Warren L. Paul

Control charting instrumental analyses

Abstract Control chart methodology has important applications in the analytical chemistry laboratory. This paper explores application of statistical process control (SPC) techniques to chemical measuring systems and highlights problems when data are correlated. Instrumental methods of chemical analysis typically generate correlated quality-control (QC) sample data owing to the effect of calibration. That is, QC sample measurements stemming from the same calibration curve will tend to be more closely related than measurements derived from different calibration curves. Data correlation will bias the estimated sample variance and essentially invalidate the control limits if not accounted for. A way of calculating statistically valid control limits in the presence of intraclass correlation is examined and an illustrative example given.

Causal modelling applied to the risk assessment of a wastewater discharge

Bayesian networks (BNs), or causal Bayesian networks, have become quite popular in ecological risk assessment and natural resource management because of their utility as a communication and decision-support tool. Since their development in the field of artificial intelligence in the 1980s, however, Bayesian networks have evolved and merged with structural equation modelling (SEM). Unlike BNs, which are constrained to encode causal knowledge in conditional probability tables, SEMs encode this knowledge in structural equations, which is thought to be a more natural language for expressing causal information. This merger has clarified the causal content of SEMs and generalised the method such that it can now be performed using standard statistical techniques. As it was with BNs, the utility of this new generation of SEM in ecological risk assessment will need to be demonstrated with examples to foster an understanding and acceptance of the method. Here, we applied SEM to the risk assessment of a wastewater discharge to a stream, with a particular focus on the process of translating a causal diagram (conceptual model) into a statistical model which might then be used in the decision-making and evaluation stages of the risk assessment. The process of building and testing a spatial causal model is demonstrated using data from a spatial sampling design, and the implications of the resulting model are discussed in terms of the risk assessment. It is argued that a spatiotemporal causal model would have greater external validity than the spatial model, enabling broader generalisations to be made regarding the impact of a discharge, and greater value as a tool for evaluating the effects of potential treatment plant upgrades. Suggestions are made on how the causal model could be augmented to include temporal as well as spatial information, including suggestions for appropriate statistical models and analyses.

Changes in soil carbon in response to flooding of the floodplain of a semi-arid lowland river

Soil C is a key factor influencing soil health and, by inference, ecosystem condition. In arid and semi-arid regions, soil moisture limits accumulation and decomposition of soil C. On floodplains, soil moisture can come from rainfall events or periodic flooding. The effect of flooding on soil C decomposition has received some attention in the literature but mostly through the study of agricultural systems or mesocosms. Field studies of actual floods are not common. We measured soil C response to a managed flood on a semi-arid lowland river floodplain in southeastern Australia before and after inundation at control (unflooded), floodplain (low hydraulic energy), and flood runner (high hydraulic energy) sites. At control sites, soil C changed little from before to 16 d after the onset of the flood. At flooded sites, most measures of soil C (total soil organic matter [floodplain sites only], permanganate-oxidizable C, persulfate-extractable C, water-extractable C [flood runner sites only], microbial C, and root material) were lower 16 d after than before the flood. However, the activity of key hydrolytic exo-enzymes associated with soil C decomposition did not change. Following the initial decline in soil C at the flooded sites, soil properties remained essentially constant for the remainder of the flood. Upon flood recession, slight increases in C pools occurred at the flood runner site (possibly associated with the formation of debris dams) but not at the floodplain or control sites. The changes in soil C pools after flood recession were at least an order of magnitude lower than those observed at the same location during more extended flooding. We argue that the loss of soil C associated with short-duration floods may adversely affect ecosystem condition of lowland river floodplains. Managed floods should be of long enough duration (months) to promote the growth of submerged and emergent aquatic macrophytes on the floodplain because aquatic macrophyte production is an important source of soil C after flood recession in lowland river floodplains.

Long‐term Monitoring of Macroinvertebrate Communities Over 2,300 km of the Murray River Reveals Ecological Signs of Salinity Mitigation Against a Backdrop of Climate Variability

We investigated the ecological effects of salinity mitigation strategies in the Murray‐Darling Basin (MDB) using macroinvertebrate data collected over 2,300 km of the Murray River between 1980 and 2012. The MDB covers 1 × 106km2 and includes both temperate and semiarid climate zones. It was extensively developed to support irrigated agriculture in the early to mid‐1900s, and the secondary salinization that followed has become a major concern. During 1975–1985 daily salinity levels, measured as electrical conductivity above the Murray River off‐take points for South Australias major urban water supplies, were above 800 μS/cm for 40% of the time, necessitating mitigation strategies that have reduced the average salinity by about 150 μS/cm since monitoring began. The MDB has also experienced several major floods and droughts during this time, and surface temperatures in the MDB have increased by 0.8 °C since 1910, mostly in the last 50 years. We hypothesized that (1) taxa richness would increase in response to floods; (2) community structure would shift toward tolerant, opportunistic taxa in response to warming; and (3) geographical ranges of species would change in response to shifting stream isotherms and reducing salinity. Our hypotheses were supported, although increases in water temperature appeared to be due principally to the 1997–2009 Millennium drought. Importantly, against a backdrop of significant climate variability, we believe we have distinguished a change in community structure along a salinity gradient and that changes over the 33 years can in part be attributed to mitigation strategies.