Paleoecological responses to avulsion and floodplain evolution in a semiarid Australian freshwater wetland

Abstract

Paleoecological responses to coupled geomorphic and hydrological changes have rarely been studied using a multiproxy approach in freshwater floodplain wetlands of semiarid Australia. The Macquarie Marshes are a large, multi-channelled and morphodynamic floodplain wetland system in the lowland interior of the Murray-Darling Basin, southeastern Australia. Evidence from historical maps, aerial photographs, geomorphology and sedimentology indicate that avulsion in the southern Macquarie Marshes caused the formation of a major new channel (Monkeygar Creek) and the abandonment of a reach of the Macquarie River in the last 200 years. Paleobiological samples from near-uniform sediment cores were studied to determine the environmental changes related to this avulsion, as well as associated floodplain evolution and hydrological changes in the adjoining flood-reliant wetlands. Fifteen macrophyte taxa, seven charophyte taxa, 28 diatom taxa and 18 aquatic invertebrate taxa were identified in sediments from the terminal reach of the Macquarie River and the active lower reach of Monkeygar Creek. Paleoecological reconstruction indicates that the slowly accreting marsh (ca 0.032 to 0.037 cm a−1) around the Macquarie River (Terminus Marsh) was more frequently and deeply inundated in the past (presence of limnetic zooplankton before ca 1626 cal. years BP) than during the penultimate pre-avulsion phase (absence of limnetic zooplankton and more diverse terrestrial and aquatic vegetation from ca 540 cal. years BP). In contrast, the floodplain near lower Monkeygar Creek was only periodically inundated prior to avulsion (high diversity of charophytes and low diversity of diatoms and invertebrates before ca 149 years BP), before experiencing disturbance, rapid accretion and ecological succession during the avulsion (paucity of wetland biota in distinct flood facies ca 149 years BP). This part of the floodplain became a substantial wetland, dominated by algae (charophytes and diatoms) and submerged macrophytes, due to a more consistent flood regime following the avulsion. The contemporary marsh on lower Monkeygar Creek has evolved in the last ca 54 years into a shallower, less frequently flooded wetland (new dominance of emergent macrophytes, charophytes and littoral cladocerans), probably due to changes in inundation caused by rapid local floodplain accretion (ca 0.409 to 0.556 cm a−1) and possibly river regulation. This multiproxy approach provides some of the first evidence of significant changes in wetland communities and ecological successions that are directly related to the geomorphic processes of avulsion and floodplain evolution in a semiarid wetland system such as the Macquarie Marshes.