Alastair J. Hirst

Classification and prediction of macroinvertebrate assemblages from running waters in Victoria, Australia

We constructed predictive models using 2 macroinvertebrate data sets (for both species and family) from bankside habitats at 49 undisturbed reference sites from 6 Victorian river basins; data were accumulated over 4 to 6 sampling occasions. Classification (by unweighted pair-group arithmetic averaging with the Bray-Curtis association measure) showed 3 site groups were evident at the species level and 4 at the family level. A subset of 5 of 22 environmental variables provided maximum discrimination (using stepwise discriminant analysis) between the 3 species site groups; these variables were: conductivity, altitude, substrate heterogeneity, distance of a site from source, and longitude. Four variables discriminated between the 4 family site groups: conductivity, catchment area upstream of site, mean annual discharge, and latitude. From the discriminant analysis, it was possible to predict the group into which an unknown site (specified only by measurements on the 4 or 5 variables just noted) would be placed and thus the probabilities of occurrence of taxa at this site. To test predictive ability, 4 sites were removed at random from the 2 data sets and the classification and discriminant models were recalculated. This process was repeated 5 times. The identity and number of taxa observed at each of these sites were compared with those predicted with a probability of occurrence >50% and the results expressed as a ratio of numbers observed to numbers expected (O/E). This ratio varied from 0.75 to 1.05 at the species level and from 0.83 to 1.12 at the family level, indicating that the fauna conformed with expectation (O/E near 1.0). To test such predictive models on independent data, O/E ratios were also calculated for family data collected in spring at 18 sites from a basin not used in the original models. Two new discriminant models based on single sets of samples from the reference sites taken in spring were constructed for this purpose. O/E ratios varied from 0.09 to 1.01 for the 18 sites and were inversely correlated (r = -0.4 to -0.8) with a range of water quality variables, the values of which increased as water quality deteriorated. The O/E ratio could thus be considered a sensitive measure of disturbance.

Broad-scale environmental gradients among estuarine benthic macrofaunal assemblages of south-eastern Australia: implications for monitoring estuaries

The importance of abiotic factors in explaining patterns of estuarine benthic macrofaunal community structure was examined on a broad spatial scale across south-eastern Australia. Macrofaunal communities were surveyed using an Ekman grab and a modified epibenthic sled (dredge) at each sampling site: data for 24 environmental variables were also collected. Twenty-eight estuaries were sampled on a single occasion during late summer at three stratified locations within each estuary (upper, mid and lower). Macrofaunal community composition was best explained by a common environmental gradient summarising variation in both salinity and longitude. Hence, although the distribution of macrofaunal taxa can be clearly linked to changes in salinity, the geographical position of the sites along an east–west axis, rather than a generalised down-stream gradient, appears to best explain the data. This association was primarily linked to broad-scale changes in estuarine morphology across the geographical range of this survey. A sediment-based environmental gradient among grab samples, but not dredge samples, reflected the largely infaunal nature of the grab samples. In general, the present survey did not support the classification of estuarine assemblages on the basis of a range of physical parameters but, instead, emphasised the continuity of estuarine benthic macrofaunal community structure on a broad spatial scale.

Multi-scale spatial variation in stable isotope and fatty acid profiles amongst temperate reef species: implications for design and interpretation of trophic studies

Stable isotopes of carbon and nitrogen and fatty acid analyses are increasingly being used in combination to determine the trophic structure of marine systems. For stable isotopes, the variability in carbon and nitrogen isotopic signatures has long been recognised and has been charac- terised for some taxa. Whilst it is known that metabolic processes may influence fatty acid profiles, the spatial variability of fatty acid profiles has not been documented. Understanding at what scale these 2 biochemical tracers vary, and if the scale of variability corresponds between tracers, is crucial for the correct design and interpretation of combined tracers in trophic studies. This study is the first to examine spatial variability in fatty acid profiles per se, and in combination with stable isotope ratios in the same organisms at multiple spatial scales. We used a spatially hierarchical design which sam- pled across broad geographic regions, reefs within regions, and also between different parts of macroalgal plants common on temperate reefs. For stable isotopes of carbon and nitrogen, variability was greatest at intermediate spatial scales (between locations within regions, and sites within loca- tions). In contrast, fatty acid profiles showed the greatest variation amongst individual replicates of lobster, abalone and macroalgae. This study demonstrates that for the increasing number of trophic studies using combined biochemical tracers, sampling design should cater to the differences in the variability of each tracer technique and allocate sampling accordingly.

