Sorcha Cronin-O'Reilly

In some estuaries, low inflow and/or isolation from the ocean can result in evapoconcentration and hypersalinity (≥40 ppt). This can create osmoregulatory and energetic challenges for the faunal community, leading to reductions in diversity as more species pass their thresholds. As climate change is increasing the magnitude and duration of hypersaline conditions, we used benthic macroinvertebrate data from 12 estuaries across a Mediterranean climatic region (southwestern Australia) to assess the influence of salinity (0–122 ppt) on the invertebrate fauna. Taxa richness and diversity were highest in salinities between 0 and 39 ppt, peaking at salinities closest to seawater, while total density peaked at 40–49 ppt. Beyond 50 ppt, these measures declined significantly. Community composition changed markedly along the salinity gradient. In lower salinities, communities were diverse, comprising polychaetes, malacostracans, hexapods, ostracods, bivalves, and gastropods. However, in salinities ≥50 ppt, many taxa declined, leading to communities dominated by polychaetes (mainly Capitella spp.) and hexapods (mostly larval chironomids). At 90 ppt, only polychaetes and hexapods remained, and at ≥110 ppt, only the latter taxon persisted. This faunal shift towards insect dominance in hypersaline conditions mirrors observations in other Mediterranean and arid/semi-arid regions, with the resulting communities resembling saline wetlands or salt lakes. This loss of invertebrates can substantially impact ecosystem functioning and trophic pathways, and the findings of this study provide a basis for predicting how these communities will respond to increasing hypersalinity driven by climate change.

Coastal environments and their associated biota provide numerous environmental, economic and societal services. Cockburn Sound, a temperate embayment on the lower west coast of Western Australia, is immensely important for the State and adjacent capital city of Perth. However, urbanisation and associated terrestrial and marine development has the potential to threaten this important ecosystem. This study collated published and unpublished data to review the current state of the ecological resources of Cockburn Sound and describe how they have changed over the past century. Post-WWII, the embayment began undergoing pronounced anthropogenic changes that limited oceanic water exchange, increased nutrient load, modified benthic habitats and increased fishing pressure. The most visual outcome of these changes was substantial eutrophication and the loss of 77% of seagrass habitats. However, the increased primary productivity from elevated nutrient inputs produced high commercial fishery yields of up to ~1,700 t in the early 1990s before improved wastewater regulation and restricted fishing access steadily reduced commercial catches to ~300 t in recent years. Despite substantial anthropogenic-induced changes, Cockburn Sound has remained a diverse and ecologically important area. For example, the embayment is a key spawning area for large aggregations of Snapper, is a breeding and feeding site for seventeen marine bird species (including Little Penguins) and, is frequented by numerous protected species such as pinnipeds, dolphins, and White and Grey Nurse sharks. In recent decades, numerous projects have been initiated to restore parts of Cockburn Sound with mixed success, including seagrass transplantation, deployment of artificial reefs and stocking of key fish species, mainly Snapper. Nevertheless, while still biodiverse, there are signs of considerable ecological stress from escalating anthropogenic pressures and the cumulative impacts of ongoing and future developments, including climate change, which may severely impact the functioning of this important ecosystem.

Starting with the Remane diagram, various conceptual models have been proposed to show how species richness varies along a salinity gradient. However, as relatively few estuaries experience extreme hypersalinity, quantitative data are lacking to evaluate the model. We used data for 1891 samples of benthic macroinvertebrates from 12 estuaries in southwestern Australia (salinity 0–122 ppt) to determine the salinities in which 257 taxa were recorded. The pattern of richness differed from the conceptual models, with relatively few species (≤20%) recorded in freshwater and oligohaline salinities. Richness peaked at 35 ppt (seawater, 44%) before declining precipitously, with 21% and 10% of taxa recorded in hyperhaline salinities of 40 and 48 ppt, respectively. Taxa were recorded across the full salinity range, and several holohaline annelids, crustaceans, and insects were identified. Descriptive statistics and the frequency distribution of each taxon along the salinity gradient are provided. These identify stenohaline taxa and those with different extents of euryhalinity and how the occurrence of these taxa changes with salinity. The results help predict how benthic macroinvertebrate species and assemblages in estuaries in southwestern Australia and other Mediterranean climatic regions may shift due to climate change, particularly increased incidences and magnitude of hypersalinity.

