The hierarchical effects of anthropogenic impacts and natural filters on the distribution of freshwater mussels in the Hawkesbury-Nepean River, NSW, Australia

In comparison with North America, there is a poor level of taxonomic diversity in the Australian freshwater mussel fauna. Seventeen endemic hyriid species have been recorded in this country, while over 30 species have been recorded from one reach of a US river. While there have been no extinctions recorded among Australian mussel species, there are several species under threat, and mussel populations in many river systems are in serious decline. One of these catchments, the Hawkesbury-Nepean River, is considered to be among the most highly impacted of the eastern coastal rivers. Mussel populations in this river are believed to have declined in density and distribution, but to date this observation has been based on anecdotal, rather than scientific, evidence. This study aims to address the dearth of knowledge regarding population ecology for hyriid species by compiling a detailed assessment of mussel species, densities and sizes for a series of 100 sites throughout the greater Hawkesbury-Nepean River. Mussel populations were sampled concurrently with a range of physical habitat variables to identify natural and anthropogenic factors that may be important for defining the observed mussel assemblage structure. The results from these assessments were used to develop a hierarchical model that ranked the relative importance of these physical habitat variables in determining mussel distribution. An initial survey of sites in the Nepean River indicated that a high level of riparian disturbance was strongly related with decreased mussel density, and often with the apparent absence of mussels from many sites. Distribution of species and density of mussel assemblages were strongly associated with geomorphic reach type, so that different reaches supported different suites of mussel species. Presence of mussels was strongly associated with annual maximum dissolved oxygen, minimum total nitrogen, and minimum conductivity. Each species was associated with different annual patterns of water quality. Velesunio ambiguus were most frequently associated with maximum dissolved oxygen, and minimum conductivity and total phosphorus. In contrast, Hyridella depressa were not linked with dissolved oxygen patterns, but were negatively affected by turbidity and total phosphorus. Hyridella australis were associated with annual maximum dissolved oxygen, and not apparently affected by other aspects of water quality. Densities of mussels were compared with single samples for water quality at a second set of sites. Data from this study suggested that at higher densities, mussel populations impacted water quality, rather than water quality impacting mussel populations. The results indicated that at higher densities, mussels were able to favourably impact on water quality, reducing levels of phosphorus in the water column. Both small and large impoundments were seen to influence the composition of mussel assemblages. The presence of a large impoundment, mid catchment, was strongly associated with the absence of mussels upstream, and the presence of lower density populations. For small impoundments, weir height and geomorphic reach type were linked with differences in species distribution and densities. Velesunio ambiguus were present in greater densities in pools above weirs, while H. depressa and H. australis were present in higher numbers below weirs. Differences in distribution were also observed among different size classes of mussels. Small mussels were virtually absent above the large impoundment. Below the large impoundment, they were mostly found below small weirs located in less impacted sandstone gorge reaches. Within reaches, mussel distribution was most frequently related with coarse sand and the presence of a roughness element characteristic for the geomorphic reach type. Small mussels were associated with boulder stabilised habitats, and medium mussels with cobble based habitats. Large mussels were rarely associated with any type of roughness element. While each species showed some microhabitat preference, this was generally associated with the geomorphic reach type, rather than a particular aspect of substrate conditions. Within a hierarchy, catchment scale variables, including position in relation to a major impoundment, tended to dominate the delineation of mussel presence. Some aspects of water quality were associated with mussel density at a reach scale. The level of riparian disturbance, a reach scale factor, was strongly associated with mussel density. In general, factors associated with larger scales for the river were more influential in determining mussel presence and mussel density. However, the exact nature of these factors varied among species. When compared with northern hemisphere mussel fauna, Australian hyriid species were affected by a similar range of impacting factors. While the distribution of mussel species throughout the catchment was originally defined by natural filters, much of their present distribution patterns can be directly linked with anthropogenic changes within this highly impacted river system. Many of these anthropogenic impacts operate at catchment and reach scales, with the result that suitable habitats may not be colonised in some reaches of the river

An exploration of the relationships between macroinvertebrate community composition and physical and chemical habitat characteristics in farm dams

