Control charts for improved decisions in environmental management: a case study of catchment water supply in south-west Western Australia

Environmental monitoring is becoming increasingly sophisticated with the widespread adoption of data loggers, sensor arrays and remote sensing, leading to larger scale, higher resolution and superior quality data. However, interpreting monitoring data and deciding when and how to apply environmental management remains a subjective and underdeveloped area of research. Control charts, developed in industrial settings to identify when manufacturing processes were beyond the acceptable bounds of production quality, represent one solution. Despite their potential utility, control charts have rarely been adopted in environmental monitoring. In theory, they are able to identify undesirable trends early and provide transparent and broadly consensual criteria for defining when management action should take place, that is action is triggered when parameter values are observed beyond the agreed control limits of the process. Once triggered, a predetermined management action is implemented. Possible actions are many and varied, and range from investigation and increased monitoring to intervention in the system. Here, the utility of control charts in monitoring water supply in south-western Australia from 1911 to 2010 is examined, and their ability to provide an early, transparent and easily understandable means of triggering management action is assessed. Two control chart types are applied: the X-bar chart and the CUSUM chart. X-bar charts varied widely in their ability to trigger action and were insensitive to many traditional threshold criteria (of which there are many to choose from). In contrast, standard CUSUM charts are specifically designed to detect subtler shifts away from a mean trend and hence provided a more consistent warning of the decline in water supply. While managers were aware of the decline in water supply from an early stage, we believe that control charts could have clearly communicated this earlier, enabling consensus among decision makers to be reached more rapidly

Effects of carp, gambusia, and Australian bass on water quality in a subtropical freshwater reservoir

Freshwater systems are frequently compromised by excessive macrophyte biomass and cyanobacterial blooms associated with eutrophication and possibly exotic fish. The effects of piscivorous Australian bass (Macquaria novemaculeata), planktivorous gambusia (Gambusia holbrooki [Girard]), and benthivorous carp (Cyprinus carpio L) on water quality in Emigrant Creek Dam (ECD), Australia, were studied in 20 experimental enclosures (depth 1 m, volume 3.2 m3), stocked at a density of 1875 kg/ha. Fish additions had immediate effects on water quality, except for bass, which merely recycled preexisting water column nutrients. Water turbidity, total suspended solids (TSS), total nitrogen (TN) and total phosphorus (TP) concentrations, and chlorophyll a (Chl-a), were all significantly higher in gambusia, but more so in carp enclosures. Assuming a linear relationship, mean daily Chl-a increases in carp enclosures were 4.6 μg/L/100 kg carp/ha. Moreover, Chl-a (\u3e90 μg/L) was positively correlated with TP (r2 = 0.91), TN (r2 = 0.89), TFe (r2 = 0.96), and TMn (r2 = 0.83) in carp enclosures; however, Fe and Mn resuspension from sediments does not seem to be a principal mechanism affecting total phytoplankton biomass. Macrophyte, epiphyte, and periphyton biomass were also significantly lower in carp enclosures, a consequence of sediment and nutrient resuspension, dietary excretion, and macrophyte damage. In the absence of carp, epiphytes, periphyton, and macrophytes competed with phytoplankton for available nutrients, providing a critical stabilizing role. Exotic fish removal may prove to be an effective management tool in subtropical systems to maintain high water quality