Todd Wallace

Provisioning of bioavailable carbon between the wet and dry phases in a semi-arid floodplain.

Ecosystem functioning on arid and semi-arid floodplains may be described by two alternate traditional paradigms. The pulse-reserve model suggests that rainfall is the main driver of plant growth and subsequent carbon and energy reserve formation in the soil of arid and semi-arid regions. The flood pulse concept suggests that periodic flooding facilitates the two-way transfer of materials between a river and its adjacent floodplain, but focuses mainly on the period when the floodplain is inundated. We compared the effects of both rainfall and flooding on soil moisture and carbon in a semi-arid floodplain to determine the relative importance of each for soil moisture recharge and the generation of a bioavailable organic carbon reserve that can potentially be utilised during the dry phase. Flooding, not rainfall, made a substantial contribution to moisture in the soil profile. Furthermore, the growth of aquatic macrophytes during the wet phase produced at least an order of magnitude more organic material than rainfall-induced pulse-reserve responses during the dry phase, and remained as recognizable soil carbon for years following flood recession. These observations have led us to extend existing paradigms to encompass the reciprocal provisioning of carbon between the wet and dry phases on the floodplain, whereby, in addition to carbon fixed during the dry phase being important for driving biogeochemical transformations upon return of the next wet phase, aquatic macrophyte carbon fixed during the wet phase is recognized as an important source of energy for the dry phase. Reciprocal provisioning presents a conceptual framework on which to formulate questions about the resistance and ecosystem resilience of arid and semi-arid floodplains in the face of threats like climate change and alterations to flood regimes.

Rapid utilisation of storm water-derived dissolved organic carbon and its fractions in an urban lake

Hypoxia occurred in the Torrens Lake, South Australia, after a 16-mm rainfall which discharged high concentrations of dissolved organic carbon (DOC) into the Lake. This work explores the hypothesis that hypoxia was correlated with the bioavailability of DOC. Carbonaceous biological oxygen demand and a decrease in DOC from samples collected immediately after stormwater was discharged into the Lake confirmed the presence of an active aerobic microbial community. In addition, the inlet and outlet of the Lake were monitored over a 10-day period. Dissolved oxygen fell from 10 to <3 mg L–1 within 44 h as the DOC increased and then decreased. A similar pattern occurred at the outlet after a lag of 20 h. At the inlet, because minimal mixing with the Lake water occurred, the rapid decrease of DOC and its fractions was interpreted as a function of aerobic microbial activity. Aquatic humic substances (AHS) were metabolised fastest, followed by hydrophilic acids (HiA) and hydrophilic neutrals (HiN). The warm nature of Mediterranean water bodies during summer may make them more susceptible to hypoxia as the frequency of extreme droughts allow accumulation of leaf litter and other debris that is subsequently mobilised by episodic floods.

Flow variability and longitudinal characteristics of organic carbon in the Lachlan River, Australia

Heterotrophic organic-carbon cycling is a major source of energy to aquatic food webs, yet there are few studies into patterns of heterotrophic productivity in large lowland rivers. The Lachlan River experienced a period of extreme flow variability from September 2010 to February 2011; for example, daily discharge (ML day–1) at one site reached >22 times its 10-year average. Heterotrophic cycling of dissolved organic carbon (DOC) and particulate organic carbon (POC) were assessed over this period at six sites on the Lachlan River. Concentrations of total organic carbon (TOC) ranged from 7 to 30 mg L–1, of which the majority was in dissolved form. Concentration of DOC was positively correlated with daily discharge. Biochemical oxygen demand of TOC over 5 days (BOD5) showed significant variability, ranging from 0.6 to 6.6 mg O2 L–1. BOD5 did not appear related to discharge, but instead to a range of other factors, including regulation via weirs, lateral and longitudinal factors. Partitioning of DOC and POC showed that POC had an influence on BOD5 comparable to DOC. This is relevant to environmental-flow management in the Lachlan River, the Murray–Darling Basin and rivers generally, by showing that flow variability influences a fundamental ecosystem characteristic, namely organic carbon.

Open water metabolism and dissolved organic carbon in response to environmental watering in a lowland river-floodplain complex

The relative importance of autochthonous and allochthonous organic material in fuelling ecosystem metabolism is increasingly understood for some river systems. However, in south-eastern Australia, the majority of studies have been conducted during low flows when the supply of allochthonous carbon was limited. Consequently, the importance of episodic inputs of terrestrially derived material in supporting these food webs remains poorly understood. We assessed the influence of return flows from two different scales of environmental watering actions on dissolved organic carbon and open-water productivity in receiving waters adjacent to the watered area. For the wetland-scale event, gross primary productivity and ecosystem respiration increased in the receiving waters during the period of return flows. During the floodplain-scale watering, differences were observed among sites. Within the managed inundation zone, values for net ecosystem productivity switched from near zero during the baseline to strongly negative during the impact period, whereas values at the river sites were either near zero or positive. The results contribute to our understanding of the relative role of allochthonous material in supporting aquatic food webs in lowland rivers, and demonstrate potential for watering actions to have a positive influence on riverine productivity during periods of low water availability.

Sediment oxygen demand in a constructed lake in south-eastern Australia

The occurrence of hypoxia and anoxia in aquatic environments is increasing, driven by changes in land use and alteration of flow regimes. Periods of low oxygen impact biodiversity and water quality for both recreational and consumptive users. We use the Torrens Lake as a case study to assess pelagic, benthic and resuspended sediment oxygen demand, and the release of sediment bound phosphorus to determine the relative role of internal and external loading on water quality in a lake within a heavily urbanised landscape. Our results indicate temporal shifts in the dominant oxygen demanding process in the lake. During periods of no-inflow, sediment oxygen demand is the dominant process; during periods of inflow resulting from wet weather conditions, pelagic rather than sediment derived oxygen demand becomes the governing process. The inlet end of the lake is a depositional zone for stormwater borne sediments. Resuspended sediments at the inlet end of the lake exert a higher oxygen demand than those from the outlet, and represent a larger pool of potentially mobile phosphorus compared to sediments at the outlet end of the lake. However, external rather than internal loading appears to be the dominant driver of water quality in this lake.