Recruitment of the infaunal bivalve Soletellina Alba (Lamark,1818) (Bivalvia: Psammobiidae) in response to different sediment types and water depths within the intermittently open Hopkins River estuary

Studies examining recruitment processes for soft-sediment macroinvertebrate fauna in intermittent estuaries are rare and most studies of active habitat selection have been tested in the laboratory rather than the field. The present field study examined whether recruitment of the infaunal bivalve Soletellina alba was influenced by water depth and sediment particle size in the intermittent Hopkins River estuary, southern Australia. The number of recruits in sediment trays differed between water depths, but active habitat selection was not evident across treatments of varying sediment particle size. The use of sediments with varying particle sizes also provided an opportunity to identify potential discontinuities in body-size distributions of recruits associated with varying habitat architecture. The length (mm) of recruits was converted to the same scale used to express sediment particle size (i.e. phi units: phi = − log2 of sediment particle size). The size of recruits differed across water depths, but did not differ across treatments with fine (phi = 3) versus coarse (phi = 1) sediment, and no relationships were apparent between bivalve size and sediments consisting of varying particle size. These patterns of recruitment do not correspond with the distribution of adult S. alba within the Hopkins River estuary. Previous sampling has shown that abundances of juvenile and adult S. alba are variable across time, site and water depth, but are often greater at the deeper water depth (1.05 m below the Australian Height Datum). However, recruitment during the present study was greatest at the shallower water depth (0.05 m below AHD), and the apparent absence of active habitat selection suggests that the distribution of adults is unlikely to be attributable to differences in recruitment associated with sediments of varying particle size.

Spatial and temporal changes in abundance of the infaunal bivalve Soletellina alba (Lamark, 1818) during a time of drought in the seasonally-closed Hopkins River Estuary, Victoria Australia

The infaunal bivalve Soletellina alba is susceptible to mass mortalities during annual winter flooding in the Hopkins River Estuary, southern Australia. Periods of low salinity (≤1) are the likely cause of these mass mortality events, which can occur in seasonally-closed estuaries when high winter flows are sufficient to flush all salt water from the estuary. Core samples of S. alba were collected from two water depths across four times and at three sites near the mouth of the estuary. Minimal to zero abundances of large S. alba (>1 mm) were expected to be sampled, particularly at the shallower water depth, during a typical winter flood event. However, the present study occurred during a period of drought, which led to the absence of winter flooding. This absence of winter flooding prevented the occurrence of lethal salinities (i.e. ≤1) in the estuary during this period and a greater number of living S. alba adults were sampled. Abundances of juvenile and adult S. alba were still variable, even in the absence of winter flooding, and reflected an interaction between date, site and water depth. However, no mass mortalities of adults were observed during the drought conditions in contrast to what occurs during typical winter flood events and provides support for the hypothesis that winter flooding is responsible for past mass mortalities.

Ecology of epifaunal caridean shrimps in the Hopkins River estuary, and the role of estuaries in the life history of the atyid "Paratya Australiensis" Kemp, 1917 in south-eastern Australia

