Effects of water intermittency on diatom (Bacillariophyta) and invertebrate (aquatic and terrestrial) communities in streams of southern Portugal

Abstract

Macroinvertebrate and diatom communities were sampled in intermittent streams (i.e. no superficial flow for more than 3 months) of southern Portugal, in spring and end summer 2017 (when carabids were also sampled). The ecological classification of the 18 sampling sites ranged from Moderate to High in spring (flow conditions), according to the Water Framework Directive. During summer, high hydrologic variability was observed and the sites have been classified in three intermittency classes: “low flow” (2 sites), “scattered pools” (11 sites) and “dry” (5 sites). In “low flow” sites, macroinvertebrate and diatoms were collected following standard protocols; in “scattered pools” sites, macroinvertebrates were sampled from pools and two benthic diatom samples were collected from hard substrates in pools and dry riverbed; and in “dry” sites, only dry biofilm on hard substrate was collected. Carabids were sampled using pitfalls placedin the margins and channels in “scattered pools” and “dry” sites, and only along margins in “low flow” sites. First results show changes in diatom communities according to intermittency, with small differences between spring and summer samples collected from “low flow” sites; whilst spring differed from summer (independently of being collected from pools or as dry biofilm). Sites classified as Good or High in spring were more affected by intermittency, with significant reduction in macroinvertebrate species richness, diversity and evenness, whilst these differences were not so evident in Moderate sites. In “scattered pools” situation, a decrease in relative abundance of Trichoptera, Plecoptera and Ephemeroptera was detected, whilst Diptera, Heteroptera and Oligochaeta (more adapted to lentic conditions) increased in relative abundance. During the dry period, 14 carabid species were identified, both in the channel and margins. Among them, few species are typical from riverine systems and the majority occurs in agricultural areas, cork-oak forests and shrublands. This study increases our knowledge about the distribution and habitats of Pheropsophus hispanicus, a carabid species poorly known in Portugal. Our preliminary findings have also highlighted the importance of improving the assessment of intermittent streams considering the whole hydrological cycle (including the period with no flow), and integrating complementary biological indicators of their ecological status, as carabids and dry biofilm.