Trophic Ecology and Energy Sources for Fish on the Floodplain of a Regulated Dryland River: Macintyre River, Australia

Drylands occupy about one-third of the world's land surface area and rivers in these regions have less predictable flow regimes than those in humid tropical and temperate regions. Australia's dryland river-floodplain systems cycle through recurrent periods of floods and droughts, oflen resulting in extreme hydrological variability. As a result, these systems have been described as having a 'boom and boost' ecology with periods of high productivity associated with flooding. Not surprisingly, flow and its variability have been recognised as major driving forces in the ecological functioning of Australian rivers and responses to flow variability from fish and aquatic invertebrates have been reasonably well described. Furthermore, the reduced amount of water reaching floodplain waterbodies due to river regulation has been held responsible for successional changes in aquatic biota and, consequently, the resources available for both fish and invertebrates. However, information regarding the impacts of water resource development has generally focused on within-channel processes of Australian rivers, not on floodplains, which are arguably more affected by water development. The following dissertation is concerned with how different types of natural and modified floodplain lagoons are able to trophically support their fish communities in the floodplain of the Macintyre River, Border Rivers catchment (QLD/NSW), a regulated dryland river. This study focuses on the influence of flooding and the implications of an extended dry period, and different levels of flow regulation, on the feeding ecology of selected fish species (Ambassis agassizil, Lelopotherapon unicolor and Nematalosa erebi) between 2001 and 2003. Food resources consumed by fish are hypothesised to vary in response to flooding, when inundation of isolated lagoons and vast floodplain areas can result in a burst of primary and secondary productivity. Given the permanently elevated water levels of some regulated floodplain lagoons, fish diets are hypothesised to be less variable in these floodplain habitats in comparison to diets of fish from floodplain lagoons with natural flow and water regime. Feeding ecology is examined firstly, in terms of diet composition of selected fish species, using stomach content analysis, and secondly, in relation to possible energy sources sustaining fish (using stable isotope analysis) in selected floodplain lagoons and a site in the main channel of the Macintyre River. The information produced should allow managers to take variations in food resources, food web structure and dietary ecology into account in management regimes for refugia and dryland systems in general. Factors such as diel and ontogenetic variations in dietary composition and food intake by fish are shown to considerably affect ovemll dietary patterns of each study species. Therefore, it is important to understand the contributions of such factors to the variability of fish dietary patterns before performing studies on resource use by fish in floodplain habitats of the Macintyre River. Major food categories consumed by the study species were zooplankton, aquatic invertebrates and detrital material. Zooplankton was of particular importance as this food item was ingested by all three study species at some stage of their life history. Spatial and temporal variation in diet composition of the study species was mostly associated with changes in prey items available across floodplain habitats and between seasons (summer/winter). The low magnitude of flooding events during the study period is arguably the most likely factor influencing the lack of patterns of variation in fish diets in floodplain habitats subject to flooding, whereas in non-flooded lagoons the observed dietary variation was a consequence of successional changes in composition of the aquatic fauna as the dry season progressed. Water regime had an important effect on differences in fish diet composition across lagoons, but further evaluation of the influence of flooding is needed due to overall lack of major flooding events during the study period. Autochthonous resources, namely plankton, were the basis of the food web and phytoplankton in the seston is the most likely ultimate energy source for fish consumers, via planktonic suspension feeders (zooplankton). Nevertheless, organic mailer could not be disregarded as an important energy source for invertebrates and higher consumers. In general, the present study does not provide support for the major models predicting the ftinctioning of large rivers, such as the River Continuum Concept and Flood Pulse Concept, which argue that allochthonous organic matter either from upstream or from the floodplain are the most important sources of carbon supporting higher consumers. In contrast, the Riverine Productivity Model would be more appropriate to describe the food web and energy sources for consumers in the Macintyre River floodplain as this model suggests that local productivity, based on autochthonous phytoplankton and organic matter, ftiels food webs in large rivers. The results of this study suggest that factors known to affect phytoplankton production in floodplain lagoons (e.g. flow regulation, turbidity and nutrient/herbicide inputs) must be seriously considered in current landscape and water management practices.