Marine and Freshwater Research

Wet-season flooding causes dietary shifts in tropical freshwater fish by regulating instream productivity, habitat structure and food availability. These dynamics have been comprehensively documented worldwide, but data are limited for Australia’s Wet Tropics rivers. The aim of the present study was to extend our earlier fish–habitat model for these systems by examining the role of trophic dynamics in determining fish assemblage composition. Chlorophyll a and phaeophytin concentrations, benthic and littoral invertebrates and fish were collected at four sites in the lower Mulgrave River under a range of flow conditions. Wet-season flooding caused significant reductions in instream productivity, whereas habitat disturbance reduced densities and abundances of littoral and benthic invertebrates. However, volumetric gut contents of 1360 fish, from 36 species, revealed seasonal shifts in guild membership by only two species, with fish moving between sites to target their preferred prey items – largely irrespective of differences in habitat structure. As a result, the food consumed by the fish community present at each site closely reflected the seasonal availability of food resources. The present paper questions whether fish community composition in small tropical rivers can be accurately predicted from habitat surrogates alone and encourages consideration of constraints imposed by the trophic dynamics and reproductive ecology of fish.

Results of a survey of metal levels in nine species of important commercial fish from New South Wales waters are reported. Muscle samples from 20-30 individuals of each species were analysed for mercury, cadmium, lead, copper and zinc. Eight to 12 individuals of each species were analysed for selenium and arsenic. Of the 232 fish analysed, 231 had concentrations of cadmium, lead, copper and zinc below the National Health and Medical Research Council (NHMRC) standards for these elements in foodstuffs. Several specimens of bream, snapper, mulloway, kingfish, Australian salmon and yellowfin tuna had total mercury concentrations in excess of the NHMRC standard of 0.5 ppm. These fish accounted for approximately 7% of the total number sampled. None of the sea mullet, flathead and tailor sampled exceeded the standard for mercury. The mercury in all species sampled occurred almost entirely as methyl mercury. Of the 95 fish analysed for arsenic and selenium, 20 fish (21 %) had arsenic concentrations equal to or greater than the NHMRC standard for selenium. The health risks associated with the presence of mercury and arsenic in these species are discussed.

The South African Hartbeespoort Dam is known for the occurrence of heavy Microcystis blooms. Although a few other cyanobacterial genera have been described, no detailed study on those cyanobacteria and their potential toxin production has been conducted. The diversity of cyanobacterial species and toxins is most probably underestimated. To ascertain the cyanobacterial composition and presence of cyanobacterial toxins in Hartbeespoort Dam, water samples were collected in April 2011. In a polyphasic approach, 27 isolated cyanobacterial strains were classified morphologically and phylogenetically and tested for microcystins (MCs), cylindrospermopsin (CYN), saxitoxins (STXs) and anatoxin-a (ATX) by liquid chromatography–tandem mass spectrometry (LC–MS/MS) and screened for toxin-encoding gene fragments. The isolated strains were identified as Sphaerospermopsis reniformis, Sphaerospermopsis aphanizomenoides, Cylindrospermopsis curvispora, Raphidiopsis curvata, Raphidiopsis mediterrranea and Microcystis aeruginosa. Only one of the Microcystis strains (AB2011/53) produced microcystins (35 variants). Forty-one microcystin variants were detected in the environmental sample from Hartbeespoort Dam, suggesting the existence of other microcystin producing strains in Hartbeespoort Dam. All investigated strains tested negative for CYN, STXs and ATX and their encoding genes. The mcyE gene of the microcystin gene cluster was found in the microcystin-producing Microcystis strain AB2011/53 and in eight non-microcystin-producing Microcystis strains, indicating that mcyE is not a good surrogate for microcystin production in environmental samples.

Leaf and other litter falling in a mangrove woodland was estimated by litter trapping for 13 months. Mean annual litter fall was 5.8 t ha-1 year-1 (dry matter) of which 79% was leaf material. Largest fortnightly collections of litter were found in the summer months, and little litter fell in winter. Data suggested that mangrove litter is rapidly incorporated into the rest of the estuarine ecosystem: the leaf litter layer in the swamp was estimated on two occasions (about 9 g m-2 in winter and about 62 g m-2 in summer, dry leaf material) and a litter-bag experiment gave a leaf decomposition half- time of about 8 weeks.