Various types of tidal barriers are used in estuaries to reduce saltwater intrusion and regulate freshwater discharge, but they often alter the physicochemical environment and faunal composition. With the use of these structures expected to increase due to climate change, there is a need to understand their impacts. A tidal exclusion barrier in the Ramsar-listed Vasse–Wonnerup Estuary (Australia) was found to act as an ecotone, fragmenting the estuarine gradient into two distinct components, a relatively stable marine-like environment downstream and a highly variable oligohaline to hypersaline (~0 to >100 ppt) environment upstream. The downstream regions contained a speciose and functionally rich estuarine fauna, comprising mainly polychaetes and bivalves. The upstream regions were taxonomically and functionally depauperate, containing insects, gastropods, and ostracods typically found in saline wetlands. The fragmentation of the estuary has likely impacted the provision of ecosystem services, with the fauna downstream mainly comprising burrowing species that bioturbate and, thus, aid in nutrient cycling. In contrast, the environmental conditions caused by the barrier and the resultant epifaunal invertebrate assemblages upstream aid little in bioturbation, but provide nutrition for avian fauna. These results may help in understanding the impacts of constructing new barriers in coastal ecosystems in response to climate change.

Through altered freshwater flow regimes and excessive anthropogenic nutrient input, many estuaries around the world are showing signs of eutrophication. As shellfish can alleviate some of these issues through their water filtration capacity, shellfish habitat restoration efforts have increased markedly in the past decade. This study quantifies, for the first time, the water filtration capacity of the Black Pygmy Mussel Xenostrobus securis and the potential for habitat enhancement to alleviate eutrophication issues in a hypereutrophic estuary in south Western Australia. Substrate, comprising coir matting, was deployed by community volunteers in four‐panel arrangements in the rivers of the Swan‐Canning Estuary onto which X. securis recruited naturally. In the Swan River, average mussel densities were 3377 individuals m −2 , based on 10% mat coverage. River water comprised relatively high particulate organic matter (POM) concentrations, particularly in spring (up to 9.2 mg L −1 ). Standardised clearance rates (CR; g −1 mussel tissue) were typically greater (> 5.0 L h −1 ) in summer when chlorophyll a concentrations, salinities and water temperature were elevated, whereas CR was often < 2.0 L h −1 in early spring. In the Swan River, it was estimated that for every square metre of habitat enhanced, 9.2 × 10 5 L of water could be potentially cleared during spring and 1.7 × 10 6 L over summer, the latter incorporating 5.3 kg of organic matter into mussel biomass. On a larger scale, 1000 m 2 of deployed habitat over the course of summer has the potential to clear 24.5% of the volume of the tidal portion of the Swan River and 64.4% of the volume of the smaller Canning River. The results thus demonstrate the efficacy of using cost‐effective soft substrates deployed by community volunteers to enhance habitat for mussels and its potential to assist in alleviating eutrophication issues.

Due to human activity, ecosystems are exceeding their ecological thresholds and shifting into undesired alternative stable states with new ecological configurations. Despite their purported ubiquity, it is uncertain whether estuaries can exist in multiple stable states. We use data from a 3.5-year study of invertebrate communities in an Australian estuary that is usually closed to the ocean to test for their existence. Sampling spanned a 1.5-year period of hypersalinity (>40 ppt) during a prolonged estuary closure, where salinity reached 122 ppt, and for 2 years during and after the estuary opened to the ocean when salinities were mesohaline (5–19 ppt). Two distinct community states occurred before and after the sandbar breached, with an intermediary period of invertebrate community impoverishment due to sediment scouring. During the closure, the community was simple (average of 1 taxa) and dominated by larvae of terrestrial insects, most notably the halotolerant, non-biting midge Tanytarsus barbitarsis. After opening, the richness and abundance of invertebrates increased (average of 4 taxa and 84 individuals 100 cm−2) as polychaetes, molluscs and crustaceans colonised the estuary, although recovery was incomplete according to previous species records. Duration of closure and salinity were the strongest drivers of composition. This study, together with evidence from the literature, suggests a salinity threshold of 60–65 ppt between states. These empirical data meet key criteria of alternative states, i.e. a clear transition between two distinct self-sustaining communities, indicating a regime shift triggered by an exogenous event. Our findings suggest that temporarily open and closed estuaries can exist in alternative stable states, with prolonged closures, hypersalinity, and sandbar breaching being key determinants of the switch between states. This situation may apply to other low-inflow estuarine systems, particularly in arid, semi-arid, or seasonally arid climates, and may become more frequent due to human-induced climate change.
[Display omitted]

Anthropogenically driven alterations to coastal sediments and their benthic macroinvertebrate communities impair ecosystem function. However, this paradigm is yet to be tested in ecosystems that typically harbour underdeveloped communities lacking larger bioturbating species. Here, we investigated the effects of sediment condition and macroinvertebrate communities on benthic metabolism, nutrient exchange and denitrification (N2 production), and assessed the relative importance of taxon richness, abundance, biomass and community bioturbation potential in influencing these processes in 2 regions of the highly modified, microtidal Peel-Harvey Estuary in temperate Western Australia. Sediment condition influenced benthic metabolism more than the macroinvertebrate community, whereas the reverse was true for nutrient exchange. Denitrification was driven by sediment condition and the community in the upper and lower estuary, respectively, highlighting the change in controls of this nitrogen-removal process within estuaries. Overall, benthic macroinvertebrates had little to no effect on many ecosystem processes, exhibiting the limited functional role played by these chronically stressed biota in this estuary. There was also no interaction between sediment condition and the community, suggesting a functional decoupling between these 2 ecosystem components. Where significant macroinvertebrate effects were detected, community biomass was the most frequently selected predictor, demonstrating its fundamental role in ecosystem function. This study reveals pressing implications of what might be expected when benthic environments become particularly degraded and the highly limited potential of the resultant benthic macroinvertebrate communities to provide key ecosystem services such as nutrient processing.

