Scenarios involving future climate and water extraction: ecosystem states in the estuary of Australia\u27s largest river

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Trophic positions of omnivores are not always flexible: Evidence from four species of freshwater crayfish

Omnivores are generally believed to be flexible in their diet and trophic position: seasonal, ontogenetic and site-based differences in trophic position have been observed. We compared consumed and assimilated diet among four species within a group of omnivorous freshwater crayfish, to determine whether species that occur together at a site occupy different trophic positions. Diets of Geocharax falcata, Gramastacus insolitus, Cherax destructor and Euastacus bispinosus (Decapoda: Parastacidae) were compared using stable isotopes (δ13C and δ15N) and gut content analysis across nine sites that varied in their species composition. Gramastacus insolitus consumed mainly plant material across all sites. Geocharax falcata consumed either plants or animals or both at different sites. Its trophic level was consistently similar to G. insolitus, despite differences in gut contents and source for dietary carbon. Cherax destructor consumed animals and had a relatively stable trophic position among sites. Relative trophic position of these three species was consistent across sites and regardless of food consumed, they were positioned as omnivores at a lower trophic level than predators but higher than primary producers and herbivores. Euastacus bispinosus occupied a higher trophic level than other invertebrate species but δ13C levels did not differ among sites. Cherax destructor and G. falcata may show flexibility in food sources and in the assimilation of food that determines their trophic position relative to other crayfish species. In contrast, G. insolitus and E. bispinosus are likely to show both a more fixed diet and less flexible trophic position. Therefore, not all omnivores show the flexible diet and trophic position generally reported in the literature. Some species of omnivorous crayfish may maintain a relatively constant trophic position across sites, seasons or changes in food availability regardless of whether their consumed diet alters or not

Benthic fluxes of nitrogen and phosphorus at southern bluefin tuna Thunnus maccoyii sea-cages

To assess the effects of southern bluefin tuna farming on benthic nutrient cycling, we measured sedimentation rates, porewater nutrients, sedimentary total nitrogen (N) and phosphorus (P) content, and benthic fluxes at 2 tuna sea-cages and associated control (non-farm) sites in South Australia over the course of a farming season. Sedimentation rates exceeded 40 g dry weight m–2 d–1 only at the sea-cage sites, and porewater concentrations of ammonium and phosphate were up to 10 and 100 times that measured at control sites, respectively. The highest ammonium and phosphate fluxes from the sediments into the water column were recorded at sea-cage sites (9962 and 2177 µmol m–2 h–1, respectively) towards the end of the season, and these were in excess of 10 times those recorded at control sites in any month. The annual average of the sediment fluxes associated with southern bluefin tuna farming produced over 7 and 40 times the daily requirements of N and P, respectively, for calculated primary productivity. The high sedimentation rates, porewater concentrations and benthic flux rates recorded at sea-cages at the end of the farming season were not observed 4 mo after the southern bluefin tuna were harvested. These data show that sedimentation rates and sediment geochemistry respond rapidly to farming activities. The risks associated with farming southern bluefin tuna include enrichment of pelagic nutrients from benthic fluxes that can lead to increased primary production. Thus continuation of site fallowing on an annual basis is strongly recommended. Sustainability of the coastal ecosystem supporting southern bluefin tuna farming must consider the regional consequences from these inputs of N and P, occurring not only through increased sedimentation but also direct nutrient availability in the water column.P. R. Lauer, M. Fernandes, P. G. Fairweather, J. Tanner, A. Cheshir