Hawkesbury River Estuary Research Articles

Investigations of the involvement of Vibrio species with Ostreid herpesvirus-1 in mass mortality events in the Pacific oyster Crassostrea gigas

Pacific oyster mortality syndrome (POMS) in Pacific oysters Crassostrea gigas is defined by mass mortality with Ostreid herpesvirus I (OsHV-1). In this study the association of Vibrio species with oysters before, during and after POMS events was investigated at population level to clarify their involvement with OsHV-1 in mass mortality events in the Georges and Hawkesbury River estuaries, Australia. In the POMS affected Georges River estuary, three patterns were observed concurrently at different sites: i) Vibrio counts and OsHV-1 DNA concentrations increased in adults and spat as mortalities started then decreased as mortalities stopped ii) bacterial counts increased in spat but not in adults associated with mortalities due to OsHV-1 iii) bacterial counts increased in the absence of mass mortality or high concentrations of OsHV-1 DNA. Although almost half of the 120 bacterial isolates were identified as an unknown Vibrio species belonging to the Splendidus clade with two predominant biotypes (A and H), there was no association between biotype and mortality. Concurrently in the Hawkesbury River estuary, neither mortality nor OsHV-1 were found, Vibrio counts fluctuated significantly and of 84 isolates identified, V. alginolyticus, V. parahaemolyticus and unidentified Vibrio sp. dominated; only five Splendidus clade isolates were identified. However, when POMS emerged in the Hawkesbury River estuary, the Splendidus-clade isolate became the dominant bacterial species. Mortality also occurred there without OsHV-1 and the dominant bacteria then were V. alginolyticus and V. harveyi, but the Splendidus-clade isolate was not detected. None of the bacterial species were specifically associated with POMS, although the Splendidus clade isolate increased in prevalence after POMS emerged in the Hawkesbury River estuary. V. aestuarianus was not detected in this study. While the findings suggest that bacterial proliferation follows infection with OsHV-1 in POMS, this was not a consistent feature. The number and diversity of bacteria within oyster tissues varied over time, between estuaries, between sites within an estuary and were associated with flooding, growing height, the age of oysters, mortality and OsHV-1.

Fifteen years of Pseudo-nitzschia in an Australian estuary, including the first potentially toxic P. delicatissima bloom in the southern hemisphere

In March 2018, an unprecedented bloom of a species of Pseudo-nitzschia occurred in Berowra Creek, a major tributary of the Hawkesbury River estuary, New South Wales, Australia (max cell abundance 5.7 × 106 cells L−1). Morphological and phylogenetic analysis showed the bloom was dominated by the species P. delicatissima, with toxin characterization by LC-MS confirming one of the seven strains isolated produced domoic acid (DA), at a concentration of 0.34 pg DA per cell.In response to this unprecedented bloom, we used a fifteen year series of phytoplankton and physico-chemical data to relate environmental factors to spatial and temporal variability of Pseudo-nitzschia species throughout the estuary. Species in the Pseudo-nitzschia delicatissima group consistently dominated all locations monitored throughout the estuary, with peak cell densities observed in the austral autumn (lowest in winter). Overall, Generalised Additive Models (GAMs) suggested that high cell concentrations of Pseudo-nitzschia were linked to an increase in soluble reactive phosphorus and a decrease in total nitrogen at all sites, with up to 55% of the deviance explained. Twenty species of Pseudo-nitzschia have now been reported in Australian coastal waters, four of which are confirmed DA producers. We hypothesise that increased urbanization, nutrient input and warmer waters could combine to increase Pseudo-nitzschia blooms in south-eastern Australia.