Understanding the influence of macroinvertebrates on ecosystem function often relies on experimental defaunation with methods that remove fauna through minimal sample disturbance. Defaunation is challenging and can lead to confounding effects and/or loss of empirical information when unsuccessful. We evaluated the ability of a deoxygenation treatment to remove macroinvertebrates from sediment cores collected in 2 regions of a microtidal estuary. Only 1 of 16 cores was fully defaunated following 3 deoxygenation cycles. To counteract confounding effects of partial defaunation, we quantified the biomass remaining in each core and used these data as a covariate in statistical models. The unremoved biomass had, in some cases, significant effects on alkalinity fluxes, with positive linear relationships evident, and net phosphate fluxes. The community in the upper estuary that regularly experiences hypoxia exhibited stronger sediment emergence responses (82-100%). The remaining fauna were spread equally among annelids, molluscs and arthropods in abundance, although arthropods dominated the biomass. In contrast, fewer macroinvertebrates emerged from sediments from the lower estuary (47-89%), with most of the remaining biomass and abundance being annelids and molluscs. These findings suggest that estuarine taxa have divergent responses to hypoxia and that regional communities are variably prone to eradication of sensitive taxa. Our study shows how the use of defaunation by deoxygenation can create systematic bias, particularly when comparing areas with disparate in situ oxygen regimes, and provides a way to quantitively account for partial defaunation without sacrificing statistical power or using overly destructive methods.

Microtidal estuaries are prone to anthropogenic degradation, with natural features of those in south-western Australia making them more susceptible. However, the benthic ecological health of these systems is rarely assessed, despite the importance of the benthos and frequent application of benthic indices in estuaries elsewhere, particularly macrotidal systems in the northern hemisphere. The aim of this research was to assess the current status of the benthic macroinvertebrate community and its role in the function and management of the microtidal Peel-Harvey Estuary. After accounting for the effects of natural hydrological conditions (e.g. salinity, temperature), the benthic macroinvertebrate community was shown to respond to anthropogenic stress as represented by sediment condition (i.e. oxygenation, organic enrichment, mud content, sulphide presence), demonstrating its potential utility for assessing estuarine health. However, existing benthic indices commonly used in macrotidal estuaries (e.g. the multivariate AZTI Marine Biotic Index) yielded results inconsistent with sediment condition, demonstrating their limitations when applied to highly adaptive, stress-tolerant macroinvertebrate communities that are common in microtidal estuaries. A new multi-metric Estuarine Benthic Community Index was developed, following a multivariate approach to select community metrics that showed greater responses to sediment condition than natural stress. Overall, the benthic macroinvertebrate community in the Peel-Harvey Estuary was typically in good to fair health, with decreased health in the summer and deeper depositional areas. It is largely dominated by small-bodied, opportunistic species, and apparently retained in early succession due to chronic natural and anthropogenic stress. This was further reflected by the community’s limited impacts on solute fluxes of benthic metabolism, nutrient exchange and denitrification, with sediment condition being more influential. These findings demonstrate that these benthic faunal communities do not play a substantial role in estuarine function, with the application of resulting benthic indices restricted to assessing more structural aspects (e.g. diversity) of benthic ecological health.

One of the most common approaches for investigating the ecology of spatially complex environments is to examine a single biotic assemblage present, such as macroinvertebrates. Underlying this approach are assumptions that sampled and unsampled taxa respond similarly to environmental gradients and exhibit congruence across different sites. These assumptions were tested for five benthic groups of various sizes (archaea, bacteria, microbial eukaryotes/protists, meiofauna and macrofauna) in Plymouth Sound, a harbour with many different pollution sources. Sediments varied in granulometry, hydrocarbon and trace metal concentrations. Following variable reduction, canonical correspondence analysis did not identify any associations between sediment characteristics and assemblage composition of archaea or macrofauna. In contrast, variation in bacteria was associated with granulometry, trace metal variations and bioturbation (e.g. community bioturbation potential). Protists varied with granulometry, hydrocarbon and trace metal predictors. Meiofaunal variation was associated with hydrocarbon and bioturbation predictors. Taxon turnover between sites varied with only three out of 10 group pairs showing congruence (meiofauna-protists, meiofauna-macrofauna and protists-macrofauna). While our results support using eukaryotic taxa as proxies for others, the lack of congruence suggests caution should be applied to inferring wider indicator or functional interpretations from studies of a single biotic assemblage.