Trophic structure and the importance of terrestrial wetland producers for aquatic food webs in tropical Australian estuaries

Estuaries support a great density and diversity of life and are traditionally considered to\ud be important nursery areas for a variety of species, providing abundant and essential\ud food supply and refuge from predation for juveniles of several fish and invertebrate\ud species. However, to date no study has provided unequivocal evidence supporting this\ud paradigm. In fact, recent studies based on the analysis of stable isotopes have shown\ud that the importance of estuarine terrestrial wetland habitats such as mangroves and\ud salt marsh in supplying energy to animals in adjacent aquatic habitats is not as\ud significant as once thought. The objective of the present thesis is use stable isotopic\ud analysis to clarify the importance of terrestrial wetland productivity as a source of\ud energy for estuarine communities in the Australian Wet and Dry Tropics and to study\ud the processes of energy flow taking place in these systems. Overall, material of\ud terrestrial wetland origin was found to be incorporated into estuarine food webs in\ud Tropical Australia. However, this importance is dependent on several physical and\ud ecological factors including productivity of the different habitats, type and extension of\ud wetland vegetation and connectivity.\ud In a first study, stable isotope analysis of carbon and nitrogen were used to analyse\ud processes of energy flow and assess the extend to which carbon fixed by terrestrial\ud plants is incorporated into adjacent aquatic food webs in two intermittently connected\ud estuarine pools in the Ross River floodplain in North Queensland, Australia. The two\ud pools differed in surrounding vegetation as one was surrounded by mangroves and the\ud other by the salt couch Sporobolus virginicus. Since δ13C values of C3 mangroves (low\ud δ13C) are very different from those of the C4 salt couch (high δ13C), it was possible to\ud determine the importance of terrestrial wetland producers by comparing isotope values\ud of consumers between sites. The IsoSource model was also used to clarify the\ud importance of the different potential sources to consumers. An incorporation mangrove\ud and S. virginicus material was detected for several fish and invertebrate species at both\ud sites, indicating that carbon of terrestrial origin is incorporated in the estuarine food\ud web. A linear negative relationship between δ13C and δ15N was also detected for\ud primary producers, primary consumers and secondary consumers at the Sporobolus\ud pool. This relationship was similar for the different trophic levels and was found to be\ud useful to calculate trophic positions. A food web of ~3.5 trophic levels was found at\ud both pools.\ud In a more detailed study, δ13C and δ15N analysis was used to determine the extent to\ud which carbon of terrestrial origin is important for nutrition of juveniles of four penaeid\ud prawn species, and to detect and describe ontogenetic variations in diet. These\ud species were selected because penaeids are known to depend on estuarine wetland\ud habitats such as mangroves and salt marsh at their juvenile stage. Although an\ud incorporation of mangrove and salt marsh carbon was detected, it was not of a major\ud importance for any species, and autochthonous sources seemed more important.\ud Ontogenetic shifts in diet were detected for Penaeus (Fenneropenaeus) merguiensis,\ud Metapenaeus bennetae and Penaeus esculentus, and corresponded to an increase in\ud mean trophic level as well as to changes in the ultimate sources of energy.\ud In a broader scale study, the incorporation of terrestrial wetland productivity in\ud estuarine food webs was studied in four open estuarine systems in Tropical Australia.\ud These included a near-pristine system in the Wet Tropics (Deluge Inlet), two impacted\ud systems in the Wet Tropics (Victoria and Half Moon Creeks), and a near pristine\ud system in the Dry Tropics (Blacksoil Creek). Incorporation of mangrove derived carbon\ud was detected for Deluge Inlet and Victoria Creek and incorporation of carbon of\ud sugarcane origin was also detected for fish from Victoria Creek. The degree of\ud incorporation of mangrove carbon into estuarine food webs seemed to relate directly to\ud the type and extent of mangrove vegetation adjacent to the estuary. Trophic structure\ud differed between estuaries, but in all areas a constant trophic length with about four\ud trophic levels was detected. Stable isotope results also suggest a high level of\ud omnivory and diet overlap between fish species at Deluge Inlet, Half Moon Creek and\ud Blacksoil Creek, but not for the agriculture impacted system of Victoria Creek, which\ud can be a reflection of the great level of anthropogenic impact in this area.\ud In a final study, the seasonality in importance of autochthonous and allochthonous\ud carbon for aquatic communities in six intermittently connected estuarine areas of the\ud Australian Dry Tropics was investigated. Results varied between sites, depending of\ud site-specific ecological conditions. The hydrology regime was a major factor controlling\ud the sources of energy in these areas, controlling the amount of terrestrial material\ud available to aquatic animals throughout the year and allowing the presence of an\ud energetic connectivity between the terrestrial and aquatic environments. An important\ud seasonal variation in the main sources of energy was detected in two systems, where a\ud greater incorporation of carbon of terrestrial origin was present after the wet season.\ud Hence, aquatic food webs may rely alternatively on autochthonous and allochthonous\ud sources of energy, depending on the season. Trophic organization, including level of\ud omnivory, diet overlap and trophic length, was also found to differ between systems\ud and seasons due to differences in species composition, resource availability,\ud connectivity, and type and level of environmental disturbances. While trophic length\ud seems to be similar between open estuarine areas, with food webs having ~4 trophic\ud levels, in intermittently connected areas trophic length was more variable between\ud systems, with between 3.2 and 4 trophic levels

