Kane T. Aldridge

Modification of sediment redox potential by three contrasting macrophytes: implications for phosphorus adsorption/desorption

Freshwater macrophytes may increase sediment redox potential and the affinity of sediments for phos- phorus through radial oxygen loss from their below-ground biomass. This study demonstrated that the ability to alter sediment redox potential differs between macrophytes, according to their capacity to transport oxygen. Of the emer- gent macrophytes, Typha domingensis increased sediment redox potential (218 mV above bare sediment) to a greater extent than Bolboschoenus caldwellii (41 mV above bare sediment). However, the inhibition of convective flow in T. domingensis reduced its oxidizing ability by 78 mV. In contrast, Potamogeton crispus, a submerged macrophyte, had no influence on sediment redox potential. The presence of T. domingensis also increased phosphorus uptake from the water column by 0.88 mg P m −2 day −1 , above that of bare sediments. In addition, inundation predictably decreased sediment redox potential from 175 mV to −176 mV over a 42-day period. Similarly, the addition of cel- lulose (10 mg L −1 ) decreased sediment redox potential by 42 mV. Consequently, deposition of organic debris may counteract the oxidizing effects of macrophytes that have a limited capacity to transport oxygen, such as P. crispus. Results suggest that macrophytes play an important role in facilitating the restoration of freshwater systems.

The Impact of Extreme Low Flows on the Water Quality of the Lower Murray River and Lakes (South Australia)

The impact of extreme low flows on the water quality of the Lower Murray River and Lower Lakes (Alexandrina and Albert) in South Australia was assessed by comparing water quality from five sites during an extreme low flow period (March 2007–November 2009) and a preceding reference period (March 2003–November 2005). Significant increases in salinity, total nitrogen, total phosphorus, chlorophyll a and turbidity were observed in the Lower Lakes during the low flow period. Consequently, water quality guidelines for the protection of aquatic ecosystems were greatly exceeded. Principal Component Analysis, empirical and mass balance model calculations suggested these changes could be attributed primarily to the lack of flushing resulting in concentration of dissolved and suspended material in the lakes, and increased sediment resuspension as the lakes became shallower. The river sites also showed significant but more minor salinity increases during the extreme low flow period, but nutrient and turbidity concentrations decreased. The most plausible reasons for these changes were decreased catchment inputs and increased influence of saline groundwater inputs. The results highlight the vulnerability of arid and semi-arid lake systems to reduced flow conditions as a result of climatic changes and/or water management decisions.

Multiple Interception Pathways for Resource Utilisation and Increased Ecosystem Resilience

Natural ecosystems consist of multiple pathways for resource capture and energy flow. As landscapes become impacted, the diversity of these pathways is reduced and ecosystem health suffers. Efficient resource processing is essential for successful ecosystem restoration and yet little information is available relating the two issues. Disturbances to terrestrial, marine, and freshwater environments have decreased the complexity of such ecosystems thus reducing their resilience to increases in resource loading from anthropogenic sources. The effects of a reduction in the number of pathways are observed in terminal water bodies where increased levels of nutrients are not successfully intercepted beforehand, and conditions often favour a single phytoplankton pathway, resulting in algal blooms. If a suitable number of pathways is restored, significant amounts of resources may be retained, reducing the impacts on downstream ecosystems. The restoration of functional diversity is likely to provide process redundancy and therefore improve ecosystem resilience to further disturbance. This concept provides a more holistic approach to management strategies that may enhance resource interception, while restoring habitat and biological diversity.

