Ecosystem engineering by primary consumers increases long-term benthic primary production in Lake Mývatn, Iceland

Abstract Ecosystem engineers impact ecosystems by changing the physical environment in which species interact. In Lake Mývatn, Iceland, larvae of the midge Tanytarsus gracilentus function as ecosystem engineers by building tubes in the sediment that solidify the benthic surface and create a 3-dimensional structure on which microalgae can grow. To assess the ecosystem-scale effect of midge engineering, we took advantage of the natural high-amplitude fluctuations in T. gracilentus over a 13-year time series. We combined larval abundance data with benthic primary production under saturating light conditions (P max ) and pelagic in situ gross primary production (GPP) measured on average 8.4 times per summer. T. gracilentus abundance had a positive effect on benthic P max , as expected from their ecosystem engineering, while pelagic GPP had a negative effect on benthic P max , reflecting competition from phytoplankton. A complementary analysis of five replicate sediment cores taken on each of 26 sampling days of the time series shows a quantitatively similar positive effect of local-scale variation in larval midge abundance on benthic net ecosystem production (NEP) in space. Finally, we reanalyzed data from an experiment manipulating light levels and the presence/absence of midge larvae to show that larval engineering increased benthic NEP more at low than at high light levels. This indicates that at least part of the positive engineering effect of midges on benthic P max arises from amelioration of light limitation through self-shading, likely due to the structure created by midge tubes. These results show that Mývatn's high benthic primary productivity is due not only to high inputs of nutrients but also to the ecosystem engineering of tube-forming midges.