The population dynamics of chironomid midges in Lake Mývatn, Iceland

Abstract Lake Mývatn experiences remarkable fluctuations in the populations of many of its 37 midge species. Here, we analyzed data from four adult traps from 1977 to 2023, and nine traps from 1999 to 2023, to characterize the annual dynamics of individual midge species, patterns of trends and cyclicity, synchrony among their fluctuations, and changes in community composition. Eighteen of the 20 common species from the 1977–2023 dataset showed quasi-periodic dynamics with periods of either 3–4 or 6–8 years. Furthermore, 12/20 species showed significant declines in abundance, while only one showed a significant increase. Although many pairs of species fluctuated in synchrony, synchrony was not greater for taxonomically related species. However, species that were more likely to have high adult abundances at the same locations (traps) were more likely to show synchrony. Finally, at the community level, a single species, Tanytarsus gracilentus , was associated with changes in the species composition of midges in Mývatn through time, while another common species, Cricotopus sylvestris , was primarily responsible for differences in community composition in space; differences occurred among traps from the two basins of Mývatn and along the Laxá outlet river. The prevalence of quasi-periodicity and synchrony in the dynamics among species suggests that many species share the same drivers of population fluctuations. We discuss the possible drivers, and although we can use the data to exclude some possible explanations of midge population dynamics, the data do not reveal a single explanation as best. Our results show the value of high-quality, long-term data on communities of organisms for revealing the existence of species-species and species-environment interactions that link the dynamics of species within the community and generate spatiotemporal changes in community composition.