Detection and analysis of unusual events in long-term zooplankton and nekton data sets from North Inlet Estuary, South Carolina, USA

Unusual events and disturbances are important sources of variability in ecological data sets, but methods for their detection and analysis are not readily available. We have adapted widely-used quality control methods to identify and quantify unusual events in ten-year-long data sets from the relatively pristine North Inlet Estuary, South Carolina, USA.-Seasonal patterns and long-term trends were first removed from biweekly zooplankton and nekton abundance and corresponding water temperature and salinity data using nonparametic smoothing algorithms. The Shewhart Control Chart Method, a quantitative technique used for quality control in industrial manufacturing processes, was used to define four types of events according to intensity and duration. Deviations of data points from the precalculated mean were determined and, based on the Shewhart criteria, unusual events were identified. Events were uncommon for most of the 39 zooplankton and nekton variables tested. Timing and frequency of events were irregular within and among years. The coincidental occurrence of biological and physical events was rare; however, unusually high or low abundances of some taxa occurred during some extreme salinity and temperature events. In general, a high degree of independence among taxa was indicated. Results of the analyses provide new insights into the ecological significance of stochasticity in the dynamic estuarine ecosystem. The Shewhart Control Chart Method is a relatively simple and unique procedure for investigating atypical variation, and its application may be useful for understanding the role of unusual events in determining long-term change in both natural and altered ecosystems.