D. Jeff Ross

Variability in the impact of an introduced predator ( Asterias amurensis : Asteroidea) on soft-sediment assemblages

The introduction and establishment of the predatory asteroid Asterias amurensis in coastal waters of southern Australia is considered a major threat to benthic marine assemblages and commercial bivalve species. We compare the impact of the seastar in experiments on three soft-sediment assemblages. The experiments were carried out immediately beyond the current range of the seastar in southeast Tasmania. This allows us to assess the repeatability, and hence predictability, of the type and magnitude of the impact of A. amurensis on soft-sediment assemblages. Responses to manipulations at the species level were dissimilar across the three experiments, reflecting marked initial differences in the three assemblages at both the species and functional group levels. However, at the functional group level, there were notable similarities in the impact of the seastar on the different assemblages. When potential prey taxa were separated into functional groups reflecting ecological availability (e.g. surface dwelling vs. deep burrowing bivalves), seastar effects were largely restricted to the surface bivalves. While the effect of seastar predation on surface bivalves was common in all three assemblages, the magnitude of the impact varied both among and within assemblages. Variability in the impact of A. amurensis at both levels appeared to be largely due to differences in relative availability of prey species. Overall, the results of these experiments indicate that while the impact of the seastar is broadly predictable at the functional group level, the exact nature of seastar effects is likely to be site- and time-specific given the inherent natural variability in soft-sediment assemblages and the seastars responses to them.

Effects of estuarine sediment hypoxia on nitrogen fluxes and ammonia oxidizer gene transcription

The effects of sediment hypoxia, resulting from increased carbon loads or decreased dissolved oxygen (DO), on nitrogen cycling in estuarine environments is poorly understood. The important role played by bacterial and archaeal ammonia oxidizers in the eventual removal of nitrogen from estuarine environments is likely to be strongly affected by hypoxic events. In this study, an analysis of the effects of different levels of sediment hypoxia (5%, 20% and 75% DO) was performed in a microcosm experiment. Changes in the nutrient fluxes related to nitrification at 5% DO were observed after 4 h. Quantification of the key nitrification gene ammonium monooxygenase (amoA) in both DNA and RNA extracts suggests that bacterial amoA transcription was reduced at both of the lower DO concentrations, while changes in DO had no significant effect on archaeal amoA transcription. There was no change in the diversity of expressed archaeal amoA, but significant change in bacterial amoA transcriptional diversity, indicative of low- and high-DO phylotypes. This study suggests that groups of ammonia oxidizers demonstrate differential responses to changes in sediment DO, which may be a significant factor in niche partitioning of different ammonia oxidizer groups.

Influence of a burrowing, metal-tolerant polychaete on benthic metabolism, denitrification and nitrogen regeneration in contaminated estuarine sediments

We investigated the effects of the burrowing cirratulid polychaete Cirriformia filigera (Delle Chiaje, 1828) on benthic respiration and nitrogen regeneration in metal-contaminated estuarine sediments using laboratory mesocosms. C. filigera is a dominant component of assemblages in the most severely contaminated sediments within the Derwent estuary, southern Australia. In the presence of C. filigera sediment O2 consumption doubled, with approximately 55% of this increase due to their respiration and the remaining 45% attributable to oxidation reactions and increased microbial respiration associated with burrow walls. Combined NO3 and NO2 fluxes were unaffected. The addition of labile organic matter did not affect benthic fluxes, in the presence or absence of C. filigera, presumably due to the short timeframe of the experiment and naturally enriched test sediments. The results suggest that a combination of tolerance and burrowing activity enables this species to provide an ecosystem service in the removal of N from contaminated sites.

Effects of Sabella spallanzanii physical structure on soft sediment macrofaunal assemblages

Effective management of introduced species requires an understanding of their effects on native species and the processes that structure the habitat. The introduced European polychaete Sabella spallanzanii dominates epifaunal assemblages in south-eastern Australia, yet little is known about how it affects the structure of the surrounding assemblages. The present study investigated the differences between infaunal assemblages in the presence and absence of S. spallanzanii using clumps of real and mimic polychaetes. Both the real and mimic clumps had the same effect on an existing assemblage with fewer numbers of small crustaceans in the sediment under the clumps. The effects of S. spallanzanii on infaunal colonisation and larval abundances above and below the S. spallanzanii canopy were also investigated. Larval effects varied among taxa, depending on position (above and below the canopy) for bivalve larvae and presence/absence of S. spallanzanii for gastropod larvae. There was no effect of the S. spallanzanii clumps on infaunal colonisation. These results suggest that the effects of S. spallanzanii on larval abundances and colonisation may not be as significant as the effects on post-colonisation processes that structure macrofaunal assemblages in soft sediment habitats.