Long-term temporal and spatial patterns of Ostreid herpesvirus 1 (OsHV-1) infection and mortality in sentinel Pacific oyster spat (Crassostrea gigas) inform farm management

Disease due to Ostreid herpes virus − 1 (OsHV-1) is a major constraint to farming the Pacific oyster Crassotrea gigas in Europe and Australasia, requiring new management strategies. The objectives of this study were to determine the seasonal limits of transmission of OsHV-1, the spatial distribution of the virus, the seasonal water temperatures associated with OsHV-1 transmission and disease and the suitability of active surveillance for OsHV-1 using sentinel oysters. Spat were deployed and sampled regularly between 2012 and 2017 at 15 sites in two large estuaries and the times of OsHV-1 exposure and mortality were identified at weekly to monthly intervals. Mortality due to OsHV-1 was observed each summer season but was significantly more likely in the Georges than the Hawkesbury River estuary. Mortality was most widespread and frequent between December and April and was not observed prior to late October or after May. However, subclinical infections with OsHV-1 that did not progress to mortality were detected from early October until late June. There was a significant reduction in mortality due to OsHV-1 as well as subclinical infection with OsHV-1 between 2012–13 and 2016–17, and viral loads in infected oysters also appeared to decline after 2012–13. Mortality due to OsHV-1 was observed in sentinel spat at 14 of 15 sites but not at every site in every season. Different outcomes were sometimes seen between spat in paired baskets that were 1 to 100 m apart at the same site, attributable to differential exposure to OsHV-1 (clustering). Fifteen different batches of spat were used and mortality due to OsHV-1 occurred in all but three; both triploid and diploid spat were affected. In 2016–17, there was a trend of better survival in selected spat with potential genetic resistance to OsHV-1 compared to unselected spat. There were no biologically significant differences in the growth of spat that subsequently died due to OsHV-1 infection and those that survived. Mortality due to OsHV-1 commenced when the mean water temperature rose above approximately 20 °C in spring, which is 4–5 °C warmer than the initiation threshold in Europe. These consistent epidemiological patterns of infection and mortality will enable strategic placement of susceptible oysters in these estuaries at certain locations and within specific time windows with predicted low risk of mortality due to OsHV-1; water temperature monitoring may be used to predict this risk. Use of sentinel oysters for surveillance for OsHV-1 can be effective, but the clustering of exposure at all scales renders this more reliable at estuary level rather than at bay or site level and it requires an intensive sampling program. Non-specific mortality of spat was relatively common, therefore tests for OsHV-1 should be applied in field trials to distinguish mortality due to OsHV-1 from other causes of mortality.

Descriptive epidemiology of mass mortality due to Ostreid herpesvirus-1 (OsHV-1) in commercially farmed Pacific oysters (Crassostrea gigas) in the Hawkesbury River estuary, Australia

Mortality of farmed triploid Pacific oysters ( Crassostrea gigas ) associated with Ostreid herpesvirus-1 (OsHV-1) was first recorded in Australia in the Georges River/Botany Bay estuary (New South Wales) in late 2010. Two years later, the first sign of possible inter-estuarine spread was observed when commercial triploid Pacific oysters in the Hawkesbury River estuary, located 50 km north of Botany Bay, were affected by mass mortality. The aim of this study was to describe the epidemiological features of the Hawkesbury outbreak via a formal investigation which was conducted in real time and comprised: an assessment of stock levels, past oyster acquisitions and a trace forward investigation to identify stock at greatest risk due to transfers of oysters; passive surveillance of the spread of mortalities in the estuary; active surveillance using PCR to identify the distribution of OsHV-1 infection on farms in the estuary and mortality estimates to identify age and size classes affected; identification of the time of first infection using data from sentinel oysters; and assessment of environmental risk factors. Mortalities were recorded in all age classes but were greater in spat and juveniles than in adults. The incubation period for mass mortality was < 4 days, however subclinical OsHV-1 infection was detected three months prior to the first signs of mortality in the index case site (first location affected), which suggests that low viral loads of OsHV-1 are insufficient to induce the disease. While inefficient oyster-to-oyster transmission occurred at two locations, a synchronous infection arising from a common environmental source was required to explain the mass mortalities at the index case site (Mullet Creek). Estuarine hydrodynamics then assisted rapid dispersal of viral particles throughout the estuary. Seawater temperatures were consistently above 24 °C during the month preceding mass mortalities with variations (± 3 °C) being observed over a few days during this period; however this did not necessarily lead to mortality events when the virus was present. There was no evidence of prior movement of potentially infected oysters or farming equipment into the Hawkesbury River estuary to explain the outbreak. • A massive OsHV-1 outbreak in Australia was monitored in real time. • Subclinical OsHV-1 infection was detected three months prior to mass mortality. • The mass mortality had an incubation period < 4 days and was not due to oyster to oyster transmission. • The outbreak was not due to introduction of infected oysters or farming equipment. • A point source epizootic and rapid dispersal by estuarine hydrodynamics is proposed.