It has been proposed that current levels of ultraviolet B (UVB) radiation could cause lethal or sublethal effects on fish embryos located in the upper layers of the water column. Observed levels of UVB off central Chile (36°S, 73°W) indicate that planktonic fish embryos could be exposed to harmful UVB radiation. From July 2011 to January 2012 embryos from anchoveta (Engraulis ringens) and common sardine (Strangomera bentincki) were used to test experimentally whether the UVB levels in central Chile produce lethal or sublethal effects in epipelagic fish embryos. Simultaneously, whether the embryos might be exposed to harmful UVB levels in the field was investigated. Our experimental results show that UVB may cause a decrease in hatching success, changes in buoyancy and embryonic malformations. These results, along with the observed vertical distribution of embryos and UVB radiation levels in the field during late spring suggest that lethal and sublethal effects may be occurring in the embryos of both species.

Recent work has evaluated the gap between current and compliant suspended sediment losses due to farming across England and Wales and the potential for change by 2015. The study adopted the guideline annual average sediment threshold of 25 mg L–1 cited by the European Union Freshwater Fish Directive. Compliance testing required national scale sediment source apportionment to assess the current contributions of diffuse agricultural and urban sector losses, channel bank erosion and point source discharges to the total suspended sediment loads delivered to all rivers. Results suggested that the agricultural sector dominates present day (year 2000) sediment inputs to rivers (1929 kt = 76%) compared with eroding channel banks (394 kt = 15%), diffuse urban sources (147 kt = 6%) and point source discharges (76 kt = 3%). Projected change in farming by 2015, represented by the Business As Usual forecast of structural developments and predicted uptake of sediment mitigation methods, suggested a 9% reduction in sediment losses from the agricultural sector across England and Wales. Further mitigation of diffuse agricultural sediment transfers to watercourses will therefore be necessary to ensure ‘good ecological status’ in some catchments. A similar modelling framework could be applied in other countries.

The MSE approach provides a simulation-based framework within which harvest strategies, stock assessment methods, performance indicators and research programmes can be compared. This approach has been used in the Australian South East Fishery (SEF) to assess harvest strategies for the over-exploited eastern gemfish resource and to compare different levels of discard monitoring for blue grenadier. The main challenges to use of the MSE approach in the SEF are poorly specified management objectives and the lack of quantitative stock assessments on which to build operating models for many of the species.

Hypotheses to explain the source of the 1011 tons of salt in groundwaters of the Murray Basin, south-eastern Australia, are evaluated; these are (a) mixing with original sea water, (b) dissolution of salt deposits, (c) weathering of aquifer minerals and (d) acquisition of solutes via rainfall. The total salinity and chemistry of many groundwater samples are similar to sea-water composition. However, their stable isotopic compositions (δ18O= –6.5 ‰; δ2H = –35) are typical of mean winter rainfall, indicating that all the original sea water has been flushed out of the aquifer. Br/Cl mass ratios are approximately the same as sea water (3.57 x 10-3) indicating that NaCl evaporites (which have Br/Cl<10-4) are not a significant contributor to Cl in the groundwater. Similarly, very low abundances of Cl in aquifer minerals preclude rock weathering as a significant source of Cl. About 1.5 million tons of new salt is deposited in the Murray–Darling Basin each year by rainfall.The groundwater chemistry has evolved by a combination of atmospheric fallout of marine and continentally derived solutes and removal of water by evapo-transpiration over tens of thousands of years of relative aridity. Carbonate dissolution/precipitation, cation exchange and reconstitution of secondary clay minerals in the aquifers results in a groundwater chemistry that retains a ‘sea-water-like’ character.

Nature-based coastal defence approaches are increasingly being explored to protect shorelines from erosion and climate change-related threats such as sea level rise, storms and coastal flooding. At Mubarraz, near Abu Dhabi in the Arabian Gulf, mass plantings of &gt;500000 nursery-reared seedlings of the mangrove Avicennia marina were conducted over 30 years in response to erosion of an artificially created, 17-km-long causeway of dredged material. Planting efforts resulted in successfully established mangrove vegetation along 6.7km of shoreline, covering an area of 16.5ha. Survival rates of planted mangrove seedlings to healthy saplings or trees were in the order of 26% averaged over all years, with best results obtained at planting sites established in artificial tidal channels excavated parallel to the causeway, ensuring appropriate tidal hydrological conditions. The mangroves and causeway have attracted a high diversity of birds (48 species), some of which breed on the island, and there are significant densities of crabs and gastropod snails in the mangroves. These results demonstrate that planting mangroves on dredged material is feasible (even under extreme climatic conditions) and may offer a cost-effective alternative for shoreline protection, with added benefits for biodiversity through the creation of new habitats.