Darren. S. Baldwin

The (re)establishment of macrophytes is a key objective for wetland managers across the Murray–Darling Basin, Australia. Numerous factors could influence wetland macrophyte germination and growth in forested floodplain wetlands, including allelopathic substances, particularly from the litter of fringing trees, but the role of allelopathy in wetland plant germination and growth is not well understood. In the current study, the allelopathic effects of aqueous leachate from the litter of the river red gum ( Eucalyptus camaldulensis Dehn.) on the emergence and growth of wetland macrophytes (and other phototrophs) were studied in a mesocosm experiment. Sediments were collected from two wetlands in Gunbower Forest, Victoria. One wetland (Reedy Lagoon) supports a diverse range and high abundance of wetland plants, while an adjacent wetland (Barapa Swamp) is humo‐eutrophic and has low diversity and abundance of wetland macrophytes. Replicate sediment samples from both wetlands were exposed to four concentrations of leachate from E. camaldulensis leaf litter: High (75 mg/L as C), Medium (37.5 mg/L as C), Low (18.75 mg/L as C) and a Control (containing no leachate). After 90 days of incubation germinants were identified and counted to the lowest taxonomic level practicable, percent cover of filamentous algae was estimated, and the concentrations of chlorophyll a and phaeophytin were measured. There were statistically significantly more germinants in the Control treatment than in the three leachate treatments from the Reedy Lagoon sediment, and a statistically significant greater number of germinants in the Control than in the High treatments for Barapa Swamp sediment. At higher concentrations, the development of germinants was inhibited, with the vast majority unable to be identified beyond monocotyledon or dicotyledon due to lack of growth and with no dicotyledons present in the High treatment from either wetland. There was no noticeable effect of leachate concentration on the emergence of free‐floating species or on filamentous algae. The impact on phytoplankton growth was more complex, with a higher algal biomass in the Low treatments compared to the Control but a decrease at higher concentrations.

Context: Fires in 2019–2020 burned over 500,000 ha in the upper catchment of Lake Hume, a major reservoir at the confluence of the Murray and Mitta Mitta Rivers. Aims: This project assessed changes in key water quality constituents entering the lake following the fires. Methods: Weekly and annual loads were estimated for 5 years post-fire at Jingellic (Murray River) and Tallandoon (Mitta Mitta River) and compared with historical loads. Results: No fire-related water quality effects were observed at Tallandoon; this was likely to be due to Lake Dartmouth (an upstream storage) buffering poor-quality runoff. At Jingellic, sediment loads remained elevated for at least 5 years, peaking at ~600,000 Mg year−1, which is 15 times the long-term average. Nutrient loads (total phoshorus, total Kjeldahl nitrogen, nitrogen oxides) were up to 3 times higher than normal for 3 years. By contrast, soluble reactive phosphorus and salt impacts lasted only weeks to months. Conclusions: These findings highlight that fire impacts on water quality vary depending on constituent type and location in the landscape. Implications: Fire impacts on water quality can last for a long time, so studies looking at the impacts should be appropriately designed and resourced, with careful consideration of sampling locations.

Following a series of fish deaths in the lower Darling River in the austral summer of 2018–19, several field trials were undertaken over two summers to determine the efficacy of various aerators to mitigate the effects of hypoxia to prevent fish deaths. The aerators evaluated included: low-powered, solar bubble-plume diffusers; high-flow, single-port Venturi systems; pontoon-mounted multi-port Venturi systems; and an ultra-fine bubble generator. The effectiveness of bubble diffusers appears related to their power, the number of units used and their running time. Pontoon-mounted multi-port Venturi aerators and high-flow, single-port Venturi aerators were much more successful in maintaining oxygen levels and may have prevented fish deaths in the Darling River in early 2019. However, the Venturi aerators were expensive to run, consuming hundreds to thousands of litres of diesel fuel per week. Trial of an ultra-fine oxygen bubble pump showed promise because it supersaturated the water with dissolved oxygen while not elevating total gas pressure to levels that would cause gas-bubble trauma in fish. Although aerators may play a role in emergency responses to hypoxic fish deaths, they have limitations regarding their area of influence (at best thousands of square metres) and their cost to deploy and operate.

