Iain M. Suthers

An avenue of eddies: Quantifying the biophysical properties of mesoscale eddies in the Tasman Sea

[1] The Tasman Sea is unique - characterised by a strong seasonal western boundary current that breaks down into a complicated field of mesoscale eddies almost immediately after separating from the coast. Through a 16-year analysis of Tasman Sea eddies, we identify a region along the southeast Australian coast which we name ‘Eddy Avenue’ where eddies have higher sea level anomalies, faster rotation and greater sea surface temperature and chlorophyll a anomalies. The density of cyclonic and anticyclonic eddies within Eddy Avenue is 23% and 16% higher respectively than the broader Tasman Sea. We find that Eddy Avenue cyclonic and anticyclonic eddies have more strongly differentiated biological properties than those of the broader TasmanSea, as a result of larger anticyclonic eddies formed from Coral Sea water depressing chl. a concentrations, and for coastal cyclonic eddies due to the entrainment of nutrient-rich shelf waters. Cyclonic eddies within Eddy Avenue have almost double the chlorophyll a (0.35 mg m 3 ) of anticyclonic eddies (0.18 mg m 3 ). The average chlorophyll a concentration for cyclonic eddies is 16% higher in Eddy Avenue and 28% lower for anticyclonic eddies when compared to the Tasman Sea. With a strengthening East Australian Current, the propagation of these eddies will have significant implications for heat transport and the entrainment and connectivity of plankton and larval fish populations. Citation: Everett, J. D., M. E. Baird, P. R. Oke, and I. M. Suthers (2012), An avenue of eddies: Quantifying the biophysical properties of mesoscale eddies in the Tasman Sea, Geophys. Res. Lett., 39, L16608, doi:10.1029/ 2012GL053091.

Dynamic estuarine plumes and fronts: importance to small fish and plankton in coastal waters of NSW, Australia

Abstract In 1990, low density estuarine plumes in the vicinity of Botany Bay, Australia, extended up to 11 km across a narrow continental shelf ( ca 25 km) on ebb tides. The shape and seaward extent of plumes varied according to a combination of state of the tide, freshwater input and the direction and intensity of coastal currents. Offshore plumes dissipated on the flood tide and fronts reformed at the entrance of Botany Bay. Major differences in the abundance and composition of ichthyoplankton and other zooplankton were found over a 400–800 m stretch of water encompassing waters of the plume, front and ocean on seven occasions. For example, highest abundances of the fishes Gobiidae, Sillaginidae, Gerreidae and Sparidae as well as barnacle larvae and fish eggs were found in plumes. Cross-shelf distribution patterns of zooplankton, therefore, are influenced by plumes. Distinct assemblages of plankters accumulated in fronts, e.g. fishes of the Mugilidae and Gonorynchidae and other zooplankters (e.g. Jaxea sp.). Accumulation in fronts was variable and may relate to variable convergence according to the tide. We argue that plumes provide a significant cue to larvae in coastal waters that an estuary is nearby. Moreover, although many larvae may be retained in the turbid waters of plumes associated with riverine input, larvae are potentially exported in surface waters on ebb tides.

To fit or not to fit: evaluating stable isotope mixing models using simulated mixing polygons

Summary Stable isotope analysis is often used to identify the relative contributions of various food resources to a consumers diet. Some Bayesian isotopic mixing models now incorporate uncertainty in the isotopic signatures of consumers, sources and trophic enrichment factors (e.g. SIAR, MixSIR). This had made model outputs more comprehensive, but at the expense of simple model evaluation, and there is no quantitative method for determining whether a proposed mixing model is likely to explain the isotopic signatures of all consumers, before the model is run. Earlier linear mixing models (e.g. IsoSource) are easier to evaluate, such that if a consumers isotopic signature is outside the mixing polygon bounding the proposed dietary sources, then mass balance cannot be established and there is no logical solution. This can be used to identify consumers for exclusion or to reject a model outright. This point-in-polygon assumption is not inherent in the Bayesian mixing models, because the source data are distributions not average values, and these models will quantify source contributions even when the solution is very unlikely. We use a Monte Carlo simulation of mixing polygons to apply the point-in-polygon assumption to these models. Convex hulls (‘mixing polygons’) are iterated using the distributions of the proposed dietary sources and trophic enrichment factors, and the proportion of polygons that have a solution (i.e. that satisfy point-in-polygon) is calculated. This proportion can be interpreted as the frequentist probability that the proposed mixing model can calculate source contributions to explain a consumers isotopic signature. The mixing polygon simulation is visualised with a mixing region, which is calculated by testing a grid of values for point-in-polygon. The simulation method enables users to quantitatively explore assumptions of stable isotope analysis in mixing models incorporating uncertainty, for both two- and three-isotope systems. It provides a quantitative basis for model rejection, for consumer exclusion (those outside the 95% mixing region) and for the correction of trophic enrichment factors. The simulation is demonstrated using a two-isotope study (15N, 13C) of an Australian freshwater food web.

