Responses in Aquatic Ecosystem Metabolism to Managed Inundation of a Regulated Dryland River Floodplain

Abstract

Environmental flows have emerged as major restoration tool in regulated rivers. When used correctly, they can restore hydrological connectivity to help improve the ecological condition of the floodplain rivers that have been impaired as a result of river regulation. However, in highly modified river systems, there are multiple constraints to providing adequate flows at large spatial scales. These constraints include conflicts between ecological requirements, social and economic expectations. As a result, management authorities are increasingly considering the use of floodplain environmental regulators to enable targeted delivery of environmental water to specific, high value floodplains. These infrastructures are intended to enable the managed inundation of larger areas of floodplains than would otherwise occur at a given discharge, and reinstate a flow regime closer to natural patterns in terms of frequency, duration and extent of flooding. Throughout the Murray-Darling Basin in Australia, the goals of flooding the floodplain through the use of environmental flows have often focussed on maintaining and improving the conditions of floodplain vegetation and structuring of floodplain biotic communities reliant on inundated floodplains. However, environmental flows also have capacity to mobilise resources such as carbon and nutrients, and hence stimulate ecosystem productivity. Therefore, understanding how to deliver environmental flows in the best possible way to maximise ecosystem productivity that can support aquatic food webs, without leading to perverse outcomes such as hypoxic blackwater events and problematic algal blooms, is crucial. Measurements of ecosystem metabolism are often used as indicators of ecosystem responses to flow variability, and to gain insights into relative health of rivers, based on a concept that they reflect energy sources and carbon fluxes in aquatic food webs. However, how information obtained from ecosystem metabolism measurements provide an understanding of the overall health of rivers is not well understood. Therefore, in this study, the utility of metabolism measurements as an indicator of riverine ecosystem health was also evaluated. Ecosystem metabolism rates in an anabranch creek of the Chowilla Floodplain adjacent to the lower River Murray in South Australia, and in the main river channel upstream and downstream of the floodplain were compared prior to, during, and after a managed inundation event. The operation of a newly constructed environmental flow regulator facilitated the managed inundation of the floodplain. Additionally, the whole ecosystem metabolism was partitioned to assess the contribution of different biotic components, by estimating the production and respiration rates of each biotic component and scaling the rates to a reach over which the whole ecosystem metabolism was measured. While there was substantial increase in ecosystem productivity rates in the anabranch sites during the managed inundation period, the responses were highly variable between sites. The results indicate substantial spatial variation in outcomes, even along a linear section of anabranch creek. Ecosystem productivity rates in the river channel also increased at sites located approximately L km and `M km downstream of the return flows, indicating the potential of managed inundations to stimulate productivity at larger spatial scale. Partitioning of whole ecosystem metabolism indicated that a large part of the whole system production and respiration was accounted for by microorganisms, while the contribution by zooplankton, invertebrates and fish was less significant; indicating low carbon flux through higher order organisms. Managed floodplain inundation events have the potential to positively influence riverine productivity, even during periods of low water availability. However, it is important to consider spatial variability across floodplains while assessing the ecosystem metabolism in response to inundation events. Greater understanding of how the mosaic of habitats present within a floodplain moderate ecological outcomes is required to enable managers to achieve desired ecological objectives at low inherent risk. Although ecosystem metabolism is a useful metric to complement monitoring of higher order organisms and to develop a reliable picture of river health, caution is required while interpreting the measurements of metabolism as an indicator of overall river health. This is because metabolism measurements may only reflect metabolic activity of phytoplankton and microbial communities, and not the ecosystem more broadly.