Status of the Sydney rock oyster in a disease-afflicted estuary: persistence of wild populations despite severe impacts on cultured counterparts

Marine diseases represent a significant threat to wild organisms and the ecosystem services they support, yet studies often consider only disease impacts to aquaculture. In eastern Australia, the Sydney rock oyster (Saccostrea glomerata) aquaculture industry is increasingly affected by outbreaks of QX disease caused by parasitic Marteilia sydneyi. The present study considered impacts of M. sydneyi infection on the structure of wild-oyster populations that are dominated by S. glomerata, but that may also include the non-native Pacific oyster, Crassostrea gigas. In the Hawkesbury River Estuary, where cultured S. glomerata has experienced up to 98% QX-induced mortality, we found that disease prevalence was comparatively low among wild S. glomerata, peaking at 14%, and annual infections did not cause seasonal patterns of mortality. Furthermore, C. gigas, a competitor of S. glomerata that is not susceptible to QX disease, was not consistently more abundant at sites with than without the parasite. Overall, our results indicated that relative to cultured counterparts, wild S. glomerata in the Hawkesbury River Estuary is minimally affected by QX disease. Nevertheless, our study showed that diseases of aquaculture stocks have the capacity to infect wild populations, and that longer-term assessment of wild populations at risk is essential.

The influence of nutrient enrichment upon mangrove seedling establishment and growth in the Hawkesbury River Estuary, New South Wales, Australia

The hypothesis that mangrove exclusion from saltmarsh is attributable to nutrient deficiency is experimentally tested on the Hawkesbury River estuary. Survivorship and growth of seedlings of the grey mangrove Avicennia marina were followed for a sixteen month period, in an experiment in which half of all seedlings were treated with a slow-releasing source of phosphorus, nitrogen and potassium. While fertilisation substantially increased growth rate, this occurred in the second summer of growth and fertilisation did not improve the low survivorship of A. Marina at the mangrove-saltmarsh boundary. Fertilisation has no influence on the root/shoot ratio where measured. It is postulated that factors other than nutrient deficiency are primarily responsible for determining the upslope distribution of mangroves in temperate Australian estuaries.

Mangroves as successional stages on the Hawkesbury River

The mangrove environments of the Hawkesbury River estuary, New South Wales, are described in relation to the geomorphic units of the marine and fluvial segments of the river. A model of seral succession is presented which sees mangrove environments replaced with saltmarsh and Casuarina associations with continued progradation during the Holocene stillstand. Preserved mangrove peats beneath the Juncus kraussii plains of Mangrove Creek, a tributary of the Hawkesbury, are presented as evidence supporting this model. Recent expansion of mangroves into the saltmarsh is therefore contrary to the longer-term successional trend of the Holocene.

Spatial Patterns of Wild Oysters in the Hawkesbury River, NSW, Australia

The native Sydney rock oyster, Saccostrea glomerata, is under increasing threat from QX disease, competition with nonnative Crassostrea gigas and coastal development. Knowledge of the distribution and population structure of S. glomerata and C. gigas is essential if oysters and their ecosystem services are to be successfully managed. We determined spatial patterns of abundance, condition, and size-structure of S. glomerata and C. gigas, across two key habitats, mangroves, and rocky shores of the Hawkesbury River, a highly modified estuary 50 km north of Sydney. Sampling of five sites per habitat, spanning a 15 km stretch of river, revealed abundant populations of S. glomerata, averaging 514 ± 185 m-2, in mangroves and on rocky shores. The native oyster accounted for 99% of all oysters sampled, with C. gigas found only at two of the five sites sampled within each habitat. Overall, rocky shores supported over eight times the oyster cover as mangroves. Among rock sites, live oyster cover and condition generally decreased with distance upstream. Although, at present, the Hawkesbury River estuary supports abundant wild oyster populations, ongoing monitoring of oyster populations is required to ensure that appropriate management strategies are established to ensure the persistence of this important component of the ecosystem. Our sampling of two key oyster habitats provides an important baseline against which future studies can assess change.

