Estuaries In Southeastern Australia Research Articles

Long-term drivers of catch variability in south-eastern Australia’s largest portunid fishery

Portunid crab fisheries are socially and economically valuable yet are characterised by high inter-annual variability. Competing hypotheses about factors contributing to this variability concern the environment, climate, and fisher behaviour. Here, we investigate variability in commercial catch, through a case study of the most heavily exploited estuary in southeastern Australia (Wallis Lake). Four main hypotheses were identified based on the broader portunid published literature: 1) Freshwater flow may cause poor recruitment due to increased offshore spawning and unfavourable dispersal of larvae, 2) Winter and spring harvesting may impact the following summer harvest by removing the spawning stock biomass (recruit overfishing); 3) Environmental factors including sea surface temperature and onshore winds may influence supply-side processes and recruitment success, contributing to catch fluctuations; and 4) Climatic indices such as the Inter Pacific Oscillation and Pacific Decadal Oscillation (PDO) may correlate with longer-term fluctuations in the fishery. We find evidence in support of hypotheses 2 and 4, with winter harvest negatively impacting the following January - April harvest and the PDO being positively correlated with January - April catch rates. Harvesting in June – November disproportionally impacts large females, and it may be possible to incorporate controls on winter fishing effort through co-management arrangements to achieve the best economic, environmental and social outcomes from the fishery. Such controls may lead to improved summer catch rates by protecting unspawned eggs during the winter months. The hypotheses explored here may provide insight in the variability observed in portunid fisheries around the world.

Mapping the development of a Dinophysis bloom in a shellfish aquaculture area using a novel molecular qPCR assay

Diarrhetic shellfish toxins produced by certain species of the marine dinoflagellate Dinophysis can accumulate in shellfish in high concentrations, representing a significant food safety issue worldwide. This risk is routinely managed by monitoring programs in shellfish producing areas, however the methods used to detect these harmful marine microbes are not usually automated nor conducted onsite, and are often expensive and require specialized expertise. Here we designed a quantitative real-time polymerase chain reaction (qPCR) assay based on the ITS-5.8S ribosomal region of Dinophysis spp. and evaluated its specificity, efficiency, and sensitivity to detect species belonging to this genus. We designed and tested twenty sets of primers pairs using three species of Dinophysis - D. caudata, D. fortii and D. acuminata. We optimized a qPCR assay using the primer pair that sufficiently amplified each of the target species (Dacu_11F/Dacu_11R), and tested this assay for cross-reactivity with other dinoflagellates and diatoms in the laboratory (11 species) and in silico 8 species (15 strains) of Dinophysis, 3 species of Ornithocercus and 2 species of Phalacroma. The qPCR assay returned efficiencies of 92.4% for D. caudata, 91.3% for D fortii, and 91.5% for D. acuminata, while showing no cross-reactivity with other phytoplankton taxa. Finally, we applied this assay to a D. acuminata bloom which occurred in an oyster producing estuary in south eastern Australia, and compared cell numbers inferred by qPCR to those determined by microscopy counts (max abund. ∼6.3 × 103 and 5.3 × 103 cells L−1 respectively). Novel molecular tools such as qPCR have the potential to be used on-farm, be automated, and provide an early warning for the management of harmful algal blooms.

Assessing the validity and sensitivity of microbial processes within a hydrodynamic model

Eutrophication due to excess anthropogenic nutrients in waterways is a significant issue worldwide. The pressure-stressor-response of a waterway to excessive nutrient loading is reliant on numerous physical and biological factors, including hydrodynamics and microbial processing. While substantial progress has been made towards simulating these mechanisms there are limited multi-disciplinary studies that relate the physical hydrodynamics of a site with the ecological response from linked laboratory and field studies. This paper presents the development of a coupled hydrodynamic and aquatic ecosystem response model, expanded to include an integrated microbial loop, that allows the explicit representation of heterotrophic bacteria growth and dissolved organic nutrient mineralisation. A unique long-term water quality dataset at an estuary in south-eastern Australia was used to validate and assess the model's sensitivity to complex biophysical processes driving the observed water quality variability. Results indicate that explicit time-varying bacterial mineralisation rates provide a substantially improved understanding of the broader aquatic ecosystem response than assigned fixed bulk rate parameter values, which are typically derived from non-local literature. Implementation of a microbial loop at the study site indicated that the model is sensitive to the boundary conditions, in particular catchment loads, with both net transport rates and the net growth rates of heterotrophic bacteria demonstrating different responses. Under average flow conditions, a smaller net transport and reduced nutrient availability has a pronounced effect of lowering net growth rates through the applied limitation factors. During high flow conditions, freshwater inflows increased net transport and nutrient loads, which resulted in higher net growth rates. Further, temporal variability in water temperature had a compounding effect on the model's response sensitivity. This approach has broader application in other riverine systems subject to eutrophication, and in interrogating linkages in hydrodynamic and microbial mediated processes (e.g., productivity). Future studies are recommended to better understand the sensitivity of aquatic ecosystem response models to microbial net growth rate kinetics at different temperatures and from top-down predation (e.g., zooplankton grazing).

