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

Abrantes, Kátya Gisela dos Santos (2008) Trophic structure and the importance of terrestrial wetland producers for aquatic food webs in tropical Australian estuaries. PhD thesis, James Cook University.

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Abstract

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

Item ID: 1989
Item Type: Thesis (PhD)
Keywords: estuaries, mangroves, salt marshes, terrestrial wetlands, aquatic habitats, trophic levels, food webs, energy flows, nutrition, stable isotope analysis, Wet Tropics, Dry Tropics, autochthonous carbon, allochthonous carbon, nitrogen, north Queensland, Ross River, sugarcane, seasonality, fishes, invertebrates, penaeid prawns, connectivity, pristine systems, agricultural impacts
Date Deposited: 01 Dec 2008 00:34
FoR Codes: 06 BIOLOGICAL SCIENCES > 0602 Ecology > 060205 Marine and Estuarine Ecology (incl Marine Ichthyology) @ 34%
06 BIOLOGICAL SCIENCES > 0602 Ecology > 060204 Freshwater Ecology @ 33%
03 CHEMICAL SCIENCES > 0304 Medicinal and Biomolecular Chemistry @ 33%
SEO Codes: 96 ENVIRONMENT > 9608 Flora, Fauna and Biodiversity > 960802 Coastal and Estuarine Flora, Fauna and Biodiversity @ 50%
97 EXPANDING KNOWLEDGE > 970105 Expanding Knowledge in the Environmental Sciences @ 50%
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