This study examined the factors affecting the distribution and abundance of epifaunal caridean shrimps in seagrass meadows of the Hopkins River estuary in south-western Victoria, Australia, and investigated the life history patterns of the freshwater Parana australiensis, found for the first time in estuaries. Adult and sub-adult shrimps were surveyed in seagrass meadows along the estuary over two years, and their planktonic larvae were surveyed in adjacent waters. Three species were collected. The marine Palaemon serenus occurred only near the mouth, summer to autumn, in high salinities. The marine/estuarine Macrobrachium intermedium occurred throughout the estuary. Adults were most abundant in late autumn, and least abundant in summer (unlike trends reported in marine meadows). Densities were higher and less variable in downstream meadows. P. australiensis occurred in the upper estuary all year, most abundantly in spring, due to migration from the river after peak discharge. Ovigerous females dominated, while males, showing less migration into the estuary, dominated above estuarine influence. Adults disappeared from the estuary in summer as salinity rose. Breeding period for P. australiensis was briefer in the estuary (September-December) than upstream (July-April). M. intermedium began breeding later in the upper estuary (November/December-March) than in the lower estuary (October-March), probably reflecting a physiological response to lower salinity, rather than an interaction with P. australiensis. No ovigerous P. serenus were found in the estuary. Larvae of P. australiensis and M intermedium occurred abundantly throughout the estuary, but P. serenus larvae did not. P. australiensis was an early coloniser to the plankton after peak discharge (November-December). Larvae concentrated in the deep saline layer at the head of the intruding salt wedge, thus probably maintaining longitudinal position. Diurnal vertical migrations were evident within the salt wedge, and in a deep pool above tidal influence. M. intermedium larvae occurred October-May in the lower estuary and November-April in the upper estuary, peaking in abundance one to two months after P. australiensis. They were associated with low surface flows and surface salinities greater than 10, over an anoxic deeper layer. All three species exhibited extended development of euryhaline larvae in the laboratory. Tolerances and optimal salinities of larvae of the three species reflected their distributions. M. intermedium was the most euryhaline species. P. australiensis larvae were tolerant of higher salinities than juveniles of adults: capable of developing in salinity of at least 15. Most P. australiensis juveniles recruited to the estuary November-December, after which numbers declined dramatically. After settlement, most recruits probably migrated upstream out of the estuary. Two cohorts of M. intermedium recruited to the estuary from larvae in summer (December and February), but some juveniles also migrated from adjacent coastal waters. Post-larval migration was at least as important a determinant of abundance as direct recruitment from estuarine, planktonic larvae in all three species. Distributions among seagrass meadows along the estuary were determined primarily by physico-chemical patterns driven by hydrological changes. Seasonal variations in salinity and temperature were strongly associated with seasonal variations in shrimp abundance. Salinity tolerances of adults of the three species reflected their distribution patterns. Biotic interactions were more important in determining distributions within meadows. P. australiensis, when abundant, were associated with seagrass biomass. M. intermedium were also, but when seagrass was sparsest and least extensive. The two species apparently partitioned the seagrass meadow according to depth in early summer. Laboratory experiments suggested P. australiensis was displaced from deeper water by M. intermedium. Preference for vegetative complexity and competition for position within meadows suggest the underlying importance of predation in regulating shrimp populations. A survey of south-eastern Australian estuaries found P. australiensis larvae abundant in all stable, open, well-developed, salt-wedge estuaries where adults were abundant. Adults were most abundant in low salinities among submerged leafy macrophytes. Reproductive traits of P. australiensis were compared in estuarine and fresh reaches of three rivers. Early in the breeding season, egg size was smaller, and (size-specific) egg number larger in estuaries than upstream. A trade-off between egg size and egg number resulted in no difference in total (size-specific) reproductive investment between locations. Reproductive investment tended to decrease at some locations over the breeding season, and this decrease was a result of decreased egg size in most cases. The decrease in reproductive investment probably reflected reduced food availability for the adult, while the reduced egg size was probably a response to improved conditions for larval development. In the Hopkins River, larger egg size at upstream sites was reflected in larger early stage larvae. Later stage larvae were larger in the estuary, suggesting more favourable conditions for larval development. Allozyme electrophoresis showed the P. australiensis populations in each of the three rivers to be distinct. Allozyme frequencies were not different within the Hopkins River, but upstream and estuarine locations in the Curdies and Gellibrand were different. Although some variation in reproductive traits within catchments may have been due to genotypic differences, trade-offs between egg size and number, and decreases in egg size over summer were probably due to plastic responses to environmental cues. It is proposed P. australiensis inhabits and reproduces in both estuarine and freshwater environments by plastic response to environmental conditions. Recruitment to estuaries is dependent on the presence of suitable adult, littoral habitat, and a stable salt wedge for larval retention. Estuaries are important recruitment sites for P. australiensis, potentially allowing an extra brood each year before riverine recruitment. Estuarine broods could constitute a large part of the total fecundity of P. australiensis females. Euryhaline larvae and estuarine recruitment of P. australiensis suggest marine transport of larvae between estuaries as a possible dispersal mechanism for Paratya species.