The geomorphological evolution of a saline coastal lagoon on the southeast coast of Australia: Swan Lake, New South Wales

Coastal lagoons and estuaries in southeastern Australia mainly occupy lowstand incised valley systems that have been impounded by post-glacial sea-level rise. Most previous research has focused on larger estuarine and deltaic systems but the Swan Lake deposits record the geomorphic evolution of a small perched saline coastal lagoon that has remained in a youthful stage of estuary infilling with very slow progradation of the bayhead delta and slow accumulation of the central basin mud facies. This can be attributed to the relatively small catchment that is covered with natural vegetation. Marine influences at the flood-tide delta have been restricted by the perched nature of the lagoon and the high wave energy on the coast that rapidly closes the entrance with sand. Comparison with other similar lagoons along the NSW coast between Sydney and Batemans Bay illustrates the general geomorphic evolution of saline coastal lagoons. Such lagoons are also very similar to small perched lagoons in South Africa and to a lesser extent with those on the east coast of South America.

Seagrass on the brink: Decline of threatened seagrass Posidonia australis continues following protection.

Seagrasses are in decline globally due to sustained pressure from coastal development, water quality declines and the ongoing threat from climate change. The result of this decline has been a change in coastal productivity, a reduction in critical fisheries habitat and increased erosion. Attempts to slow this decline have included legislative protection of habitat and direct restoration efforts. Monitoring the success of these approaches requires tracking changes in the abundance of seagrasses, but such monitoring is frequently conducted at either too coarse a spatial scale, or too infrequently to adequately detect changes within individual meadows. Here, we used high resolution aerial imagery to quantify the change in meadows dominated by Posidonia australis over five years at 14 sites in five estuaries in south-eastern Australia. Australia has some of the world's most diverse and extensive seagrass meadows, but the widely distributed P. australis has a slow growth rate, recovers poorly after disturbance, and suffers runaway attrition if the conditions for recovery are not met. In 2010, after declines of 12–57% between the 1940s and 1980s, P. australis was listed as a threatened ecological community in New South Wales. We quantified changes in area at fine spatial scales and, where loss was observed, describe the general patterns of temporal decline within each meadow. Our results demonstrate that seagrass meadows dominated by P. australis underwent declines of ~ 2–40% total area at 11 out of 14 study sites between 2009 and 2014. In the iconic Sydney Harbour, our analyses suggest that P. australis meadows are declining at an average rate greater than 10% yr-1, exceeding the global rate of seagrass decline. Highlighting these alarming declines across the study region should serve as means to prioritise management action and review the effectiveness of legislative listing as a method to limit impacts at an ecosystem level.

Effects of urbanisation on macroalgae and sessile invertebrates in southeast Australian estuaries

The influence of anthropogenic and environmental factors on the composition, cover and dominance of macroalgae and sessile invertebrates was assessed in three capital city estuaries in south-eastern Australia. Heavy metals and proximity to ports showed the strongest relationships to the distribution of sessile reef biota after accounting for natural environmental gradients. The densities of laminarian, fucoid, brown and red foliose algae were negatively correlated with heavy metals, both in Port Phillip Bay (Melbourne) and the Derwent (Hobart), while turf, filamentous algae and some invertebrates were favoured. Sydney Harbour possessed a different pattern, with the laminarian kelp Ecklonia radiata most abundant near the main shipping port, probably because of biotic interactions involving urchin grazing in the lower estuary. Identifying drivers of benthic community pattern represents a key challenge for effective conservation management, particularly for estuaries affected by multiple anthropogenic impacts.

Catchment land use predicts benthic vegetation in small estuaries.