Recently, Australian interest in farm dams has focused on rates of harvest of surface waters (runoff), and the impact this has on nearby natural systems. Little research has been directed towards the role of these artificial water bodies in sustaining biological reserves within the wider ecosystem. Macroinvertebrate communities in three farm dams in the Central Tablelands of New South Wales were surveyed, and water quality variables were correlated with species richness and abundance. Community responses to habitat factors including sediment depth, stock use, vegetation and debris were also examined. Communities were described at several taxonomic levels in addition to allocation to trophic groups and primary functional feeding groups. Species richness and abundance of communities were found to vary between dams and between habitat types within dams. The extent of these differences was decreased when communities were described by either trophic status or functional feeding mechanisms. Habitats were influenced by water quality and by physical features of the habitat, with the two factors interacting to define equilibrium conditions. Localised conditions resulted in different macroinvertebrate communities. Physicochemical parameters that correlated most closely with communities included light penetration, chlorophyll-a and conductivity. Habitat factors that were most frequently linked with communities were sediment depth and canopy cover, with localised disturbances related to stock use affecting feeding groups rather than specific taxa. One of the major problems associated with increasing modification of landscapes by agriculture or urbanisation is the fragmentation of undisturbed habitats. Creation of joint aquatic and woodland habitats enhances biodiversity corridors. The recognition of the potential for farm dams as reservoirs of biodiversity and development of management practices that optimise this neglected biodiversity reserve may have much wider benefits biologically, aesthetically and productively

Temporal variations in water quality of farm dams : impacts of land use and water sources

Three farm dams near Raglan, New South Wales (Australia), were compared to investigate diurnal and seasonal patterns in water quality over a period of 1 year. The source of water in one dam was predominantly influenced by groundwater, another showed links with both groundwater and agricultural runoff, and the third collected runoff from urban and agricultural lands with no apparent groundwater input. Patterns in chemical profiles were compared to identify level of similarity in macro-trends of water quality. Within dams, micro-trends were contrasted with known chemical relationships in dam waters to explore links between land use and water quality. The three dams were shown to have quite distinct patterns of water chemistry. Within dams, trends were strongly linked with the differing water sources, evidenced by chemical patterns that matched those expected from the different dominant ion transfer pathways associated with surface water and groundwater flow processes. Phosphates were primarily linked with groundwater fluctuations, nitrogen as ammonium ions with urban runoff, and nitrates with storm events resulting in runoff from pastures. We conclude that, for farm dams, the combination of land use and preferential flow paths gives a more complete description of water quality impacts than land use alone

The role of geomorphology in substratum patch selection by freshwater mussels in the Hawkesbury-Nepean River (New South Wales) Australia

1. Microhabitat preferences of freshwater mussels and associated substrate characteristics were investigated across a range of geomorphic reaches in the Hawkesbury-Nepean River, Australia. 2. The structure of substratum patches available was strongly influenced by geomorphic reach type. In each reach type, mussel distribution was most frequently correlated with coarse sand and a roughness element characteristic for that reach. Roughness elements such as boulders and cobbles create a flow refuge and were linked with mussel size. 3. Small mussels tended to be associated with boulder-stabilized habitats and medium sized mussels with cobble habitats. Large mussels rarely co-occurred with any particular roughness element. Individual species were strongly linked to geomorphic reach type. This association may be due to species' differences in ability to colonize available microhabitat types. 4. The highly tolerant Velesunio ambiguus dominated shale reaches, characterized by fine sediments and human impacts. In contrast, Hyridella depressa dominated in gorges, utilizing small flow refuges among boulders, while H. australis were present in low densities across a range of substrate conditions. 5. The persistence of multispecies assemblages in mussel beds throughout the Hawkesbury-Nepean River implies similar niche utilization among species. Partitioning of habitats across species on the basis of size suggests some degree of habitat selection, or differential survival. At the local scale, microhabitat characteristics influenced the size distribution and densities of mussel assemblages. Continuing declines in mussel densities are likely to result from ongoing channel modification and increased siltation resulting from changes to riparian vegetation