Impact of a drought on nutrient concentrations in the Lower Lakes (Murray Darling Basin, Australia)

Abstract Nutrient concentrations increased in 2 lakes (Lake Alexandrina and Lake Albert) located at the downstream end of the Murray Darling Basin, Australia, as a result of water level drawdown and salinisation associated with a severe drought. Between January 2007 and March 2008 the salinity difference between the inlet and outlets (5 barrages) increased from 0.9 to 21.0 g L−1, resulting primarily from seawater leakage through the barrages. Subsequently, in relatively sheltered areas upstream of the barrages, permanent salinity-derived density stratification developed, leading to the development of an anoxic hypolimnion. This seemingly favoured the regeneration of dissolved nutrients from the sediments, with standing stocks of ammonium and filterable reactive phosphorus increasing by 250 and 142%, respectively. However, the source of leakage water through the barrages also contributed to the increase. While dissolved organic carbon concentrations also increased, this was a result of evapoconcentration because calculated standing stocks changed little during the study period. In the open water areas, vertical density stratification was not evident, but sediment resuspension seemed to increase during the drawdown. Total organic nitrogen and total phosphorus concentrations were closely related to light attenuation, suggesting increased resuspension of particulate nutrients during the water level drawdown or increased assimilation of dissolved nutrients by phytoplankton. Overall, sediment resuspension seemed to have had a greater impact on nutrient concentrations in open water areas of Lake Alexandrina and Lake Albert, while saline intrusions were more significant in relatively sheltered areas located close to the lake outlets.

Floodplain connectivity facilitates significant export of zooplankton to the main River Murray channel during a flood event

Abstract This study assessed the role that floods play in providing lateral connectivity between riverine habitats and floodplains, stimulating productivity and contributing zooplankton from the floodplain to the river channel. The study took place on the Chowilla Floodplain of the River Murray, Australia, and the adjacent River Murray Channel throughout the 2010–2011 floods. We found that a considerable transfer of zooplankton from the floodplain into the river channel occurred. Average zooplankton abundance was higher on the floodplain than the main river channel and increased the zooplankton abundance in the river channel downstream. At the peak of the flood, flows reached ~93 000 megalitres per day (ML d−1), inundating ~67 km2 of floodplain. At the time of this study, up to 6.3 ± 1.6 (SD) tonnes per day of zooplankton (dryweight) was being exported from the Chowilla floodplain. Differences in species assemblages were also observed within the River Murray, which seemed to be caused by the influence of the Chowilla Floodplain. This study demonstrated that floodplains provide significant zooplankton biomass, which constitutes a resource input into the riverine food web. These results provide some evidence for the Flood Pulse Concept, which highlights the importance of lateral hydrological connectivity between riverine habitats and floodplains in stimulating productivity and providing a linkage between habitats for biota. Management of regulated lowland rivers should consider not only the provision of water to the floodplain, but also the return of the floodplain waters to the river to sustain riverine food webs .

Fish productivity in the lower lakes and Coorong, Australia, during severe drought

Anthropogenic modification of catchments and river flow can significantly alter estuarine habitats, hydrology and nutrient delivery with implications for fisheries productivity. The Coorong estuary at the terminus of Australia’s River Murray supports an economically important fishery as well as being recognised internationally as a critical site for migratory birds. Salinity near the Murray Mouth varies between fresh and marine depending upon river flow, but the Coorong becomes increasingly saline along its 120 km length. Freshwater flow to the Coorong is naturally variable but has significantly reduced by extraction for irrigated agriculture and domestic use upstream. Extreme drought from 2000 to 2010 and over-allocation of water resources resulted in the cessation of freshwater flow to the Coorong, significantly increasing salinity. During this period the diversity and abundance of organisms in the Coorong declined which reduced food web complexity. During lower flows the system generally becomes less productive as evidenced by: lower nutrient concentrations and loads, lower chlorophyll and primary productivity, a decrease in the abundance of fish-prey items (zooplankton, macroinvertebrates and small fish), a decrease in fish abundance, although this is not well reflected in fishery catch data because of the concentration of fishing in available habitat. The maintenance of flow is the only management strategy that stimulates recruitment, delivers nutrient resources to the estuary and ensures maintenance of habitable area by maintaining appropriate salinity.

