How do long-term patterns affect time-limited environmental monitoring programmes?

A five year Environmental Monitoring Programme (EMP) was set up in east-Australian coastal waters to determine the environmental effects of diverting most of Sydney's primary treated sewage from cliff face outfalls to diffuser outfall systems some 3 km offshore in 60–80 m of water. Many component EMP studies adopted Before and After Control Impact (BACI and ‘beyond BACI’) designs, to isolate outfall impacts on the marine ecosystem from background variability. Although seasonal patterns dominated ambient conditions in coastal waters, long-term climatic signals, such as the El Nino Southern Oscillation (ENSO), were apparent during the EMP. The Southern Oscillation Index (SOI) indicated the two year post-commissioning study period of 1991–1993 was dominated by an El Nino episode, while the 1989–1990 pre-commissioning period experienced a La Nina episode. Anomalous rainfall conditions and ocean temperature structure were associated with these episodes with respect to long-term records. Rainfall patterns suggested that the pre-commissioning period experienced high rainfall causing an increased incidence of stormwater pollution in coastal waters. Effects of the lower rain levels during post-commissioning phase were, however, outweighed by the dramatic improvements due to the commissioning of the deepwater outfalls. Decreased thermal stratification was experienced during the winter months of the 1992–1993 El Nino episode, and promoted increased dilutions associated with a greater frequency in sewage plume surfacing. There was, however, no apparent short-term effect of this change in plume behaviour on the marine environment as measured by EMP indicators.