Growth rates of the infaunal bivalve Soletellina alba (Lamarck, 1818) (Bivalvia: Psammobiidae) in an intermittent estuary of southern Australia

Caging and a mark–recapture design were used to estimate the growth rate of the brittle, infaunal bivalve Soletellina alba in the Hopkins River estuary. The growth of both caged and uncaged individuals was monitored at three sites near the mouth of the estuary over 180 days. Growth rates did not differ for caged and uncaged bivalves, or for bivalves subject to different amounts of handling, or between sites. Growth did differ between consecutive time intervals, which was attributable to negligible growth occurring during the colder months of autumn/winter. Comparisons of the condition (as indicated by total mass for length³) of S. alba were inconsistent between sites for caged and uncaged bivalves and for those subject to different amounts of handling. Soletellina alba is a rapidly growing bivalve with mean growth rates for the three time intervals being 0.04±0.002 mm day⁻¹ in summer, 0.02±0.001 mm day⁻¹ in autumn and 0.03±0.001 mm day⁻¹ from summer to winter. Using existing literature, it was shown that a significant relationship exists between maximum shell length and onset of sexual maturity in bivalve molluscs. This relationship predicts that S. alba should reach the onset of sexual maturity at 15.8 mm length. Therefore, it appears that it may be possible for juvenile S. alba (<1 mm) to grow, reach sexual maturity and reproduce in between annual mass-mortality events caused by winter flooding.

Effect of flooding on estuarine bivalve populations near the mouth of the Hopkins River, Victoria, Australia

The densities of two common intertidal/shallow subtidal bivalves, Soletellina alba and Arthritica helmsi, were sampled in vegetated and unvegetated habitats of the Hopkins River estuary on three occasions during the  autumn/winter 1995.Winter flooding coincided with mass mortalities of the infaunal bivalve S. alba, but not A. helmsi. Mortalities were apparent for individuals living deeper in the sediment (≈35 cm) in vegetated and unvegetated habitats, but small S. alba (<1mm) were less susceptible to mortality than larger individuals (>1mm). Mortalities were similar across different habitat types and sediment depths, and at multiple sites within close proximity to the estuary mouth.

Annual phytoplankton and nutrient fluctuations in the Hopkins Estuary, South-West Victoria, Australia

The Hopkins River drains grazing farmland for most of its length. Nutrients and phytoplankton populations in the Hopkins Estuary are strongly affected by the hydrodynamic cycle. The findings of this research show that nutrient and chlorophyll a levels in the Hopkins Estuary are high enough to indicate that future algal problems may occur and that management of nutrients entering its catchment and improvement of river banks are of high importance.

Spatial and temporal variation of infauna in an Australian estuary

Macroinvertebrates living within the sediments of the Hopkins River estuary were examined during 1997. Differences in the abundance and composition of faunal assemblages were identified between vegetated and unvegetated habitats and between the lower, middle and upper sections of the estuary. The numbers of taxa and individuals present were found to be similar to that of other periodically opened estuaries.

Feeding ecology of benthic invertebrates in an intermittently open estuary

Abundances of benthic invertebrates in the Hopkins River estuary were not affected by connection to the ocean, but by freshwater discharge and sediment properties. The bivalve Soletellina alba was identified as a suspension feeder. Invertebrate food sources were derived mainly from estuarine organic matter, but also contained marine and freshwater/terrestrial plants despite reduced river flow during drought conditions.

