Development of invertebrate community indexes of stream quality for the islands of Maui and Oahu, Hawaii

Abstract

In 2009-10 the U.S. Geological Survey (USGS) collected physical habitat information and benthic macroinvertebrates at 40 wadeable sites on 25 perennial streams on the Island of Maui, Hawaiʻi, to evaluate the relations between the macroinvertebrate assemblages and environmental characteristics and to develop a multimetric invertebrate community index (ICI) that could be used as an indicator of stream quality. The macroinvertebrate community data were used to identify metrics that could best differentiate among sites according to disturbance gradients such as embeddedness, percent fines (silt and sand areal coverage), or percent agricultural land in the contributing basin area. Environmental assessments were conducted using land-use/land-cover data and reach-level physical habitat data. The Maui data were first evaluated using the previously developed Preliminary-Hawaiian Benthic Index of Biotic Integrity (P-HBIBI) to determine if existing metrics would successfully differentiate stream quality among the sites. Secondly, a number of candidate invertebrate metrics were screened and tested and the individual metrics that proved the best at discerning among the sites along one or more disturbance gradients were combined into a multimetric invertebrate community index (ICI) of stream quality. These metrics were: total invertebrate abundance, Class Insecta relative abundance, the ratio of Trichoptera abundance to nonnative Diptera abundance, native snail (hihiwai) presence or absence, native mountain shrimp (′δpae) presence or absence, native torrent midge ( Telmatogeton spp.) presence or absence, and native Megalagrion damselfly presence or absence. The Maui ICI classified 15 of the 40 sites (37.5 percent) as having "good" quality communities, 17 of the sites (42.5 percent) as having "fair" quality communities, and 8 sites (20 percent) as having "poor" quality communities, a classification that may be used to initiate further investigation into the causes of the poor rating. Additionally, quantitative macroinvertebrate samples collected from 31 randomly selected sites on Oʻahu in 2006-07 as part of the U.S. Environmental Protection Agency's Wadeable Stream Assessment (WSA) were used to refine and develop an ICI of stream quality for Oʻahu. The set of metrics that were included in the revised index were: total invertebrate abundance, Class Insecta relative abundance, the ratio of Trichoptera abundance to nonnative Diptera abundance, turbellarian relative abundance, amphipod relative abundance, nonnative mollusk abundance, and nonnative crayfish ( Procambarus clarkii ) and/or red cherry shrimp ( Neocaridina denticulata sinensis ) presence or absence. The Oʻahu ICI classified 10 of the 31 sites (32.3 percent) as "good" quality communities, 16 of the sites (51.6 percent) as "fair" quality communities, and 5 of the sites (16.1 percent) as "poor" quality communities. A reanalysis of 18 of the Oʻahu macroinvertebrate sites used to develop the P-HBIBI resulted in the reclassification of 3 samples. The beginning of a statewide ICI was developed on the basis of a combination of metrics from the Maui and Oʻahu ICIs. This combined ICI is intended to help identify broad problem areas so that the Hawaii State Department of Health (HIDOH) can prioritize their efforts on a statewide scale. Once these problem areas are identified, the island-wide ICIs can be used to more accurately assess the quality of individual stream reaches so that the HIDOH can prioritize their efforts on the most impaired streams. By using the combined ICI, 70 percent of the Maui sites and 10 percent of the Oʻahu WSA sites were designated as "good" quality sites; 25 percent of the Maui sites and 45 percent of the Oʻahu WSA sites were designated as "fair" quality sites; and 5 percent of the Maui sites and 45 percent of the Oʻahu WSA sites were designated as "poor" quality sites.