Diatom and dinoflagellate assemblages of the Hawkesbury River, N.S.W., over the last two centuries: evidence for changes in hydrology

Diatom and dinoflagellate cyst analysis of a 77 cm long sediment core from Cowan Creek, Hawkesbury River estuary, N.S.W., revealed changes in the catchment hydrology over the last 266 years. High abundances of the freshwater/brackish diatom genus Cyclotella at the base of the core imply sustained periods of reduced salinity that now no longer occur. Reduction of freshwater flow after approximately circa 1800 (60 cm) has allowed the development of marine planktonic diatoms Thalassiosira spp.,Ditylum brightwellii, Rhizosolenia setigera, Pseudo-nitzschia pungens and Chaetoceros spp. Benthic diatom diversity has remained relatively unchanged. The toxic dinoflagellate Gymnodinium catenatum, although identified in a cyst survey in April 1995, was not found in the sediment cores. Changes in dinoflagellate assemblage are consistent with the effects of increasing urbanisation and eutrophication.

The upper Hawkesbury River, New South Wales, Australia: a Holocene example of an estuarine bayhead delta

ABSTRACTHolocene deposits of the Hawkesbury River estuary, located immediately north of Sydney on the New South Wales coast, record the complex interplay between sediment supply and relative sea‐level rise within a deeply incised bedrock‐confined valley system. The present day Hawkesbury River is interpreted as a wave‐dominated estuarine complex, divisible into two broad facies zones: (i) an outer marine‐dominated zone extending 6 km upstream from the estuary mouth that is characterized by a large, subtidal sandy flood‐tidal delta. Ocean wave energy is partially dissipated by this flood‐tidal delta, so that tidal level fluctuations are the predominant marine mechanism operating further landward; (ii) a river‐dominated zone that is 103 km long and characterized by a well developed progradational bayhead delta that includes distributary channels, levees, and overbank deposits. This reach of the Hawkesbury River undergoes minor tidal level fluctuations and low fluvial runoff during baseflow conditions, but experiences strong flood flows during major runoff events. Fluvial deposits of the Hawkesbury River occur upstream of this zone.The focus of this paper is the Hawkesbury River bayhead delta. History of deposition within this delta over the last c. 12 ka is interpreted from six continuous cores located along the upper reaches of the Hawkesbury River. Detailed sedimentological analysis of facies, whole‐core X‐ray analysis of burrow traces and a chronostratigraphic framework derived from 10 C‐14 dates reveal four stages of incised‐valley infilling in the study area: (1) before 17 ka BP, a 0–1 m thick deposit of coarse‐grained fluvial sand and silt was laid down under falling‐to‐lowstand sea level conditions; (2) from 17 to 6·5 ka BP, a 5–10 m thick deposit composed of fine‐grained fluvial sand and silt, muddy bayhead delta and muddy central‐basin deposits developed as the incised valley was flooded during eustatic sea‐level rise; (3) during early highstand, between 6·5 and 3 ka BP, a 3–8 m thick bed of interbedded muddy central‐basin deposits and sandy river flood deposits, formed in association with maximum flooding and progradation of sandy distributary mouth‐bar deposits commenced; (4) since 3 ka BP, fluvial deposits have prograded toward the estuary mouth in distributary mouth‐bar, interdistributary‐bay and bayhead‐delta plain environments to produce a 5–15 m thick progradational to aggradational bayhead‐delta deposit. At the mouth of the Hawkesbury estuary subaqueous fluvial sands interfinger with and overlie marine sands. The Hawkesbury River bayhead‐delta depositional succession provides an example of the potential for significant variation of facies within the estuarine to fluvial segment of incised‐valley systems.

Above- and below-ground biomasses of two species of mangrove on the Hawkesbury River estuary, New South Wales

Above- and below-ground biomasses of two species of mangrove, Avicennia marina and Aegiceras corniculatum, were estimated in a range of intertidal environments along the Hawkesbury River. Estimates of biomass of Avicennia communities in freshly accreted brackish substrata were in the order of 40 kg m–2, the highest figure ever recorded for temperate mangrove communities. The above-ground biomass communities of each species declined with increasing substratum salinity, whereas root/shoot ratios increased with increasing substratum salinity.