Do exotic salmonids feed on native fauna after escaping from aquaculture cages in Tasmania, Australia?

Atlantic salmon, Salmo salar, and rainbow trout, Oncorhynchus mykiss, are farmed in Tasmania, Australia, where fish sometimes escape into the natural environment. If escapees are able to survive and feed on native fauna, it is likely that they will have ecosystem impacts. Stomach content, body condition (muscle lipid content and Fulton' K), stable isotope, and fatty acid analysis were used to determine if escaped salmonids feed on native fauna. Results indicate that, in general, escaped salmonids do not feed on native fauna. Salmonids loose condition after escaping, and escapee stomachs were mostly empty or contained non-nutritious material or feed pellets. Nevertheless, almost a quarter of rainbow trout stomachs contained native fauna. The majority of escapees had biochemical composition similar to caged animals, indicating that these fish had not switched to feed on local food sources. However, a small fraction of escapees conclusively showed changes in biochemical parameters indicative of a shift to feeding on native fauna. Given the numbers and frequency of escapes, this can have an important impact on native species and on the ecology of Macquarie Harbour

Importance of freshwater flow in terrestrial–aquatic energetic connectivity in intermittently connected estuaries of tropical Australia

δ13C was used to identify seasonal variations in the importance of autochthonous and allochthonous sources of productivity for fish communities in intermittently connected estuarine areas of Australia’s dry tropics. A total of 224 fish from 38 species were collected from six intermittently connected estuarine pools, three in central Queensland (two dominated by C3 forest and one by C4 pasture) and three in north Queensland (one dominated by C3 and two by C4 vegetation). Samples were collected before and after the wet season. Fish collected in the two forested areas in central Queensland had the lowest δ13C, suggesting a greater incorporation of C3 terrestrial material. A seasonal variation in δ13C was also detected for these areas, with mean δ13C varying from −20 to −23‰ from the pre- to the post-wet season, indicating a greater incorporation of terrestrial carbon after the wet season. Negative seasonal shifts in fish δ13C were also present at the pasture site, suggesting a greater dependence on carbon of riparian vegetation (C3 Juncus sp.) in the post-wet season. In north Queensland, terrestrial carbon seemed to be incorporated by fish in the two C4 areas, as δ13C of most species shifted towards slightly heavier values in the post-wet season. A two-source, one-isotope mixing model also indicated a greater incorporation of carbon of terrestrial origin in the post-wet season. However, no seasonal differences in δ13C were detected for fish from the forested area of north Queensland. Overall, hydrologic connectivity seemed to be a key factor in regulating the ultimate sources of carbon in these areas. It is therefore important to preserve the surrounding habitats and to maintain the hydrologic regimes as close to natural conditions as possible, for the conservation of the ecological functioning of these areas