Past water resource management strategies have changed the biogeochemical cycling in
floodplain wetlands. Permanent water cover in low-lying floodplain areas has changed the carbon cycle in these environments. Coupled with rising salinity, in particular sulfate pollution, the more labile form of carbon has allowed microbial metabolism to play a larger role in the
biogeochemical cycles of floodplain wetlands.
This study has highlighted the effect that changing the hydrological régime can have on salinity and the biogeochemical cycles of iron, sulfur, nitrogen and phosphorus within floodplain wetlands. Water resource management strategies need to encompass these changes as the
implications of this study are that the inundation of sulfidic sediments may neutralise acidity, but may also increase the quantity of sulfide present in these waterways.
Changes in the redox potential, brought about by the inundation of these sediments after a drying phase, exert considerable influence on the cycling of nutrients. Due to the elevated sulfate concentrations, it is hypothesised that the rate at which sulfidic material form in these systems is limited by the supply of readily metabolisable organic matter formed during primary production. In turn, the cycling of nutrients resulting from the formation of these same sulfidic material may trigger algal blooms leading to the rapid accumulation of sulfidic material. This positive feedback process is of considerable significance to future managers.
Management strategies, for example, which aim to reinstitute a natural cycle of wetting and drying on these aquatic ecosystems, may compound the acidity risk associated with the presence of sulfide through its influence on salinity. Furthermore, repeated cycles of wetting and drying redistribute carbonate minerals away from areas where sulfide accumulates, resulting in a net deficit of acid-neutralising capacity where it is desirable, and lead to the formation of acid sulfate soils with sulfuric material.
Response of the Sediments to
Inundation
Management strategies aimed at reducing the formation of sulfides through the imposition of wetting-drying cycles will need to account for the rapid response of the sediments to
resuspension. Whilst these studies were ‘bench top’ laboratory experiments, at the wetland scale, the response would be comparatively fast and significant quantities of sulfide would accumulate in weeks to months following inundation. The algal growth would also be rapid. This has been demonstrated at the Loveday Disposal Basin where experimental reflooding in May and June
2006 for the North and South Basins respectively, was followed by the generation of algal mats, noted by Lamontagne et. al. (2008), in October 2006.
Physicochemical Properties of the
Sediments
The changes in water chemistry brought about through the inundation of the sediments depended on their physicochemical properties. The extent of these changes was related to the pH and salinity of the sediments, with the pulses of iron being higher in the more acidic samples, and the pulse of phosphate being more pronounced in Mussel Lagoon where the salinity was significantly lower.
These changes following inundation result from the changes in redox potential at the sediment - water interface. If the sediments contain significant quantities of sulfidic material, particularly AVS (metastable, reactive iron monosulfide), the shift in redox from high potential to low
potential will be rapid as this material will oxidise and lower the redox potential more rapidly than would occur from respiration alone. Ideally, management strategies aimed at reducing the
quantity of reduced sulfur in floodplain wetlands need to not only consider the frequency of wetting and drying cycles, as noted above, but also the timing. This may be achieved by exposing the sediments to the atmosphere to reduce the quantity of reduced sulfur present in the sediments, which would prolong the onset of anoxia. By contrast, the wetting cycle should have a rapid onset and be of short duration. A prolonged wetting cycle will induce anoxia, and when this occurs the positive feedback of sulfate reduction and algal growth will result in the rapid accumulation of sulfidic material.
Wetting and drying cycles will also need to account for the solubility of carbonate minerals, which may lead to a heterogeneous distribution of carbonate and acid neutralising capacity away from areas of high sulfide content. This may ultimately lead to the formation of acid sulfate soil with sulfuric material in saline wetlands.
The effect of sulfur cycling in these wetland ecosystems has been to enhance the productivity of these ecosystems. This in turn has led to positive feedback where the conditions conducive to the accumulation of sulfide are maintained and enhanced when the sediments are inundated. The research suggests that whilst the sediments are not unusually enriched in nutrients, the solubility of phosphorus and nitrogen, as phosphate and ammonium respectively, is enhanced by the onset of anoxia and anaerobic metabolism through iron and are established in freshwater wetlands, sulfur cycling can become the dominant biogeochemical cycle in these systems.
Future Research
This research has identified a potential feedback process involving the onset of anoxia and the liberation of nutrients from sediments when inundated. Future research needs to be focused on the cycling of carbon in these systems. The cycling of carbon was touched on superficially in this project. The potential trigger for the rapid accumulation of reduced sulfur in these inland systems is the availability of metabolisable organic carbon. It needs to be determined if carbon is limiting the rate of sulfate reduction in these systems.
The experiments conducted for this research, were small scale, benchtop microcosms under controlled conditions. In reality, the inundation of the sediments found in inland ecosystems will not occur with deoxygenated water. Larger scale experiments, designed to replicate the field environment need to be conducted. The apparatus used to achieve these conditions imposed limitations on what variables could be measured during the experimental work. Encouraging microbial sulfate reduction, due to the formation of inorganic carbon, is used in management strategies to combat the acidity generated through the oxidation of sulfide minerals. The effect of microbial sulfate reduction on the pH of the waters and sediments was not determined in this study.
During the kinetics experiments, there was an appreciable quantity of ferrous iron liberated from the sediments, which remained in solution to the end of the experiments. The observance of ferrous iron in solution was most pronounced in the Mussel Lagoon sample and the Loveday South (400) sample, with more modest amounts of ferrous iron mobilized from Loveday North (200) and Berri samples. Mussel Lagoon and Loveday North were acidic, which may explain the mobilization of iron from these sediments. Further research needs to be conducted into the mobilization of other metals from the sediments when they are inundated and the redox potential is lowered.
The proposed future research, if completed, would allow for a more complete conceptual model to be developed with regard to the onset of anoxia in these inland systems and would allow land managers to allocate increasingly scarce resources to managing the problem of sulfidic material in floodplain wetlands in the Murray-Darling Basin.
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