Effects of Dry‐Phase Frequency and Duration on Macroinvertebrate Assemblages in an Alpine Stream

ABSTRACT Although temporary streams are a common feature of fluvial networks, the prevalence of flow intermittency of perennial streams, especially in alpine landscapes, has increased with climate change. The fluvial network of the alpine Val Roseg catchment in eastern Switzerland experiences ca. 80% surface flow contraction over an annual cycle. Our aim was to determine how repeated dry phases of different durations and frequencies influence organic resources and macroinvertebrate assemblages in a perennial stream in this catchment. We conducted a field experiment in which replicated mesocosms experienced varying dry‐phase durations (1 or 9 days) and frequencies (every 3, 7 or 15 days, respectively) comprising six treatments and a control (continuous flow). A fine‐meshed net was installed midway across the experimental reach to limit drift to a downstream section of mesocosms to determine the contribution made by drift to recolonisation when flows resumed. The upstream section contained other mesocosms in which drift was not limited by the drift net. At the end of the experiment, we measured coarse particulate organic matter (CPOM), periphyton biomass and taxonomic and functional metrics as well as compositional attributes of macroinvertebrates in each mesocosm. CPOM (leaves and larch needles), periphyton chlorophyll‐a and macroinvertebrate abundance and taxon richness decreased with increasing dry‐phase duration and frequency, whereas Shannon's diversity and taxon evenness increased. Metrics of functional richness and diversity also decreased with increasing dry‐phase duration and frequency. Most macroinvertebrate metrics, and functional measures in particular, were lower in the drift‐limited downstream section. Lastly, assemblage composition differed in mesocosms in relation to increasing dry‐phase frequency (15 days and controls) in both sections. We conclude that dry‐phase duration and especially frequency are strong drivers of organic resources and macroinvertebrate assemblages in intermittent alpine streams. Levels of CPOM and periphyton chlorophyll‐a were reduced with dry phases. Macroinvertebrates had low resistance to dry phases but had resilience traits that enabled rapid recolonisation once flow resumed. This study provides a mechanistic explanation of the observed spatial patterns of macroinvertebrate assemblages in alpine fluvial networks exposed to intermittent drying.