Tracking a century of change in trophic structure and dynamics in a floodplain wetland: integrating palaeoecological and palaeoisotopic evidence

Abstract

1. The palaeoecological assessment, and the use of stable isotopes of carbon in subfossils of herbivores and omnivores, represents a novel approach to understand transitions in past food-web structure and the dynamics of lake ecosystems in response to natural perturbations and human impacts. Combined with records of subfossil assemblages of cladocerans and chironomids, it may be possible to decipher whether changes are attributable to external forces or internally derived system shifts. 2. A sediment record taken from the shallow (2.3 m depth) Kings Billabong in the River Murray floodplain (Australia) was analysed to explore changes in trophic dynamics over the past century. 3. The palaeoecological assessment revealed that littoral assemblages of cladocerans and benthic diatoms were gradually replaced by planktonic (planktonic and facultative planktonic) assemblages after river regulation in the 1920s. 4. The stable isotopic composition of carbon (δ¹³C), derived from chironomid head capsules, was relatively constant down-core, ranging between -26.1⁰⁄₀₀ and -24.0⁰⁄₀₀, and coincided largely with the δ¹³C of bulk sediment samples (-25.6⁰⁄₀₀ to -22.0⁰⁄₀₀). The δ¹³C values of pelagic (Daphnia) and ubiquitous (Bosmina, Alona) cladocerans, however, varied markedly, with that for Daphnia between -29.8⁰⁄₀₀ (10–20 cm) and -23.2⁰⁄₀₀ (60–70 cm), and for ubiquitous cladocerans, between -29.4⁰⁄₀₀ (20–30 cm) and -24.5⁰⁄₀₀ (80–70 cm). 5. The temporal changes in the δ¹³C values of cladocerans also suggest a gradual transition from a macrophyte-dominated state to a phytoplankton-dominated state after river regulation and further indicate changes in the horizontal migration behaviour of Daphnia depending on macrophyte abundance and predation risk. 6. Our study demonstrates the potential of reconstructing, more precisely, the trophic dynamics of large river floodplain lakes and their ecological resilience by combining subfossil analyses with stable isotope analyses of selected subfossil groups.