New Zealand's National River Water Quality Network: Long‐term trends in macroinvertebrate communities

Abstract

Aquatic macroinvertebrates were sampled annually at 66 sites of the National Rivers Water Quality Network (NRWQN) from 1989 to 1996. At each site, temporal trends in number of taxa, total abundance, Macroinvertebrate Community Index (MCI), the quantitative version of MCI (QMCI), number and percent of Ephemeroptera, Plecoptera, and Trichoptera, Shannon's Diversity Index (H'), Jaccard's similarity index, and Bray‐Curtis similarity index were analysed using non‐parametric Spearman rank correlations. Sixty‐nine significant (P < 0.10) trends were found out of a total of 594 analyses. There were 27 positive trends (i.e., showing increases in invertebrate community measures through time) and 42 negative. MCI and Jaccard's Index exhibited the greatest number of trends (13), and Shannon's diversity the least (1). A number of sites (8) showed a significant trend of decreasing taxa richness, particularly pollution‐sensitive Ephemeroptera, Plecoptera, and Trichoptera (EPT). However, MCI values, which are strongly linked to numbers of EPT taxa, had the highest number of positive trends (12), with only one site showing a negative trend. Overall the trend in MCI scores was particularly strong for “baseline” sites, and less so for “impact” sites. The percentage of baseline sites with MCI scores indicative of clean water increased from 3% in 1989 to 46% in 1996. At two sites trends in MCI and number of EPT taxa gave conflicting signals. Regional patterns were apparent, with trends in MCI in particular being more common in the east and lower half of the North Island, and the lower half of the South Island than in other areas. Observed trends in some macroinvertebrate community measures, particularly MCI, coincided with reported general improvements in water quality.