Rain reverses diel activity rhythms in an estuarine teleost

Activity rhythms are ubiquitous in nature, and generally synchronized with the day–night cycle. Several taxa have been shown to switch between nocturnal and diurnal activity in response to environmental variability, and these relatively uncommon switches provide a basis for greater understanding of the mechanisms and adaptive significance of circadian (approx. 24 h) rhythms. Plasticity of activity rhythms has been identified in association with a variety of factors, from changes in predation pressure to an altered nutritional or social status. Here, we report a switch in activity rhythm that is associated with rainfall. Outside periods of rain, the estuarine-associated teleost Acanthopagrus australis was most active and in shallower depths during the day, but this activity and depth pattern was reversed in the days following rain, with diurnality restored as estuarine conductivity and turbidity levels returned to pre-rain levels. Although representing the first example of a rain-induced reversal of activity rhythm in an aquatic animal of which we are aware, our results are consistent with established models on the trade-offs between predation risk and foraging efficiency.

Spatial and ontogenetic variation in the diet of wild and stocked mulloway (Argyrosomus japonicus, sciaenidae) in Australian estuaries

Prey importance and ontogenetic transitions in the diet of stocked and wild mulloway (Argyrosomus japonicus) were compared between a southeast Australian reverine estuary and a coastal lagoon. Stomach content analysis of fish captured from these estuaries in 1997–1979, 1997–1998, and 2003–2005 revealed size-specific and estuary-specific diets. Mysid shrimp were most common in diets of fish <250 mm total length (TL), and prawns were common in diets of fish measuring 301–450 mm. Forage fish were most abundant in diets of mulloway >500 mm. Index of Relative Importance (IRI) of forage fish increased with TL, while IRI of mysids decreased with TL. Prawn IRI was greatest for fish 150–600 mm TL. Comparisons between benthic resources and dietary composition revealed that Georges River mulloway consumed prey categories in proportions similar to those in their environment. No mysid shrimp were detected in the coastal lagoon or in the diet of mulloway captured there; growth was comparable to the Georges River. Hatchery-reared fish fed<16 d after stocking, indicating normal behavioural adaptation after release. Dietary information can be used to optimize stocking locations, times, and densities, as well as estimate potential effects of mulloway on potential prey and wild conspecifics.

Comparative persistence of marine fish larvae from pelagic versus demersal eggs off southwestern Nova Scotia, Canada

Two groupings of larval fish were repeatedly identified by principal component analyses of larval densities from four broad-scale surveys during the spring and summer of 1985–1987 off southwestern Nova Scotia, Canada. Larvae originating from pelagic eggs (four species within Gadidae and Pleuronectidae) constituted one group, which were uniformly distributed over the sampling area with densities not correlated with bathymetry, although nearly all spawning occurs on the shallow western cap of Browns Bank, 100 km offshore. Larvae from demersal eggs (five species within Pholidae, Stichaeidae, Cottidae, Agonidae) constituted the second group, which dominated the shallow-water environments both inshore and on Browns Bank. Lower patchiness indices were evident amongst larvae from pelagic eggs in small and large sampling-gear collections (average 3.4 and 3.1, respectively) compared to fish hatching from demersal eggs (average 5.1 and 4.6). Fine-scale nearshore surveys over a 5 wk period in 1987 also showed that larvae of demersal eggs had a less variable distribution along an inshoreoffshore transect. Larvae from demersal eggs appear spatially persistent through the release of well-developed larvae from non-drifting eggs. These conclusions are consistent with other studies over a range of spatial scales in temperate and tropical environments, demonstrating that single-species models of larval dispersal are inadequate to account for the distributional patterns of larval fish in general.

Fish Assemblages on Estuarine Artificial Reefs: Natural Rocky-Reef Mimics or Discrete Assemblages?

