Average score per metric: an alternative metric aggregation method for assessing wadeable stream health

Integrating multiple metrics derived from stream macroinvertebrate communities into single scores that reflect ecological condition can bridge the needs of multiple groups using biomonitoring data. Macroinvertebrate metrics from 511 Waikato, New Zealand, stream samples were standardised by their maximum observed value. Metric redundancy and optimisation processes identified three standardised metrics from an initial set of 17 that provided greatest discrimination between reference sites and those influenced by urbanisation or high levels of pastoral development: the richness of Ephemeroptera, Plecoptera and Trichoptera excluding Hydroptilidae (EPT*), % EPT* and the macroinvertebrate community index (MCI). The mean of these standardised metrics was used to calculate the average score per metric (ASPM). Narrative condition bands were developed based on the lower SD of individual metrics for reference samples to distinguish “very high” values, and metric quartiles between this point and a hypothetical worst-case community were used to define “high”, “moderate”, “low”, and “very low” bands. When compared with its component metrics, the ASPM distinguished reference conditions and low–moderate levels of catchment modification and local habitat degradation more accurately than EPT* richness or MCI, and displayed lower temporal variability within reference sites than either EPT* metric. The ASPM was calculated for an independent test data set of urban and reference sites which, respectively, were allocated to low–very low and high–very high narrative condition bands. This analysis suggests that prudent application of the ASPM can provide a parsimonious and effective screening tool for assessing the condition of wadeable streams where more complex methods are not practical