Environmental aspects of acid sulphate soils - ROOT of content

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evidence suggesting that these outbreaks occur only when susceptible fish are exposedto both Aphanomyces sp. propagules (spores and/or hyphae) and water having specificattributes. Further, there is an association between these water quality attributes andthe presence of significant areas of acid sulphate soils in the lower floodplains ofaffected rivers. Such areas are known to be present in floodplains of most rivers inAustralia from which EUS has been reported.Studies on the tidal floodplain of the Richmond River system in northern NewSouth Wales have demonstrated some relationships between major rain events andseveral water quality parameters. A submersible automatic water quality data logger,programmed to measure pH, dissolved oxygen concentrations, conductivity, temperatureand depth at 15-minute intervals, was placed in a tributary of the RichmondRiver immediately downstream of a typical, drained backswamp containing significantareas of acid sulphate soils. Water quality changes associated with a major rainevent are shown in Figure 1.The low pH of the water after the rain event was probably due to acidity leachedfrom acid sulphate soils in the catchment. The cause(s) of the low dissolved oxygenconcentrations are more difficult to explain; they may be associated with microbialdecomposition of suspended organic matter or from soil water from the active rootzone, but the clear temporal association between reductions in both pH and dissolvedoxygen concentrations suggests a common mechanism. One such mechanism involvesthe oxidation of iron IT (from pyrite oxidation) to ferrihydrite. Concentrations of toxicaluminum species were not measured but, given findings elsewhere, were likely to havebeen high. Following the pH decline, a moderate fish kill occurred at the study site.Related studies on the Richmond and Clarence Rivers have shown that, duringthe 2 weeks after major rain events, significant falls in dissolved oxygen concentrationsconsistently occur in most representative floodplain tributaries and at some main-22ODAYSFigure 1 Changes in pH, dissolved oxygen (D.O.) concentrations and depth in a tributary of the RichmondRiver, New .South Wales, downstream of an acid sulphate soils area. Data were recorded at15-minute intervals by a submersible automatic data logger. Rainfall at the site is also shown.406
stream sites. Falls in pH at mainstream sites may also occur. For example, on theRichmond River in May 1987 at Site 4 (Figure 2), dissolved oxygen concentrationsfell from 5.8 mg I-' to 3.3 mg I-' and pH fell from 7.2 to 5.6 within 9 days of a majorrain event. However, at Site 4 in April 1988, dissolved oxygen concentrations fell from5.0 mg 1-' to 1.2 mg I-' but pH fell only marginally, from 6.6 to 6.2, within 10 daysof a major rain event.Clearly, runoff or drainage from acid sulphate soil areas contributes to thesechanges, to an extent dependent on a complex of factors including rainfall patternspreceding the major rain event and the distribution of rainfall over the catchmentduring the major rain event. However, it is likely that a 'pulse' of acidic, deoxygenatedwater is discharged from these areas during the early stages of most major rain events.Experimental exposure of fish to water with low pH (Daye & Garside 1976, Linnenbachet al 1987) and high aluminum concentrations (Tngersoll et al 1990b, Tandjunget al 1982) results in alterations to epidermal structure and, in extreme cases, epidermalnecrosis. Low concentrations of dissolved oxygen may also injure fish epidermis.Plumb et al (1 976) described apparantly sterile haemorrhagic and necrotic lesions inskin and skeletal muscle of channel catfish Ictalurus punctatus, exposed to dissolvedoxygen concentrations of less than 1 mg I-' for several days. They suggested that thisepidermal damage allowed subsequent bacterial invasion and septicaemia. It is likelythat, in some sections of lower river systems, and for varying periods after major rainevents, 'first-flush' runoff water from acid sulphate soil areas causes sublethal waterFigure 2 Location on the lower Richmond River of mainstream sampling sites for EUS prevalence (1-14)and known areas of acid sulphate soils (A-D)407
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