For many floodplain rivers, reinstating wetland connectivity is necessary for ecosystems to recover from decades of regulation. Environmental return flows (the managed delivery of wetland water to an adjacent river) can be used strategically to facilitate natural ecosystem connectivity, enabling the transfer of nutrients, energy, and biota from wetland habitats to the river. Using an informal adaptive management framework, we delivered return flows from a forested wetland complex into a large lowland river in south-eastern Australia. We hypothesized that return flows would (a) increase river nutrient concentrations; (b) reduce wetland nutrient concentrations; (c) increase rates of ecosystem metabolism through the addition of potentially limiting nutrients, causing related increases in the concentration of water column chlorophyll-a; and (d) increase the density and species richness of microinvertebrates in riverine benthic habitats. Our monitoring results demonstrated a small increase in the concentrations of several key nutrients but no evidence for significant ecological responses was found. Although return flows can be delivered from forested floodplain areas without risking hypoxic blackwater events, returning nutrient and carbon-rich water to increase riverine productivity is limited by the achievable scale of return flows. Nevertheless, using return flows to flush carbon from floodplains may be a useful management tool to reduce carbon loads, preparing floodplains for subsequent releases (e.g., mitigating the risk of hypoxic blackwater events). In this example, adaptive management benefited from a semi-formal collaboration between science and management that allowed for prompt decision-making.

Global acknowledgement of climate change and its predicted environmental consequences has created a need for practical management techniques that increase a landscape’s ability to capture and store atmospheric carbon (C). Globally, wetlands sequester disproportionally more C per unit surface area than many other components of the landscape. However, wetlands vary in their capacity to store C and regulate greenhouse gas emissions. Hydrology, in particular, is a critical driver of wetland C capture and storage. Rain-filled wetlands offer a challenge for the management of C sequestration and storage because the hydrology of these systems is almost entirely driven by rainfall. We present a conceptual model of how management options, including weed and pest control, grazing and crop management and revegetation, will affect C sequestration and storage in rain-filled wetlands. Given the intensive nature of agricultural activities in areas where rain-filled wetlands are common, further work is needed to increase our understanding of the effects of these activities on wetland C capture and storage. Key knowledge gaps relating to the effect of management actions on wetland C sequestration include: (a) the benefits of integrated wetland management; (b) the appropriateness of different grazing regimes and the effect of total grazing pressure; (c) the effects of fire; and (d) the extent to which wetland function (C storage) can be restored following agricultural activities, such as cropping.

Environmental flows are managed events in river systems designed to enhance the ecological condition of aquatic ecosystems. Although not traditionally seen as important in lowland rivers, there is mounting evidence that terrestrial subsidies can be an important energy source in aquatic metazoan food webs. We argue that the apparent lack of importance of terrestrial subsidies to many lowland river food webs may reflect an artefact resulting from historical anthropogenic changes to lowland river–floodplain ecosystems, including the loss of lateral connectivity between rivers and their floodplains, changes in floodplain land use and carbon stores, and loss of sites of transformation within the main channel. The loss of floodplain subsidies to the main river channel can be partially redressed using environmental flows; however, this will require mimicking important aspects of natural high-flow events that have hitherto been overlooked when targeting environmental flows to a limited suite of biota. We suggest that key biotic targets for environmental flow releases may not be achievable unless river–floodplain subsidies are sufficiently restored. Environmental flows can go some way to addressing this shortfall, but only if floodplain subsidies to river channels are explicitly included in the design and management of environmental flows.

Phosphorus is an important nutrient for plants and algae, and can be the limiting nutrient in aquatic ecosystems. However, oversupply can lead to significant water quality issues. The largest source and sink of P in most aquatic systems is the sediment. As a consequence of drought, in many places sediments that normally would have remained inundated are now being desiccated. Based on previous studies, it is often difficult to predict what impact drying will have on the cycling of P. This is because most of these studies have looked at drying across a chronosequence in the field, where there may be differences in sediment composition or microbial community structure. In this paper we present the results of a study where sediment was exposed to progressively more severe drying in the laboratory – starting with wet sediment, followed by air drying and then sequential oven drying at 30, 50 and 85 °C. Drying resulted in a shift in P speciation, notably with an increase in NaHCO₃-extractable reactive P and a decline in NaHCO₃-extractable unreactive P, likely indicating an increase in bioavailable, easily exchangeable P. Drying also resulted in a decline in the microbial-P fraction. Drying significantly affected the P adsorption characteristics of the sediment. The total amount of P adsorbed by the sediment and the linear adsorption co-efficient both declined, while the amount of native P adsorbed to the sediment and the equilibrium P concentration both increased. Drying also affected iron speciation with a shift from more reactive oxalate-extractable Fe to more recalcitrant citrate–dithionate–bicarbonate-extractable Fe, suggesting an increase in iron crystallinity and hence decrease in P adsorption capacity. The increase in crystallinity is consistent with Fe EXAFS results, which showed that drying resulted in an increase in edge-sharing neighbours. We hypothesise that the shifts in P speciation, the decline in P adsorption capacity, the increase in the equilibrium P concentration, as well as the death of micro-organisms (as evidenced by a decline in microbial P) on drying all contribute to the Birch effect – the initial pulse of P and/or N upon inundation of dried soils or sediments.

