Getting the Biggest Bang for Your Duck: Understanding the Influence of Hydrology on Trophic Dynamics and Resource Provision in a Managed Wetland to Inform Optimal Water Management

Abstract

<p>Floodplain wetlands are among the most threatened ecosystems in the world. As such, they are frequent targets for management with environmental water with programs aimed at conservation of higher-order, iconic species such as waterbirds or fish. Very few studies have investigated the mechanisms underlying these population level responses, and as a consequence these mechanisms remain poorly understood. There are increasing calls to incorporate trophic dynamics into design and monitoring of environmental flow programs, with a focus on identifying functional rather than population-level responses to management. This thesis aims to use existing and emerging techniques in food web ecology and ecosystem energetics to investigate mechanisms of response to inundation in floodplain wetlands to better inform environmental water management.</p> <p>Following an extensive review of existing literature, a novel conceptual model of wetland productivity was developed identifying potential links between long and short-term hydrology and trophic dynamics, with specific consideration of resource quality. It proposes that long-term hydrologic maintenance of core wetland habitats will provide detrital-based energy refuges, and that, when hydrologically connected to intermittently inundated habitats that are fuelled by high-quality algal food resources, will substantially improve the trophic carrying capacity of wetlands. The results of a field-based observational study conducted over the course of a managed inundation event partially support this hypothesis.</p> <p>Hydroperiod is recognised as an important driver of community and food web structure in temporary water bodies. I tested the hypothesis that increased hydroperiod would result in increased food web complexity and food chain length and that these changes would relate to predictable patterns of community assembly related to life history strategies of invertebrate taxa. The study results supported this hypothesis.</p> <p>I found that the invertebrate community was initially dominated by small-bodied crustaceans and molluscs which likely emerged from desiccation-resistant life stages. This community resulted in a low-complexity food web of short chain length. Immigration and reproduction of largely non-predatory insect taxa in the Filling stage resulted in a mostly lateral expansion of the food web. A subsequent influx of immigrant insect predators in the Drying stage resulted in a significant peak in food chain length and contraction in breadth as the system approached collapse and predation pressure impacted community structure in the lower and intermediate trophic levels.</p> <p>In addition to investigating the influence of hydroperiod on food web structure, I quantified the total amount of energy produced in the food web at each of three stages of inundation and found a similarly strong influence of increasing hydroperiod on increasing energy production. I investigated energy density on a volumetric basis (i.e. per litre of inundation) and also on a per taxon basis. There was a peak in energy density in the Drying stage as might be expected as a result of habitat contraction. However, energy density was also found to increase over time on a per taxon or functional feeding group (FFG) basis, irrespective of inundation volume. I found a strong correlation between energy density and fatty acid concentration (ρ = 0.669, p = 0.0001), which in turn was related to taxon/FFG occurrence through time. Fatty acid concentration (and therefore energy density) was highest in shredders and predators. The occurrence of these taxa at different stages also had a strong influence on food web structure with shredders and predators contributing to the lateral and vertical expansion of trophic niche space, respectively.</p> <p>To my knowledge, this is the first study of its kind, tracing energy production and food web structure through time in an Australian wetland and it highlights an interesting area of future research. Understanding the production and transfer of energy, and therefore potential carrying capacity of a system, is vital in setting realistic management goals. Incorporating measures of energy density into food web metrics would provide a valuable additional dimension in understanding trophic dynamics in temporary aquatic systems.</p> <p>I also modelled predator-prey interactions for aquatic insects using multiple tracers to identify resource use in the lower orders of the aquatic food web, and sought links to hydrology. To do this, I employed a relatively new method incorporating stable isotopes of δ13C, δ15N and selected fatty acids in the Bayesian mixing model MixSIAR. A significant challenge in employing this method is the lack of robust calibration co-efficients (or trophic discrimination factors [TDF]) for fatty acid metabolism for many taxa. I identified a set of TDFs that performed reasonably well when considered against known diets. However, this study served to highlight the need for more experimental work to develop robust data for model parameterisation.</p> <p>Overall, this project identified several interesting areas for further research including incorporation of energy density measures into food web studies, identifying links between trophic level and energy density, and experimental feeding trials to identify energy transformation pathways in aquatic insect taxa. Several important links to hydrology were revealed and the results offer some potentially valuable considerations for water management in floodplain wetlands.</p>