Many estuaries are becoming increasingly eutrophic from human activities within their catchments. Nutrient loads often are used to assess risk of eutrophication to estuaries, but such data are expensive and time consuming to obtain. We compared the percent of fertilized land within a catchment, dissolved inorganic nitrogen loads, catchment to estuary area ratio and flushing time as predictors of the proportion of macroalgae to total vegetation within 14 estuaries in south-eastern Australia. The percent of fertilized land within the catchment was the best predictor of the proportion of macroalgae within the estuaries studied. There was a transition to a dominance of macroalgae once the proportion of fertilized land in the catchment exceeded 24%, highlighting the sensitivity of estuaries to catchment land use.

Geoinformatics vulnerability predictions of coastal ecosystems to sea-level rise in southeastern Australia

ABSTRACTCoastlines are dynamic environments, with their Eco-geomorphology controlled by a complex range of natural and anthropic processes. Estuarine environments and associated wetland ecosystems are a critical shoreline types with regards to biodiversity, and are particularly susceptible to the influence of sea-level rise.This project applied future sea-level rise of Intergovernmental Panel on Climate Change (IPCC) hydro-scenarios to assess its impact on the eco-geomorphic aspects of coastal ecosystems in terms of risk assessment and sustainability. Comerong Island is used as a case study and is compared with other surrounding ocean-influenced and lagoonal deltas to assess the regional effects of sea-level rise. Applying the IPCC scenarios to the chosen geomorphic coastal data-sets resulted in a hydro-geomorphic model that shows the study site was already under pressure in 2015, with significant land area projected to be lost by 2050 and 2100. These findings are also expected to occur across the remaining estuaries in southeastern Australia. Applying this broad-scale, multi-strand application of geoinformatics simulation (GIS & RS), together with the various IPCC sea-level rise scenarios, will be necessary to assess future ecosystem sustainability management plans for coastal zones worldwide.

Drivers of sulfide intrusion in Zostera muelleri in a moderately affected estuary in south-eastern Australia

The seagrass Zostera muelleri Irmisch ex Asch. is abundant in estuaries in Australia and is under pressure from coastal developments. We studied sulfide intrusion in Z. muelleri along a gradient of anthropogenic impact at five stations in the Wallis Lake estuary, Australia. Results showed differences in sediment biogeochemical conditions, seagrass metrics as well as nutrient content and sulfide intrusion along the gradient from the lower estuary (affected) to the lagoon (unaffected). Sulfide intrusion was driven by complex interactions and related to changes in seagrass morphology and sediment biogeochemistry and was modified by the exposure to wind and wave action. The sediments in the lower estuary had high contributions from phytoplanktonic detritus, whereas the organic pools in the lagoon were dominated by seagrass detritus. Despite high concentrations of organic matter, sulfide intrusion was lower at stations dominated by seagrass detritus, probably because of lower sulfide pressure from the less labile nature of organic matter. Porewater diffusive gradients in thin-film (DGT) sulfide samplers showed efficient sulfide reoxidation in the rhizosphere, with high sulfur incorporation in the plants from sedimentary sulfides being likely due to sulfate uptake from reoxidised sulfide. This is a unique adaptation of Z. muelleri, which allows high productivity in estuarine sediments.

Large wood in the Snowy River estuary, Australia

In this paper we report on 8years of data collection and interpretation of large wood in the Snowy River estuary in southeastern Australia, providing quantitative data on the amount, sources, transport, decay, and geomorphic actions. No prior census data for an estuary is known to the authors despite their environmental and economic importance and the significant differences between a fluvial channel and an estuarine channel. Southeastern Australian estuaries contain a significant quantity of large wood that is derived from many sources, including river flood flows, local bank erosion, and anthropogenic sources. Wind and tide are shown to be as important as river flow in transporting and stranding large wood. Tidal action facilitates trapping of large wood on intertidal bars and shoals; but channels are wider and generally deeper, so log jams are less likely than in rivers. Estuarine large wood contributes to localised scour and accretion and hence to the modification of estuarine habitat, but in the study area it did not have large-scale impacts on the hydraulic gradients nor the geomorphology.

An ecological risk assessment for managing and predicting trophic shifts in estuarine ecosystems using a Bayesian network

Estuaries are dynamic systems at the transition between freshwater and marine ecosystems. In this study, a spatially and temporally explicit Bayesian network (BN) was developed for a tidally connected estuary in southeastern Australia. The BN provides an environmental risk assessment (ERA) for the probability of a shift to a eutrophied state based on markers of pelagic and benthic primary production. The model was created to provide an initial framework of system knowledge based on empirical data, with the intention that the model and its linkages be iteratively developed as more information becomes available. The BN was investigated for its potential to predict trophic shifts and provide a framework for evidence-based decision making. Model assessment was conducted through both sensitivity analysis and scenario tests. Through evaluation and updating, the BN can provide information on the key nutrients and bio-physical mechanisms regulating changes in trophic state in estuarine ecosystems.