Lowland riverine–floodplain systems often have significant but irregular inputs of allochthonous carbon. However, the importance of this carbon to riverine systems remains poorly understood. We assessed open water dissolved organic carbon (DOC) concentrations, metabolism and biofilm stable isotope (d13C) signatures, upstream and downstream of an extensive floodplain forest on the Murray River, Australia, before and after a flood event. Prior to flooding, all sites had similar concentrations of DOC, rates of metabolism and biofilm d13C signatures. During the flood DOC concentration increased up to three-fold downstream of the forest, gross primary production (GPP) increased at all sites, but community respiration (CR) increased only at the downstream sites, resulting in decreased in NPP downstream and a slight increase upstream. Biofilm d13C signatures became depleted by between 4 and 7‰ downstream of the forest during the flood, reflecting a rapid incorporation of allochthonous carbon into the biofilm. These results indicate that flooding led to a substantial increase to the energy budget of the Murray River through the provisioning of large quantities of allochthonous carbon and that terrestrial carbon was processed within the river biofilms. Allochthonous carbon assimilation within biofilms during flooding provides a potential pathway for allochthonous carbon to be incorporated into the metazoan foodweb.

Optical absorbance and fluorescence spectroscopies have the potential to play an important role in monitoring aquatic ecosystems. This paper explores the relationship between the optical absorption and fluorescence characteristics of water samples taken from 18 sites (spanning a range of aquatic environments including lowland rivers, small and large dams, and floodplain wetlands) with their dissolved organic carbon (DOC) concentration and 28 days bioavailability Both optical absorbance in the ultraviolet region and fluorescence above excitation wavelengths of about 240 nm and emission wavelengths above about 350 were correlated with DOC concentration (r² > 0.8). The initial (pre-incubation) optical absorbance in the UV region (r² ≈ 0.7–0.8) and fluorescence attributed to both ‘humic like’ (r² = 0.84) and ‘tryptophan-like’ (r² = 0.87) fluorophores correlated with DOC bioavailability. These correlations were used to develop empirical linear models relating the initial optical properties of water with DOC quantity and quality. The robustness of these models was then tested against a second suite of water samples from 12 different sites, collected independently of those used in the model development. Although based on strong correlations, the empirical models were not particularly good at predicting the bioavailability of DOC in the model validation samples. We suggest that one of the reasons for the low predictive power of the models is that the strong correlations observed between DOC bioavailability and optical absorbance in the UV region or fluorescence and are co-incidental rather than causal. Changes in UV-absorbance or fluorescence during the incubation experiments are not consistent with changes in DOC concentration. One of the best predictors of DOC bioavailability is the initial concentration of DOC. We argue the strong correlation between DOC bioavailability and initial fluorescence intensity or UV absorption simply reflects the strong correlation between initial DOC concentration and initial fluorescence intensity or UV absorption. We argue that unless there is an underlying causal relationship between two components (the component of interest and a surrogate measure for that component) care should be taken in extrapolating correlative models beyond the data set used to create them.

Dissolved organic carbon export from floodplain litter during flood events is an important energy subsidy for lowland rivers. Temperature affects both the release and decomposition of dissolved organic carbon from floodplain plant litter. Unseasonally warm flood conditions have resulted in the release and consumption of carbon at a rate so rapid that water column oxygen has been depleted, causing the death of aquatic organisms upon exposure to this ‘hypoxic blackwater’. To date, there has not been a systematic investigation of the role of temperature on hypoxic blackwater dynamics. In a series of laboratory experiments, we investigated the temperature dependence of both leaching and decomposition of carbon from a common floodplain litter component in south-eastern Australia – the leaves of the river red gum Eucalyptus camaldulensis. The leaching rate increased with temperature and approximated Arrhenius kinetics. Additionally, the solubility of the leaf carbon increased substantially at temperatures above ∼25°C. The rate of organic carbon respiration also increased with temperature, and the relationship with temperature was approximately linear. These data can be used to improve models for the prediction of hypoxic blackwater risk.