Microsatellite primer development for the seagrass Zostera nigricaulis (Zosteraceae)

Seagrasses are marine angiosperms with a worldwide distribution that form conspicuous beds in nearshore habitats. Despite being universally recognised as a foundation species that performs a number of important ecosystems functions (incl. sediment stabilisation, facilitation of biodiversity, nutrient cycling and carbon sequestration), global seagrass habitats are in decline. Resilience—the ability to recover from disturbance without switching to an alternative state—is paramount to the maintenance and persistence of seagrass habitats. Genetic diversity is a key component of seagrass resilience and contributes to an understanding of population structure, connectivity between populations, and reproductive strategies. Microsatellite primers were developed to investigate the resilience of the seagrass Zostera nigricaulis, which dominates subtidal habitats in the bays of south-eastern Australia. We also tested for cross-amplification of markers between Z. nigricaulis and previously developed markers for the sympatric species Z. muelleri to assess their applicability for use in assessing patterns of genetic diversity, population structure, and mating system. Using next-generation sequencing we isolated 11 novel microsatellite loci for Z. nigricaulis, 8 of which were polymorphic for the samples tested. Allelic diversity ranged from 1 to 8. None of the primer pairs developed for Z. nigricaulis cross-amplified in Z. muelleri; but 14 of 24 primer pairs previously developed for Z. muelleri amplified clearly in Z. nigricaulis samples with six of these showing polymorphism. The results demonstrate the applicability of the Z. nigricaulis microsatellite primers for use in the study of population genetics and limited cross-amplification with Z. muelleri.

Spatial variation in reproductive effort of a southern Australian seagrass.

In marine environments characterised by habitat-forming plants, the relative allocation of resources into vegetative growth and flowering is an important indicator of plant condition and hence ecosystem health. In addition, the production and abundance of seeds can give clues to local resilience. Flowering density, seed bank, biomass and epiphyte levels were recorded for the temperate seagrass Zostera nigricaulis in Port Phillip Bay, south east Australia at 14 sites chosen to represent several regions with different physicochemical conditions. Strong regional differences were found within the large bay. Spathe and seed density were very low in the north of the bay (3 sites), low in the centre of the bay (2 sites) intermediate in the Outer Geelong Arm (2 sites), high in Swan Bay (2 sites) and very high in the Inner Geelong Arm (3 sites). In the south (2 sites) seed density was low and spathe density was high. These regional patterns were largely consistent for the 5 sites sampled over the three year period. Timing of flowering was consistent across sites, occurring from August until December with peak production in October, except during the third year of monitoring when overall densities were lower and peaked in November. Seagrass biomass, epiphyte load, canopy height and stem density showed few consistent spatial and temporal patterns. Variation in spathe and seed density and morphology across Port Phillip Bay reflects varying environmental conditions and suggests that northern sites may be restricted in their ability to recover from disturbance through sexual reproduction. In contrast, sites in the west and south of the bay have greater potential to recover from disturbances due to a larger seed bank and these sites could act as source populations for sites where seed production is low.

Response to “Comment on ‘Seagrass Viviparous Propagules as a Potential Long-Distance Dispersal Mechanism’ by A. C. G. Thomson et al”

[Extract] Our original article (Thomson et al. 2015) presented data exploring Zostera nigricaulis asexually produced vegetative propagules as a potential long-distance dispersal mechanism for seagrasses. We found that the vegetative propagules of Z. nigricaulis were able to maintain buoyancy and photosynthetic health for more than 85 days, which suggested capacity for long-distance dispersal. While long-term establishment of propagules in situ was not successful due to poor seasonal conditions, highly successful establishment and growth in mesocosm-based experiments gave support for positive establishment opportunities. Resilience of seagrass meadows relies on the ability of seagrass to successfully recolonise denuded areas or disperse to new areas (Macreadie et al. 2014), and this research demonstrated that although successful establishment may be rare, vegetative propagules show re-establishment potential for declining seagrass populations. These results are consistent with results found by Stafford-Bell et al. (2015), where prolonged dispersal of Zostera muelleri vegetative fragments was predicted.

Using macrofaunal communities to inform estuarine classification

Worldwide, geomorphological classifications of estuaries are often used to guide the design of monitoring programs and management strategies. However, if classifications do not reflect biotic patterns, the effectiveness of monitoring and management is potentially reduced. In this study, we consider the effectiveness of one classification scheme in describing biotic patterns by examining and comparing spatial variation of macrofaunal assemblages and their relationship with the environment in 12 estuaries of 2 geomorphological types (mesotidal river dominated and permanently open barrier estuaries). Estuaries were sampled at three locations (upper, mid and lower) for macroinvertebrates and environmental characteristics. Differences in macrofaunal assemblages occurred between the estuary types at the lower and mid locations, but not the upper. Similarities in the upper locations were related to sediment, whereas at the mid and lower locations differences were linked to salinity, dissolved oxygen concentrations and seagrass area. Within estuaries, location effects were definitive and unique within each estuary type, correlating to changes in sediment particle size, nitrogen concentration, microphytobenthos and percentage organic carbon. These results suggest that estuarine classification based on physical attributes alone does have the potential to capture important biological attributes if the biological scales of variability within these systems are well understood.