Litterfall in Casuarina glauca Coastal Wetland Forests

Litterfall was measured over 3 years at two sites in coastal wetland forests dominated by Casuarina glauca Sieb. ex Spreng. in New South Wales. One site was in an incised river valley adjacent to the Hawkesbury River estuary, and the other site was in an open embayment adjacent to Botany Bay. Branchlets were the major litter component, followed by stems and infructescences. Significant year to year variation in branchlet fall correlated with storm events in one year. Overall, no significant differences in annual total litterfall were detected among sites and, averaged over all sites and years, the mean annual litterfall was 848 g m-2. This suggests that coastal wetland forests dominated by C. glauca have some of the highest recorded annual litterfall rates for Australian temperate and subtropical forests. A flood event in the river valley site removed about 75% of the standing litter accumulated on the forest floor prior to flooding (1244 g m-2). This suggests that accumulated organic matter and nutrients can be directly transported to estuarine ecosystems. Under non-flood conditions, however, these forests appear to act as organic matter and nutrient sinks.

Heavy metals, TBT and DDT in the Sydney rock oyster ( Saccostrea commercialis) sampled from the Hawkesbury River estuary, NSW, Australia

Heavy metals, TBT and DDT in the Sydney rock oyster ( Saccostrea commercialis) sampled from the Hawkesbury River estuary, NSW, Australia

Holocene environments in the Hawkesbury valley, new South Wales: A comparison of terrestrial and marine records

Rapid sedimentation of sands, silts and muds associated with seaward prograding tidal delta and central basin deposits, characterise the early Holocene sediment infill of the Hawkesbury River's estuary. From about 6 ka BP fluvial muds and sands dominate the estuary and sedimentation rates are much reduced. Micro-fossil records contained in sediments from the middle Hawkesbury-Colo Rivers region support the sedimentological-lithostratigraphic interpretation of estuary development. Information from fossil diatoms indicates that for a period of about 300 years around 6 ka BP, there was an increased marine influence in this part of the present estuary. This may have resulted from the probable peak in the rate of relative sea-level rise, although there may also have been a contemporaneous increase in freshwater flow down the river. Pollen and charcoal records from Mill Creek, downstream from the Colo sequence, show that sclerophyll forest, occasionally with rainforest elements, became more dominant from 8 ka BP. In contrast, the upper tidal Casuarina glauca forest decreased in importance during the phases of sea-level rise and the subsequent ‘still-stand’. Together the palaeoenvironmental records show broad agreement in defining the patterns of regional environmental changes and linked phases of estuary development during the Holocene. Differences in records appear to relate to local site factors and to the limitations in provenance, persistence and resolving power of the data types. The rapid accumulation rates provide an opportunity to investigate high magnitude but low frequency events in estuarine environments, including floods and fire.

Diet of the Australian bass, Macquaria novemaculeata (Perciformes : Percichthyidae), in the Sydney Basin

Stomach contents of Australian bass, M. novemaculeata, sampled from the Hawkesbury River and Georges River in the Sydney Basin, New South Wales, between November 1977 and January 1982 were analysed by the occurrence and points methods. Stomach fullness was also recorded. A total of 143 aquatic and terrestrial animal taxa were present in the diet, and these were grouped into 19 food types for analysis. M. novemaculeata is a euryphagic carnivore. Season and habitat type had significant effects on composition of the diet. Insects were the most important food type, followed by fish and large crustaceans. A large proportion of the diet of bass was derived from allochthonous sources, mainly during summer, and especially in lotic habitats. Mean stomach fullness was highest in spring and lowest in winter. Young M. novernaculeata (TL 11-47 mm) from the Hawkesbury River estuary fed on far fewer prey taxa (mainly chironomids and copepods) than did adults. Dietary overlap occurs between M. novemaculeata and many other carnivorous freshwater vertebrates in the Sydney Basin. However, persistent competitive clashes are generally avoided, either by differences in microhabitat preference and feeding behaviour or by larger-scale habitat partitioning.