Brief floodplain inundation provides growth and survival benefits to a young-of-year fish in an intermittent river threatened by water development.

Riverine floodplains are highly productive habitats that often act as nurseries for fish but are threatened by flow regulation. The Fitzroy River in northern Australia is facing development, but uncertainty exists regarding the extent to which floodplain habitats deliver benefits to fish, particularly given the brevity of seasonal floodplain inundation. We investigated the growth rate of young-of-year bony bream (Nematalosa erebi) in main channel and ephemeral floodplain habitats using age derived from otolith daily increments. We also investigated potential mechanisms influencing growth and modelled the consequences of differential growth rate on survival. Our results revealed higher growth occurred exclusively on the floodplain and that zooplankton biomass was the best predictor of growth rate. Modelling indicated that elevated growth rate in high-growth floodplain pools (top 25th percentile) could translate into substantial increases in survivorship. The positive effect of zooplankton biomass on growth was moderated under highly turbid conditions. Temperature had a minor influence on growth, and only in floodplain habitats. Our results indicate ephemeral floodplain habitats can deliver substantial growth and survival benefits to young-of-year fish even when floodplain inundation is brief. This study highlights the need to ensure that water policy safeguards floodplain habitats due to their important ecological role.

New insights into the food web of an Australian tropical river to inform water resource management.

Rivers around the world are threatened by altered flow due to water resource development. Altered flow can change food webs and impact riverine energetics. The Fitzroy River, in northern Australia, is targeted for development but uncertainty remains about the sources of carbon supporting the food web, particularly in the lowlands-the region most likely to be impacted by water extraction. This study used stable isotopes to investigate if algal biofilm is the main carbon source sustaining fish in lowland habitats. We also sought evidence that large-bodied migratory fish were transporting remote carbon around the system. Our results revealed that local algal biofilm carbon was the dominant source of energy sustaining fish in wet season floodplain habitats, but that fish in main-channel pools during the dry season were increasingly dependent on other carbon sources, such as leaf litter or phytoplankton. We found no evidence that large-bodied fish were transporting remote carbon from the floodplain or estuary into the lower main-channel of the river. We recommend that water planners take a precautionary approach to policy until sufficient food web evidence is amassed.

Multi-scale characterisation of stream nutrient and carbon dynamics in sandy near coastal catchments of south-western Australia.

Managers tasked with repairing degraded stream ecosystems require restoration strategies that are tailored to local and regional characteristics. Emerging evidence suggests that local reach-scale approaches may be as effective, if not more so, than catchment-scale actions in highly permeable coastal landscapes, particularly if there is hydraulic connectivity to shallow groundwater and where recharge is strongly seasonal. This study assessed the relative influence of catchment-scale land use and reach-scale vegetation structure on the distribution of carbon and nutrient concentrations of streams within urban and agricultural catchments of the Perth region of south-western Australia. We used linear mixed-effects models to evaluate the extent to which phosphorus, nitrogen and carbon concentrations in different stream zones (streamwater, and fluvial and parafluvial sediments) were explained by catchment and reach-scale attributes and moderated by high versus low-flow periods, i.e., in wet versus dry months. We found that reach-scale vegetation (woody plant cover, annual plant cover) was a better predictor of nutrient concentrations than catchment-scale land use, particularly total imperviousness, a common measure of urbanisation. Flow was also important, with carbon and nutrient concentrations better described by reach- or catchment-scale attributes during the low flow period. The extent to which individual catchment and reach attributes influenced the distribution of nutrients in different stream zones was complex. However, our results suggest that planting woody vegetation can reduce nitrogen concentrations in surface water and fluvial sediments. Reducing the abundance of weedy annual species and restoring woody perennial species may further reduce phosphorus concentrations in surface water. We conclude that local riparian restoration can be a cost-effective strategy for managing excess nutrients and carbon in flat and permeable urban landscapes, particularly during low flow periods.