Predicting the buffering of acid plumes within estuaries

The acid buffering capacity of an estuary is directly proportional to the volume of buffering agents within the system. In areas with limited upstream inflows of buffering agents, the primary buffering agents are sourced from the diffusion of marine constituents. During dry periods the buffering capacity increases with tidal diffusion, whereas buffering decreases following estuarine flushing from high inflows. In acid sulphate soil environments, acidic discharges are generated immediately following rainfall events. Thereby, the diffusion of buffering agents throughout an estuary plays an important role in sustaining biogeochemical processes. Prediction of the transport and buffering dynamics of acidic plumes within estuaries allows severely impacted acid areas to be identified and catchment management strategies developed.To simulate acid transport and tidal buffering processes within an estuary, a numerical model was modified and an acidic buffering module included within the hydrodynamic routine. An estuarine wide case study was then undertaken to demonstrate the effectiveness of the acid plume module in simulating onsite conditions and the role of buffering versus dilution. Acid discharged from the study domain, located on a tidal barrier estuary in south-eastern Australia, was monitored using fixed and boat mounted instrumentation to track far-field plume behaviour. The acidic plume monitoring results were converted to hydrogen proton concentrations and included within the water quality module to simulate acid buffering with bicarbonate ions diffusing from the marine boundary.The validated model can be applied to single or multi-drain estuary networks to assess possible management options for reducing the impact of acid plume events. This enables the most effective management approach to be identified and assists in prioritising acid sulphate soil sites for remediation. This is particularly useful to natural resource managers in order to assess remediation sites that have the most environmental impact, or can be most effectively buffered, via model scenario tests.

Molecular evidence supports coastal dispersal among estuaries for two benthic marine worm (Nephtyidae) species in southeastern Australia

Understanding patterns of dispersal of marine organisms among estuaries is important for the conservation of biodiversity and the design of marine park networks. Whereas numerous studies have recently assessed dispersal potential among key marine vertebrates and habitat-forming macroalgae, relatively few have assessed the potential for dispersal in ecologically important benthic polychaete worms. Here, we used phylogeographic analyses to test for evidence of genetic disjunctions among populations of polychaete worms from different estuaries in southeastern Australia. Our study focused on two species from the family Nephtyidae (Aglaophamus australiensis and Nephtys longipes) that are found intertidally in soft sediments in estuaries. Both species have planktonic larvae, but little is known about the survival times of the larvae, or their potential to disperse to other estuaries rather than settling locally. Genetic analyses of two mitochondrial (cytochrome c oxidase subunit I and 16S rDNA) markers in both species and a nuclear marker (28S rDNA) in A. australiensis were carried out to assess whether geographically distinct populations show genetic differences. Little evidence of genetic differentiation among populations was found, despite a high level of genetic diversity within each species. Although some significant population pairwise FST differences were detected for both species via AMOVA, these appeared largely driven by singleton haplotype diversity, whereas several common haplotypes were shared among all populations. Our results suggest that sedentary, benthic estuarine organisms with planktonic larvae can disperse to distant estuaries with the aid of tidal flushing and coastal ocean currents.

Sponges as sentinels: Metal accumulation using transplanted sponges across a metal gradient.

To be effective sentinels, organisms must be able to be readily translocated to contamination hotspots. The authors sought to assess metal accumulation in genetically identical explants of a relatively common estuarine sponge, Suberites cf. diversicolor. Explants were transplanted to 7 locations across a metal contamination gradient in a large coastal estuary in southeastern Australia to establish, first, that explants of this species could be successfully translocated; second, that explants accumulated metals (cadmium, copper, lead, selenium, and zinc) sufficiently rapidly to be effective sentinels; third, that rates of metal accumulation in explants were in agreement with metal concentrations within sediments (<63-µm fraction) at each of the transplant locations; and finally, that changes in explant biomass correlated with overall metal load. Suberites were readily transplanted, with no mortality observed for the 2 mo of transplantation. Metal accumulation for lead, cadmium, and zinc was in close agreement with sediment metal concentrations, and explants showed dramatic increases in these metals in the heavily contaminated northern sections of the estuarine lake. No striking patterns were apparent for copper and selenium. Finally, growth was negatively correlated with total metal load and standardized total metal load in our explants. Taken together, these outcomes confirm that explants of this sponge are amenable to translocation and show considerable promise as biomonitors.

