Iceland: The Land of Fire and Ice and Midges, Oh My!

Annual emergences of chronomid (non-biting) midges at the subarctic Lake Myvatn in northeastern Iceland are being studied for their interesting effects on arthropod food webs. Understanding the aquatic-terrestrial linkages in the near-shore ecosystems will shed light on the role midges play in the nitrogen cycle. 

Iceland is widely known for its beautiful landscape of volcanoes and glaciers, but few understand the curious arthropod food web surrounding Lake Myvatn. Claudio Gratton, someone who understands this system well, is a Professor of Entomology at the University of Wisconsin, Madison. He is currently studying the aquatic-terrestrial linkages in the landscape, specifically the role midges play in the nitrogen cycle. Lake Myvatn, located in northern Iceland just below the Arctic Circle, is known as “Midge Lake” due to the massive numbers of these insects that emerge from this lake in some years. Midge (Tanytarsus gracilentus and Chironomus islandicus, family: Chironomidae) emergences fluctuate year to year, with major emergences seen every 5 to 8 years (Einarsson et al. 2002; Ives et al. 2008). In high midge emergence years, they can contribute approximately 55 

Figure 1. Midge life cycle. Source: NC State Extension

metric tons of dry biomass to the surrounding landscape, which is “roughly equivalent to 510,000 Big Macs” according to Gratton. These flies swarm around the lake in what are known as mating pillars, made up of mostly males, and can be seen up to a few miles away from the lake.

Figure 2. C. Gratton sweep netting midges. Source: C. Gratton

These midges have important impacts on the surrounding environment by providing food for predators, distracting predators from other possible prey, and altering plant composition. The main predators of these midges are web-building spiders. On years when midge density is extraordinarily high, these will aggregate in larger numbers near the lake. Increased midge numbers reduce the predation of leafhoppers by web-building spiders. When spider density was increased, midges still reduced spider predation on other prey species by distracting the predators (Dreyer et al. 2016).

Increased midge biomass has also been shown to increase the density of graminoid plant species (herbaceous, grass-like) and decrease the number of health plant

(low growing woody) found close to the lake (M. Raudenbush, MS Thesis). This change in plant composition near the lake has been suggested to be the result of a “fertilization effect” as midges quickly decompose and their bodies are made up of 10% nitrogen, which becomes available to the environment (Gratton et al. in review). The impacts of increased nitrogen are seen as far away as 500 m from Lake Myvatn (Dreyer et al. 2015). To better understand the effect that midge deposition has on plant communities, Gratton set up a multi-year addition experiment on a midge-less lake where small plots were supplemented with dried midges (biomass) and changes in the plant community were documented. Some plots received midge biomass multiple consecutive years (the press treatment) and others received midges for just one year (the pulse treatment). Results from this eight-year study indicate that plots that received the pulse treatment saw short-lived effects in terms of total nitrogen and small and inconsistent increase in plant biomass that was gone by the second growing season. While these effects were short-lived they were still comparable to conventional fertilizer. In the press treatments, total nitrogen continues to increase but eventually declines, while the plant biomass continues to grow. . Over the eight-year study, plant community composition

shifts, with graminoids dominating in the press treatment plots (Gratton et al. in review). Midge populations at Lake Myvatn are known to influence the surrounding landscape and fluctuate year to year, therefore it is likely that similar press and pulse effects are impacting the plant communities surrounding Lake Myvatn.

While it seems that the Lake Myvatn study system is unique, there are many other instances where insects impact the landscape. Some examples include: periodic cicadas, locust swarms, monarch migration, and mayfly emergences. These insect phenomena can have long- and short-term impacts on the environment and can link ecosystems.

Figure 3. Midge. Source: C. Gratton.

About the Authors:

Olivia Bernauer is a Master’s student in Dennis vanEngelsdorp’s bee lab working to better understand the floral preferences of Maryland’s wild, native pollinators. 
 
Meghan McConnell is a Master’s student in Dennis vanEngelsdorp’s Lab studying honey bees with a focus on non-chemical control of varroa mites. After 5 years at UMD with the bee lab and Bee Informed Partnership, she will be the Delaware State Apiarist.
 
References:
Dreyer, J., Townsend, P. A., III, J. C. H., Hoekman, D., Vander Zanden, M. J. and Gratton, C. (2015), Quantifying aquatic insect deposition from lake to land. Ecology, 96: 499–509. doi:10.1890/14-0704.1
 
Dreyer, J., Hoekman, D. and Gratton, C. (2016), Positive indirect effect of aquatic insects on terrestrial prey is not offset by increased predator density. Ecol Entomol, 41: 61–71. doi:10.1111/een.12272
 
Einarsson, Á., Gardarsson, A., Gíslason, G. M. and Ives, A. R. (2002), Consumer–resource interactions and cyclic population dynamics of Tanytarsus gracilentus (Diptera: Chironomidae). Journal of Animal Ecology, 71: 832–845. doi:10.1046/j.1365-2656.2002.00648.x
 
Ives, Anthony R., Árni Einarsson, Vincent A. A. Jansen, and Arnthor Gardarsson. 2008. “High-Amplitude Fluctuations and Alternative Dynamical States of Midges in Lake Myvatn.” Nature 452 (7183): 84–87. doi:10.1038/nature06610.