Water requirements for estuaries: relationships between freshwater inflows, hydrodynamics and larval fish dynamics in estuarine lagoons, south-western Australia

[Truncated abstract] Estuaries and Intermittently Closed and Open Lakes and Lagoons (ICOLLs) are ecologically important ecosystems that link catchment and oceanic processes through their role in biological production, sediment transport, and biogeochemical processing. They are influenced by both freshwater and marine exchanges, and depending on the nature of these, display a variety of physiographic forms and associated biophysical functions. Because they "accumulate" the effects of upstream disturbances, estuaries and coastal lagoons are particularly vulnerable to anthropogenic influence. Alteration to freshwater flow regimes represents one critically important, contemporary disturbance influencing the health and sustainability of estuarine ecosystems. Although relationships between streamflow and ecology are relatively well studied in freshwater reaches, those issues in estuaries remain poorly understood. River regulation in south-western Australia represents a major environmental management concern. Consequently, estuaries and ICOLLs on the south coast of Western Australia are currently the focus of regional environmental flow assessment. They represent a variety of estuarine geomorphologies, including permanently open estuaries, ICOLLs and normally closed lagoons, reflecting spatially variable rainfall, runoff and oceanic exchange. Consequently, while there is substantial knowledge on some estuaries and larger ICOLLs, transferability of this knowledge among south coast estuaries is problematic. This is especially true for small transitional ICOLLs on the central south coast where significant knowledge gaps have restricted environmental flow assessment. ... Iterative feedback mechanisms, undertaken in a Bayesian framework, allow for a-priori understanding and hypotheses to be refined and updated. The approach allows for the inclusion of various data sources, including expert judgment. Key steps in the framework include examination of the links between multiple ecosystem assets, description of the current and desired future states of those resources, investigation of existing and/or potential alteration to the natural dynamics of freshwater inputs and the establishment of clearly-stated and measurable objectives for environmental flow management. The research outcomes described here represent substantial advances, not only to the understanding of the ecology of small, transitional ICOLLs (and more broadly ICOLLs in general), but also to the development of key hydro-ecology links in estuarine ecosystems. The results illustrate that although there are broad similarities with other larger temperate ICOLLs, smaller transitional ICOLLs located on the central south coast of Western Australia are strongly influenced by distinctive dynamics in freshwater and oceanic exchange. Larval fish assemblages demonstrated that these distinctive environmental conditions are likely to influence generally more depauperate biodiversities compared with those of larger ICOLLs. The relationships between freshwater inflows, hydrodynamics, oceanic connection and larval fish dynamics detailed here addresses previously identified knowledge gaps that restricted the determination of water requirements of priority estuaries of Western Australias south coast. As such, the research outcomes facilitate prediction of important ecological responses in small transitional ICOLLs of Western Australia to alteration in freshwater inflows.