A biotic index for New Zealand's soft‐bottomed streams

Abstract

Versions of the Macroinvertebrate Community Index (MCI) and its semi‐quantitative (SQMCI) and quantitative (QMCI) variants used extensively in New Zealand to assess organic pollution in hard‐bottomed (HB) streams have been developed for assessing the health of soft‐bottomed (SB) streams dominated by fine sediments, woody debris, and macrophytes. Aniterative rank correlation procedure developed in Australia was used to derive tolerance values for 124 macroinvertebrate taxa using a data set of 224 samples from 50 sites in the Auckland region. Deriving SB tolerance values to the nearest 0.1 (range 0.1–10.0) improved index performance and eliminated confusion with the integer tolerance values of the HB indices. The new indices (MCI‐sb and SQMCI‐sb), exhibited a greater range of scores overall and across gradients of forestry, rural, and urban land‐use intensity compared to the MCI and SQMCI when applied to SB streams. Higher scores for reference sites, lower scores for severely degraded sites, and high rank correlations (rs) with environmental variables, including % developed land in the catchment (rs = ‐0.842), a water quality index (rs = 0.818), and a habitat quality index (rs = 0.648), confirmed the superiority of MCI‐sb for classifying SB sites. Tolerance values for 35 additional taxa were derived using data from six other regions of New Zealand combined with the Auckland data. The interpretation of HB and SB versions of the MCI, SQMCI, and QMCI are the same (i.e., “excellent”: >120 or >6.00, “good”: 100–119 or 5.00–5.99, “fair”: 80–99 or 4.00^.99, “poor”: <80 or <4.00), simplifying the use of these indices in State of the Environment reporting. The MCI‐sb performed better than the SQMCI‐sb and is recommended for assessing SB stream health. Standard methods now exist for macroinvertebrate sampling, sample processing, quality control, and reporting (biotic indices and quality classes) on the health of HB and SB streams in New Zealand.