If the primary goal of artificial reef construction is the creation of additional reef habitat that is comparable to adjacent natural rocky-reef, then performance should be evaluated using simultaneous comparisons with adjacent natural habitats. Using baited remote underwater video (BRUV) fish assemblages on purpose-built estuarine artificial reefs and adjacent natural rocky-reef and sand-flat were assessed 18 months post-deployment in three south-east Australian estuaries. Fish abundance, species richness and diversity were found to be greater on the artificial reefs than on either naturally occurring reef or sand-flat in all estuaries. Comparisons within each estuary identified significant differences in the species composition between the artificial and natural rocky-reefs. The artificial reef assemblage was dominated by sparid species including Acanthopagrus australis and Rhabdosargus sarba. The preference for a range of habitats by theses sparid species is evident by their detection on sand-flat, natural rocky reef and artificial reef habitats. The fish assemblage identified on the artificial reefs remained distinct from the adjacent rocky-reef, comprising a range of species drawn from naturally occurring rocky-reef and sand-flat. In addition, some mid-water schooling species including Trachurus novaezelandiae and Pseudocaranx georgianus were only identified on the artificial reef community; presumably as result of the reefs isolated location in open-water. We concluded that estuarine artificial reef assemblages are likely to differ significantly from adjacent rocky-reef, potentially as a result of physical factors such as reef isolation, coupled with species specific behavioural traits such as the ability of some species to traverse large sand flats in order to locate reef structure, and feeding preferences. Artificial reefs should not be viewed as direct surrogates for natural reef. The assemblages are likely to remain distinct from naturally occurring habitat comprised of species that reside on a range of adjacent natural habitats.

The 13C, 15N and 34S signatures of a rocky reef planktivorous fish indicate different coastal discharges of sewage

We assessed the effects of primary, secondary and tertiary treated sewage on the stable isotope composition of the viscera and muscle of the zooplanktivorous eastern hula fish (Plesiopidae: Trachinops taeniatus). Fish were collected from three regions during three consecutive summer months for 2 years. In comparison to fish from control sites, the muscle δ15N of fish at outfall sites was significantly enriched by secondary and tertiary treated effluent. We estimate that 30–50% of nitrogen in hula fish near outfalls may be sewage-derived. The δ34S content of muscle was similar at all regions and sites (20–21‰), although it was significantly depleted by 1–2‰ at the tertiary treated outfall site. Detection of a comparatively minor volume of effluent ( 1000 ML day–1. With increasing upgrades to sewage treatment, stable isotopes may become useful tracers of anthropogenic nutrients in an oligotrophic environment.

Consistent timing of juvenile fish recruitment to seagrass beds within two Sydney estuaries

Recruitment patterns of juvenile Rhabdosargus sarba (Sparidae) and Pelates sexlineatus (Terapontidae) were examined by frequent (1–4 weeks) beach seining of seagrass beds in Sydney, south-east Australia. Two sites within each of two estuaries (Botany Bay and Pitt Water) were sampled for one year. One site within Botany Bay was sampled for 3 years. A total of 12 824 juveniles of R. sarba and 7037 juveniles of P. sexlineatus were collected. R. sarba recruited in 4 pulses during winter/spring, while P. sexlineatus recruited in 6 pulses during summer/autumn, and the timing of recruitment events was consistent among locations and years. P. sexlineatus recruitment coincided with new moons, but R. sarba recruitment dates were less precisely determined. Predictable annual recruitment patterns result in temporal partitioning of seagrass habitat between these two abundant estuarine species. Spatial differences in magnitude of recruitment events among sites reflected patterns of estuarine circulation.

Effects of sewage effluent discharge on the abundance, condition and mortality of hulafish, Trachinops taeniatus (Plesiopidae)

The effects of sewage effluent on the abundance, condition (length and reproduction) and mortality of hulafish Trachinops taeniatus were investigated at multiple outfall and control locations on the central coast of New South Wales, Australia. Underwater visual surveys found consistently fewer T. taeniatus at locations where sewage was discharged compared to control locations. The condition of T. taeniatus was investigated by comparing mean length and reproductive indices of fish from two outfall and two control locations. Fish from the largest outfall location were significantly smaller in comparison with fish from control locations. Gravid female fish from outfall locations had similar gonadosomatic indices but a significantly greater number of eggs and smaller size of eggs in comparison with fish from control locations. Mortality of T. taeniatus was investigated during 2-week, in situ, caging experiments at multiple locations and times and 80% of fish survived, although mortalities of up to 73% per cage were recorded at one sewage outfall. T. taeniatus may be a suitable environmental indicator species of sewage pollution.