Spatial and temporal variation in the structure of estuarine macroinvertebrate assemblages: implications for assessing the health of estuaries

As human impacts in estuaries are often pervasive (estuary-wide) and/or pre-existing, the identification of suitable reference points, from which to assess the extent of impacts, is problematic. One solution is to compare potentially degraded estuaries with estuaries deemed to be largely unmodified by human activities. However, there is a perception that individual estuaries are too spatially and temporally dynamic to allow valid comparisons to be made using such an approach. We tested this idea for a commonly used indicator, benthic macroinvertebrates, using a factorial design incorporating both temporal and spatial scales between and within three adjacent meso-tidal river estuaries in northern Tasmania. Variation in macroinvertebrate assemblage structure was analysed using permutational multivariate analysis of variance. Most variance occurred within estuaries (68–82% variance) relative to spatial differences between estuaries (24–14%) corresponding with a strong upstream estuarine gradient and small-scale spatial patchiness. Seasonal variation accounted for 9–4% of total variance indicating that temporal differences were relatively insignificant when contrasted against spatial variability within and between estuaries. We suggest that with sufficient spatial replication at the within estuary-scale, entire estuaries may act as whole reference systems, allowing studies to examine potential impacts within estuaries with spatially diffuse, pre-existing human impacts.

Modern estuarine siliceous spiculites, Tasmania, Australia: A non-polar link to Phanerozoic spiculitic cherts

Biosiliceous sedimentary rocks are well known from the geologic record and many are correctly interpreted to have formed in deep-water or cold-water environments. Shallow non-polar spiculites are also known from the rock record, yet no modern analog has been documented for such environments. Bathurst Harbour, an estuarine system in southwest Tasmania, provides this much-needed modern analog. In this system a sharp halocline separates tannin-rich low-salinity surface waters from clear marine bottom waters. Tannins supply few nutrients and substantially reduce light penetration to bottom environments, resulting in a thinned photic zone and the mixing of deeper-water sub-photic biotas of soft corals, bryozoans, and sponges with other organisms more typical of this temperate shallow-water environment. The well-defined halocline allows a typically marine biota, including echinoderms, to live in bottom waters of this estuarine setting. The bioclastic factory, producing both carbonate and siliceous particles, exists in marine subphotic bottom waters of incised channel and shallow rocky environments along the shoreline. Extensive organic-rich soft sediments in protected embayments generate few bioclasts, but contain allochthonous sponge spicules transported from the adjacent bioclastic factory. Trapping of organic material within the estuarine system lowers sediment pH and promotes dissolution of carbonate biofragments, resulting in preferential preservation of siliceous sponge spicules. This situation implies that many biosiliceous neritic deposits in the rock record may be the result of similar preferential preservation.

Multiple dispersal vectors drive range expansion in an invasive marine species

The establishment and subsequent spread of invasive species is widely recognized as one of the most threatening processes contributing to global biodiversity loss. This is especially true for marine and estuarine ecosystems, which have experienced significant increases in the number of invasive species with the increase in global maritime trade. Understanding the rate and mechanisms of range expansion is therefore of significant interest to ecologists and conservation managers alike. Using a combination of population genetic surveys, environmental DNA (eDNA) plankton sampling and hydrodynamic modelling, we examined the patterns of introduction of the predatory Northern Pacific seastar (Asterias amurensis) and pathways of secondary spread within southeast Australia. Genetic surveys across the invasive range reveal some genetic divergence between the two main invasive regions and no evidence of ongoing gene flow, a pattern that is consistent with the establishment of the second invasive region via a human‐mediated translocation event. In contrast, hydrodynamic modelling combined with eDNA plankton sampling demonstrated that the establishment of range expansion populations within a region is consistent with natural larval dispersal and recruitment. Our results suggest that both anthropogenic and natural dispersal vectors have played an important role in the range expansion of this species in Australia. The multiple modes of spread combined with high levels of fecundity and a long larval duration in A. amurensis suggests it is likely to continue its range expansion and significantly impact Australian marine ecosystems.

Determination of the physical drivers of Zostera seagrass distribution using a spatial autoregressive lag model

Seagrass mapping has become a key tool in understanding the causes of change in seagrass habitats. The present study demonstrates a method for examining relationships between seagrass habitat polygons and environmental data generated by hydrodynamic, wave, catchment and dispersion models. Seagrass abundance data are highly spatially autocorrelated and this effect was corrected using a spatially simultaneous autoregressive lag model (SSARLM). The physical processes that determine the spatial distribution of seagrass in Port Phillip Bay, Australia, were investigated by examining the links between seagrass distribution and abundance and broadscale hydrodynamic (waves, currents), physical (light, depth, salinity and temperature) and catchment (nutrient and suspended sediment concentrations) processes. The SSARLM indicated that the distribution of Zostera spp. meadows is principally constrained by two physical thresholds, namely, wave height or exposure and light. The former excludes seagrasses from colonising wave-exposed coastlines, whereas the latter directly determines the depth profile of seagrasses through its influence on light availability. In total, 95% of all seagrass occurred within grid cells with a mean significant wave height of 33% surface levels. By comparison, variation in water quality, represented by variables such as modelled total nitrogen, suspended solids or salinity, had little influence on seagrass distribution.