Integration of palaeo-and-modern food webs reveal slow changes in a river floodplain wetland ecosystem

Giri R Kattel,Peter A Gell,Bradley D Eyre

doi:10.1038/s41598-020-69829-8

Abstract

Large rivers, including the Murray River system in southeast Australia, are disturbed by many activities. The arrival of European settlers to Australia by the mid-1800s transformed many floodplain wetlands of the lower Murray River system. River impoundment and flow regulation in the late 1800s and, from the 1930s, resulted in species invasion, and elevated nutrient concentrations causing widespread eutrophication. An integrated palaeoecology, and palaeo-and-modern food web approach, incorporating mixing models, was undertaken to reveal changes in a regulated wetland (i.e. Kings Billabong). The lack of preserved sediment suggests the wetland was naturally intermittent before 1890. After this time, when used as a water retention basin, the wetland experienced net sediment accumulation. Subfossil cladocerans, and δ13C of Daphnia, chironomid, and bulk sediment, all reflected an early productive, likely clear water state and shifts in trophic state following river regulation in the 1930s. Food web mixing models, based on δ13C and δ15N in subfossil and modern Daphnia, fish, and submerged and emergent macrophytes, also indicated a shift in the trophic relationships between fish and Daphnia. By the 1970s, a new state was established but a further significant alteration of nitrogen and carbon sources, and trophic interactions, continued through to the early 2000s. A possible switch from Daphnia as a prey of Australian Smelt could have modified the food web of the wetland by c. 2006. The timing of this change corresponded to the expansion of emergent macrophytes possibly due to landscape level disruptions. The evidence of these changes suggests a need for a broader understanding of the evolution of wetlands for the management of floodplains in the region.

Highlights

Large rivers, including the Murray River system in southeast Australia, are disturbed by many activities
The constant rate of sediment accumulation model (CRS) suggested that, at 51 cm sediment depth, the rate of accumulation was 0.6 g cm−2 year−1 yielding a corresponding age of c. 1970 (Fig. 2a,b)
Change in the ecosystem of Kings Billabong wetland was revealed by shifts in the composition of assemblages of subfossil cladocerans over time (Fig. 3)

Summary

Introduction

Large rivers, including the Murray River system in southeast Australia, are disturbed by many activities. The integration of palaeoecology, modern ecology and carbon energy and nutrient mass flow approaches to understand food web dynamics can resolve many key questions of the modern ecosystem structure and function of shallow floodplain wetland systems[3,4]. This combined approach provides an historic perspective that should facilitate best practices for the management of the hydrology and ecology of regulated river s ystems[5]. The lower Murray River system in southeast Australia has witnessed rapid hydrological transformation over the past century This has impacted biodiversity and ecosystem structure and the function of wetlands across temporal and spatial s cales[6]. The increased disturbance of terrestrial and pelagic or periphytic habitats has altered carbon energy and nitrogen mass flows from the base of the food web to higher trophic levels[17,18]

Methods

Results

Conclusion