Quantifying aquatic insect deposition from lake to land

Adjacent ecosystems are influenced by organisms that move across boundaries, such as insects with aquatic larval stages and terrestrial adult stages, which transport energy and nutrients from water to land. However, the ecosystem-level effect of aquatic insects on land has generally been ignored, perhaps because the organisms themselves are individually small. At the naturally productive Lake Mývatn, Iceland, we used two readily measured quantities: total insect emergence from water and relative insect density on land, to demonstrate an approach for estimating aquatic insect deposition (e.g., kg N·m⁻²·yr⁻¹) to shore. Estimates from emergence traps between 2008 and 2011 indicated a range of 0.15–3.7 g·m⁻²·yr⁻¹, or a whole-lake emergence of 3.1–76 Mg/yr; all masses are given as dry mass. Using aerial infall trap measurements of midge relative abundance over land, we developed a local-maximum decay function model to predict proportional midge deposition with distance from the lake. The dispersal model predicted midge abundance with R² = 0.89, a pattern consistent among years, with peak midge deposition occurring 20–25 m inland and 70% of midges deposited within 100 m of shore. During a high-midge year (2008), we estimate midge deposition within the first 50 m of shoreline to be 100 kg·ha⁻¹·yr⁻¹, corresponding to inputs of 10 kg N·ha⁻¹·yr⁻¹ and 1 kg P·ha⁻¹·yr⁻¹, or about three to five times above background terrestrial N deposition rates. Consistent with elevated N input where midges are most dense, we observed that soil available nitrate in resin bags decreases with increasing distance from the lake. Our approach, generalizable to other systems, shows that aquatic insects can be a major source of nutrients to terrestrial ecosystems and have the capacity to significantly affect ecosystem processes.