Finite dormancy and declining resistance in invertebrate resting stages: Setting thresholds for arid-zone floodplain where drying is extended by river regulation and abstraction
Many arid-zone floodplain wetlands suffer extended dry periods due to water abstractions for irrigation and town supply affecting their biodiversity and their supporting ecological processes. Drought further exacerbates the imbalance between water allocations to the environment versus human uses. For example, the internationally recognised Macquarie Marshes has not flooded for 6 years, while irrigation allocations continued during one of Australia’s worst recorded droughts. Hydrological analysis reveals that under natural conditions the Macquarie Marshes would have flooded at least 1 in 2 years during this period. To understand impacts of extended drying on biotic responses from dormant stages in floodplain sediments, we compared emergence from sediments that had not flooded for 1, 4 and 14 years. Microinvertebrate taxonomic richness was significantly reduced when floodplain drying extended from 4 to 14 years. Rotifers, cladocerans and ostracods were absent from more samples from floodplain last flooded 14 years ago compared to samples flooded more recently. These biotic patterns echoed the loss of soil organic matter, dissolved organic carbon and the shift from heterotrophy to autotrophy as floodplain drying increased. The declines have implications for fish and waterbirds that are supported by these rich food webs. These patterns allow us to set drying thresholds of concern to trigger environmental allocations to floodplain wetlands and provide a biotic indicator of the success of restorative flooding.