Use of a high resolution 3D fully coupled hydrodynamic, sediment and biogeochemical model to understand estuarine nutrient dynamics under various water quality scenarios

We use a high resolution, three dimensional, calibrated and coupled hydrodynamic, sediment and biogeochemical model to demonstrate and quantify the nutrient dynamics of an estuary. Observations alone cannot be used to understand estuarine dynamics and the impact of various management scenarios. Calibrated models and model scenarios are useful to provide additional understanding of estuarine systems and can be used to explore and quantify alternative management scenarios. Using the biogeochemical model we identified several management scenarios to demonstrate a range of water quality impacts associated with variation in river flow, sewage and aquaculture nutrient loads and marine flushing. The model was used to inform strategic management of waterways and also gives insight into the biogeochemical and physical dynamics of a temperate salt wedge estuary in South Eastern Australia. A low river flow simulation with high anthropogenic nutrient loads resulted in extension of the estuarine salt wedge upstream due to lower river input. The higher levels of denitrification in this scenario helped to offset increased nutrient fluxes across the marine boundary and the increases in sewage treatment plant and industry loads. However the simulation suggested there would be higher concentrations of nutrients and chlorophyll in the surface layers and lower relative levels of dissolved oxygen in the bottom waters and sediment. The simulation with the lowest loads and with no anthropogenic input resulted in low nutrient concentrations indicating that under these conditions the nutrients are retained and recycled in the estuary. The use of our 3D model allowed greater detail of understanding of the estuary dynamics and loads which is essential for testing the realistic state of future management possibilities when running scenarios.

Detecting benthic community responses to pollution in estuaries: A field mesocosm approach

Biological stress responses in individuals are used as indicators of pollution in aquatic ecosystems, but detecting ecologically relevant responses in whole communities remains a challenge. We developed an experimental approach to detect the effects of pollution on estuarine communities using field-based mesocosms. Mesocosms containing defaunated sediments from four estuaries in southeastern Australia that varied in sediment contamination were transplanted and buried in sediments of the same four estuaries for six weeks. Mesocosm sediment properties and metal concentrations remained representative of their source locations. In each estuary, fauna communities associated with sediments derived from the site with the highest metal concentrations were significantly different from other communities. This pattern was evident for some of the individual taxa, in particular the polychaete Capitella sp. Consistent responses across estuaries suggest numbers of individuals, and especially Capitella sp., could be used to identify contaminated sediments in estuaries with similar fauna and site characteristics.

Fluctuations in natural and synthetic estrogen concentrations in a tidal estuary in south-eastern Australia

Estuaries are often the final repositories for aquatic pollutants but how estuarine hydrology influences the availability of marine- and freshwater-derived pollutants is not well understood, particularly for micro-pollutants such as endocrine disrupting chemicals. To address this knowledge gap, this study measured natural and synthetic estrogen concentrations within the Little River, a tidal estuary in close vicinity to the major discharge point of Melbourne's largest waste water treatment plant (WWTP), the Western Treatment Plant. Quantitative enzyme-linked immunosorbent assays (ELISA) were used to determine concentrations of natural estrogens (ES: ∑E1 (estrone), E2 (17β-estradiol), E3 (estriol)) and the synthetic estrogen, 17α-ethinylestradiol (EE2). The highest concentrations were measured in samples taken from the WWTP effluent discharge channel (29.0 ng/L and 0.35 ng/L, respectively). Within the estuary, concentrations of ES (2.25–23.16 ng/L) varied somewhat between locations and sampling periods (p < 0.05), however patterns were not consistent. Significant spatial variation was observed on only one sampling occasion, and likewise temporal variation was only observed once. In the upstream freshwaters, ES (2.95–7.26 ng/L) concentrations were lower than in the estuary, although their presence suggests an additional source of ES to the environment, most likely of agricultural origin. The EE2 concentrations measured in both the estuarine and freshwater areas were all low (mostly below 0.20 ng/L), which created difficulties in interpretation due to problems associated with trying to measure such low concentrations with confidence. However, some patterns did emerge, with EE2 concentrations exhibiting significant temporal and tidal variation (p < 0.05), with concentrations greatest during low and flooding (incoming) tides. Physico-chemical properties explained 30% of the variation in ES concentrations, whereby concentrations increased with decreasing pH and DO and increasing salinity. Given the higher concentrations observed during flooding tides and the association of higher estrogen concentrations with increased salinity and low DO, we suggest that estrogens might accumulate in estuarine bottom waters and upon disturbance from the incoming tidal flows, may be a contributing source of estrogens into the estuary. This study contributes the first comprehensive investigation of estrogen dynamics in an Australian estuary, and provides the foundation for further research aimed at identifying which compounds are present in estuarine waterways, where they are coming from and how their concentrations vary through space and time.