Hydrological-niche models predict water plant functional group distributions in diverse wetland types

Human use of water resources threatens environmental water supplies. If resource managers are to develop policies that avoid unacceptable ecological impacts, some means to predict ecosystem response to changes in water availability is necessary. This is difficult to achieve at spatial scales relevant for water resource management because of the high natural variability in ecosystem hydrology and ecology. Water plant functional groups classify species with similar hydrological niche preferences together, allowing a qualitative means to generalize community responses to changes in hydrology. We tested the potential for functional groups in making quantitative prediction of water plant functional group distributions across diverse wetland types over a large geographical extent. We sampled wetlands covering a broad range of hydrogeomorphic and salinity conditions in South Australia, collecting both hydrological and floristic data from 687 quadrats across 28 wetland hydrological gradients. We built hydrological-niche models for eight water plant functional groups using a range of candidate models combining different surface inundation metrics. We then tested the predictive performance of top-ranked individual and averaged models for each functional group. Cross validation showed that models achieved acceptable predictive performance, with correct classification rates in the range 0.68-0.95. Model predictions can be made at any spatial scale that hydrological data are available and could be implemented in a geographical information system. We show the response of water plant functional groups to inundation is consistent enough across diverse wetland types to quantify the probability of hydrological impacts over regional spatial scales.

Integrated science informs forest and water allocation policies in the South East of Australia

Abstract Groundwater-dependent ecosystems are particularly vulnerable to groundwater decline. The South East of South Australia has experienced considerable hydrological change due to drainage, clearance of native vegetation, irrigated agriculture, and forestry. Recent hardwood plantings in the Limestone Coast Prescribed Wells Area (PWA) have increased considerably since 2000, coincident with groundwater decline; however, the introduction of hardwood plantations coincided with a period of lower than average rainfall. Given the economic implications of modifying water allocation policy based on apparent anthropogenic causes, establishing the contributions to the observed declines in groundwater level was necessary. The water balance shows a net gain in water across the entire Lower Limestone Coast Prescribed Wells Area, yet sites with forestry land use show the water table has been declining at a rate up to 0.7 m yr−1 since 2004, whereas no long-term change has occurred in the water table depth at adjacent pasture sites. Forested areas have 4050 wetlands at risk with groundwater decline >0.3 m yr−1, covering a total wetland area of of 212 km2. A series of 15 policy scenarios were simulated using a groundwater flow model to examine how climate, land use, and particularly forestry would impact groundwater, with an imperative to enact policy to protect remaining wetlands. Water allocation policy accounted for groundwater extraction for irrigated agriculture, but other activities, such as forestry, were not considered in water allocation even though they were found to be a major water user. Following this study, the South Australian Government amended its Natural Resources Management act to stipulate that forestry must have a water licence and a water allocation to account for water used by the forest.

Physicochemical influences on Ruppia tuberosa abundance and distribution mediated through life cycle stages

Abstract This study determined and contrasted the effect of physicochemical conditions on the distribution and abundance of the life stages of Ruppia tuberosa J. Davis and Tomlinson in neighbouring permanent (Coorong) and ephemeral (Pipe Clay and Cantara) lakes in southeastern South Australia. The distribution and abundance of shoots, flowers, seeds, and turions of R. tuberosa were monitored. Salinity and water depth were identified as the principal physicochemical conditions driving changes in the distribution and abundance of the life stages of R. tuberosa. Extremely high salinity led to reduced germination, flower and seed production, and shoot abundance in the Coorong. Salinity levels in Lake Pipe Clay were favourable for germination but not for flowering; however, Lake Cantara was not only favourable for the establishment of R. tuberosa shoots, but also for flower and seed production. In ephemeral systems, if favourable salinities for germination and growth occur during autumn and winter, R. tuberosa is able to survive elevated spring salinities and persist through either sexual reproduction or the production of turions, depending on salinity levels. The reduced abundances of R. tuberosa at water depths >0.6 m suggest light limitation and that R. tuberosa is a high-light– adapted species. Because salinity is an easily measured parameter with significant biological implications, it is useful for setting environmental flow targets for the Coorong and River Murray estuary.