Downstream spawning migration by the amphidromous Australian grayling (Prototroctes maraena) in a coastal river in south-eastern Australia

Understanding the reasons and cues for migration is crucial for developing effective conservation and management strategies of diadromous fishes. Spawning and movement patterns of the threatened diadromous Australian grayling (Prototroctes maraena) were investigated in the Bunyip River, Victoria, using drift sampling (2008–2011) and acoustic telemetry (2009–2010) during the autumn–winter spawning period of each year. Fifty-five adult fish (2009: n = 21; 2010: n = 34) were tagged and released in February ~15–30 km upstream of the Bunyip River estuary. Thirteen fish (2009: n = 7; 2010: n = 6) undertook rapid downstream migrations from March to April to reaches immediately upstream of the estuary. Drifting eggs were detected at multiple sites between April and July; however, the majority (78.8%) were collected in the lower reaches within ~0.5 km of the estuary in early–mid-May. Tagged adult fish arrived in this area 1–4 weeks before eggs were detected and usually moved back upstream within 2 weeks following the peak egg abundance. Downstream migration and peak egg abundance were associated with increased river flows. Although the proportion of fish that undertook migrations was low, low rates of tag retention in this species likely account for the failure to detect migration by many of the tagged individuals.

Utilization of organic matter by invertebrates along an estuarine gradient in an intermittently open estuary

In intermittently open estuaries, the sources of organic matter sustaining benthic invertebrates are likely to vary seasonally, particularly between periods of connection and disconnection with the ocean and higher and lower freshwater flows. This study investigated the contribution of allochthonous and autochthonous primary production to the diet of representative invertebrate species using stable isotope analysis (SIA) during the austral summer and winter (2008, 2009) in an intermittently open estuary on the south-eastern coast of Australia. As the study was conducted towards the end of a prolonged period of drought, a reduced influence of freshwater/terrestrial organic matter was expected. Sampling was conducted along an estuarine gradient, including upper, middle and lower reaches and showed that the majority of assimilated organic matter was derived from autochthonous estuarine food sources. Additionally, there was an input of allochthonous organic matter, which varied along the length of the estuary, indicated by distinct longitudinal trends in carbon and nitrogen stable isotope signatures along the estuarine gradient. Marine seaweed contributed to invertebrate diets in the lower reaches of the estuary, while freshwater/terrestrial organic matter had increased influence in the upper reaches. Suspension-feeding invertebrates derived large parts of their diet from freshwater/terrestrial material, despite flows being greatly reduced in comparison with non-drought years.

Some morphometric and biochemical features of ready-to-migrate silver and pre-migratory yellow stages of the shortfin eel of south-eastern Australian waters

The mean total length (L_T), mass and age of ready to migrate female silver shortfin eels Anguilla australis from the Hopkins River estuary and the mouth of the Merri River in south-eastern Australia, were 83·2 ± 1·2 cm, 1051 ± 51 g, and 17·2 ± 1·79 years, respectively. The eye index (I_E) of the silver shortfin eels was < 5·2 (mean 7·64 ± 0·29) and differed significantly from that of the yellow shortfin eels collected from two other sites. The I_E increased with L_T (mm) and was related by log I_E= 2·656 log L_T6·925. The per cent moisture, protein and ash content of the liver of silver shortfin eels was significantly lower than in yellow shortfin eels, but lipid content was significantly higher in the former (35·5 ± 2·0%). The mean mass μg mg lipid ‾) of saturates (230·4 ± 2·6 v. 181·7 ±2·6), monoenes (367·4 ± 6·3 v. 290·8 ± 8·9) and PUFA (177·3 ± 5·3 v. 159·7 ± 4·6) in muscle was significantly higher, and the great majority of individual fatty acids was found also in higher quantities in silver shortfin eels. In the liver, the PUFA found in the highest quantity was 22:6n-3, except in shortfin eels from Hopkins River estuary, and the amount of 18:2n-6 in the liver of silver shortfin eels was significantly higher than that in yellow shortfin eels but the reverse was true of 20:4n-6. In both muscle and liver tissues the saturate 16:0 and the monoene 18:ln-9 collectively accounted for >50% of all the fatty acids in the lipid.