Comparison of baited remote underwater video (BRUV) and underwater visual census (UVC) for assessment of artificial reefs in estuaries

Fish communities associated with a series of artificial reefs deployed in three estuaries in southeastern Australia (151° 34′ E, 33° 7′ S to 150° 37′ E, 35° 8′ S) were surveyed using both Baited Remote Underwater Video (BRUV) and Underwater Visual Census (UVC). Abundance estimates, frequency of observations, and species indicators (richness and diversity) provided the basis for comparison between methods. UVC recorded significantly greater numbers of species in all estuaries and significantly greater species richness and diversity at two of the three estuaries. Variation in the number and frequency of species detected by each method directly related to the ecological niche and behaviour of each species. UVC provided better estimates of the rare or cryptic reef associated species. BRUV sampled a smaller proportion of species overall but did observe key recreational species such as Acanthopagrus australis, Pagrus auratus and Rhabdosargus sarba with increased frequency, although the presence of large numbers of schooling species such as Pelates sexlineatus reduced the detection frequency of these species. In summary, results indicate that BRUV is an effective method for recording species associated with artificial reefs with the exception of cryptic species that are located within the reef structure itself. BRUV techniques complement UVC by providing increased coverage of species known to be diver averse as well as providing important information regarding behaviour of the species identified. Given the limitation of each method, it is recommended that monitoring plans for artificial structures should adopt a multi-method approach utilising BRUV and UVC where possible.

APPLICATION OF A SIMPLE HYDRODYNAMIC MODEL TO ESTUARY ENTRANCE

Tidal inlets which link a tidal basin to the sea via a constricted entrance are common on the south-east Australian coast. Closure, or even significant constriction, raises water levels but restricts tidal range within the basin, while open entrances provide regular and significant tidal exchange with the ocean. A rapid assessment procedure with minimal data requirements has been shown to be informative for monitoring and a useful component of any Decision Support System set up as part of a management structure. Such a system is presented in this paper. It is based on one permanent water level gauge inside the inlet plus the use of a simple, first-order hydrodynamic model to relate the tide range, mean water level and river flow to the inlet cross sectional area. The method is tested against data from the&#x0D; Snowy River Estuary in south-eastern Australia but would be suitable over a range of estuaries. In addition, the framework presented can also provide a mechanism to explore conditions over the range of expected data, thus allowing better selection of model schematization and runs in estuarine systems where the use of 2 or 3D modeling can be justified.

Differences in soft-sediment macrobenthic assemblages invaded by Caulerpa taxifolia compared to uninvaded habitats

Caulerpa taxifolia is a habitat-forming green alga that has invaded several temperate regions worldwide. Although C. taxifolia covers large areas of soft-sediment habitat, little is known about its effects on soft-sediment invertebrate assemblages. We compared soft-sediment macroinver- tebrate assemblages in 2 estuaries in southeastern Australia invaded by C. taxifolia to examine 2 main predictions: (1) areas covered with C. taxifolia will have different assemblages compared to unvegetated sediment because infauna are inhibited but epifauna are facilitated, and (2) areas with C. taxifolia will have different assemblages compared to those with native seagrasses (Halophila ovalis and Zostera capricorni) because infauna are inhibited but epifauna are not. Multidimensional scaling and ANOSIM showed differences in invertebrate assemblages between all habitats. In C. tax- ifolia, infauna were less abundant and epifauna were more abundant compared to unvegetated sed- iment. However, when compared to native seagrasses, epifauna in C. taxifolia were more abundant than in H. ovalis in one estuary but less abundant than in Z. capricorni in another estuary, while infauna in C. taxifolia were less abundant than in both seagrass species. The consistently low infau- nal abundance in C. taxifolia, irrespective of infaunal feeding mode, suggests C. taxifolia impacts infauna generally. Examination of environmental factors potentially responsible for the low abun- dance of infauna indicated that differences in redox potential (and associated chemical changes) may explain patterns in abundance of infauna among habitats. Our findings indicate that invasion by C. taxifolia causes important changes to soft-sediment macroinvertebrate assemblages and suggest that infauna may be particularly vulnerable to invasion because of changes to sediment chemistry.