Diatom communities as indicators of ecological status in Mediterranean temporary streams (Balearic Islands, Spain)

摘要

Diatoms are widely used for the bioassessment of permanent rivers and streams due to their broad distribution and their ability to integrate changes occurring in water composition and quality, but they have rarely been investigated in temporary streams. Water and epilithic diatom samples were collected from 60 sites in temporary streams distributed among the three largest Balearic Islands (Majorca, Minorca and Ibiza). The sampling design included only temporary streams that have water for at least 4-5 months during the annual period. The selected sites included minimally disturbed (reference sites) and other sites influenced by different degrees of human pressures. Each study site was sampled at least twice in different seasons to take into account the temporal variability in diatom assemblages (n = 255). Non-metric multidimensional scaling (NMDS) and analyses of similarities (ANOSIM) were used to assess variation in the communities between a priori defined types and along to the degradation gradient, and to study the species tolerance to pollution. A diatom multimetric index (DIATMIB) was developed to assess the ecological status of temporary streams, as a combination of three metrics: abundance of sensitive taxa, abundance of tolerant taxa and a metric of chlorophyll a values. Individual diatom metrics and the multimetric DIATMIB were correlated in a significant way with phosphate, nitrate, ammonia, silicon dioxide, ash free dry mass and water electric conductivity. Cymbella vulgata, Diploneis oblongella and Encyonop-sis microcephala appeared only in reference sites and in high and good status classes. These taxa were replaced by others such as Navicula veneta, Nitzschia inconspicua, N.frustulum and Planothidium frequen-tissimum that appeared in moderate, poor and bad status classes, which were characterized by high levels of organicutrient pollution. Our study validates the application of a diatom multimetric index as a good approach to classify the ecological status of temporary streams due to changes in the structure of diatom communities. We concluded that the multimetric index DIATMIB is a valuable tool for the assessment and future monitoring of temporary Mediterranean streams.