1. Background: The impact of climate change on the longitudinal distribution of macroinvertebrates in glacier‐fed streams has been the subject of increasing awareness in recent years. Despite prior intensive research on the longitudinal distribution of macroinvertebrates in glacier-fed ecosystems, long-term studies in glacier-fed rivers of arctic regions, that investigate the temporal change of macroinvertebrate communities during periods of rapid climate change, are largely missing, although climate change is expected to cause large shifts in arctic ecosystems. Especially in the arctic rivers of Iceland, where the aquatic macroinvertebrate fauna is very poor in species, and glacier-fed rivers have the lowest density and diversity of benthic invertebrates of all river groups, the response of benthic macroinvertebrates to future environmental change may differ distinctly from those in low-latitude climate regions. Here we present the results of a pilot study investigating the changes of the macroinvertebrate communities along the longitudinal gradient of the glacial river ecosystem Vestari-Jökulsá in Iceland between the years 1996 and 2018, where environmental conditions changed rapidly, especially including the recession of its source glacier.

2. Methods: At the 9 respective locations invertebrate sampling took place along a 15 m river reach, where 10 stones were removed while holding a surber sampler with a mesh size 250 μm downstream of the stone. Electrical conductivity, water temperate, pH-value, dissolved oxygen, turbidity and chlorophyll α were measured in-situ and water samples were taken for analyzing DOC (concentration, optical properties), POC and nutrients.

3. Results: The total number of species and diversity decreased significantly from July 1996 to July 2018. The longitudinal distribution of macroinvertebrates within the studied Arctic glacial stream did not resemble long-term changes as their occur in low-latitude glacial streams. While 36 taxa related to 12 different species were found in 1996, their number has decreased to 8 in 2018.

4. Conclusions: At present, it is difficult to firmly determine the exact mechanisms which control the reduction of macroinvertebrates along the unique ecosystem of Vestari-Jökulsá with its shows comparatively low biodiversity. Glacier retreat may be crucial, but it is imperative to carry out further comparative studies, especially investigating the influence of changing hydrological conditions on the longitudinal changes of macroinvertebrate in artic glacier-fed rivers.

At the 9 respective locations invertebrate sampling took place along a 15 m river reach, where 10 stones were removed while holding a surber sampler with a mesh size 250 μm downstream of the stone. Electrical conductivity, water temperate, pH-value, dissolved oxygen, turbidity and chlorophyll α were measured in-situ and water samples were taken for analyzing DOC (concentration, optical properties), POC and nutrients.