxia represents a growing threat to biodiversity in freshwater ecosystems. Here, aquatic surface respiration (ASR) and oxygen thresholds required for survival in freshwater and simulated blackwater are evaluated for four lowland river fishes native to the Murray-Darling Basin (MDB), Australia. Juvenile stages of predatory species including golden perch Macquaria ambigua, silver perch Bidyanus bidyanus, Murray cod Maccullochella peelii, and eel-tailed catfish Tandanus tandanus were exposed to experimental conditions of nitrogen-induced hypoxia in freshwater and hypoxic blackwater simulations using dried river red gum Eucalyptus camaldulensis leaf litter. Australias largest freshwater fish, M. peelii, was the most sensitive to hypoxia but given that we evaluated tolerances of juveniles (0.99±0.04 g; mean mass ±SE), the low tolerance of this species could not be attributed to its large maximum attainable body mass (>100,000 g). Concentrations of dissolved oxygen causing 50% mortality (LC50) in freshwater ranged from 0.25±0.06 mg l−1 in T. tandanus to 1.58±0.01 mg l−1 in M. peelii over 48 h at 25–26°C. Logistic models predicted that first mortalities may start at oxygen concentrations ranging from 2.4 mg l−1 to 3.1 mg l−1 in T. tandanus and M. peelii respectively within blackwater simulations. Aquatic surface respiration preceded mortality and this behaviour is documented here for the first time in juveniles of all four species. Despite the natural occurrence of hypoxia and blackwater events in lowland rivers of the MDB, juvenile stages of these large-bodied predators are vulnerable to mortality induced by low oxygen concentration and water chemistry changes associated with the decomposition of organic material. Given the extent of natural flow regime alteration and climate change predictions of rising temperatures and more severe drought and flooding, acute episodes of hypoxia may represent an underappreciated risk to riverine fish communities.

Stable isotope analysis of aquatic invertebrate communities in irrigated rice fields cultivated under different management regimes

In this study we have used stable isotope analysis to identify major food resources driving food webs in commercial rice agroecosystems and to examine the effects of agricultural management practices on the trophic structure of these food webs. Potential carbon sources and aquatic macroinvertebrate consumers were collected from large-scale rice farms in south-eastern Australia cultivated under three different crop management regimes conventional-aerial (agrochemicals applied, aerially sown), conventional-sod (agrochemicals applied, directly sown) and organic-sod (agrochemical-free, directly sown). Evidence from stable isotope analysis demonstrated the importance of food sources, such as biofilm and detritus, as the principal energy sources driving aquatic food webs in rice agroecosystems. Despite the greater diversity of potential food sources collected from the organic-sod regime across all sampling occasions, the range of food resources directly assimilated by macroinvertebrate consumers did not differ substantially across management regimes. Trophic complexity of aquatic food webs, as evidenced by the number of trophic levels identified using δ15N data, differed across management regimes at the early season sampling. Sites with low or no agrochemical applications contained more than two trophic levels, but at the site with the highest pesticide application no primary or secondary consumers were found. Our data demonstrates that the choice of agricultural management regime has a season-long influence on aquatic food webs in rice crops, and highlights the importance of conserving non-rice food resources that drive these trophic networks.