Quantifying aquatic insect deposition from lake to land

Abstract

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 x m(-2) x yr(-1)) to shore. Estimates from emergence traps between 2008 and 20.11 indicated a range of 0.15-3.7 g x m(-2) x 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 xha(-1) x yr(-1), corresponding to inputs of 10 kg N x ha(-1) x yr(-1) and 1 kg P x ha(-1) x 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.

Importance of Riparian Zone: Effects of Resource Availability at Land-water Interface

“…Aquatic ecosystems often transfer large quantity of resource subsidies to terrestrial ecosystems, such as adult aquatic insects [9,18]. The majority deposition of emerging adult aquatic insects occurs in riparian area and the deposition rate into the terrestrial habitats is related to the distance to the water edge [52].…”

“…Plants -While most studies focused on the importance of riparian forests as donor ecosystems to transfer leaf litter, large wood, seeds, pollen and terrestrial insects to aquatic ecosystems [5,10,39,56,108,109], or as recipient ecosystems for terrestrial consumers to enjoy the aquatic subsidy feast [7,9,23,27,54], ecologists investigated effects of aquatic subsidies (e.g. organic matter, emerging aquatic insects, salmon carcasses) on riparian plants [97,100,110,111].…”

“…organic matter, emerging aquatic insects, salmon carcasses) on riparian plants [97,100,110,111]. As most adults of aquatic insects deposited within 100 m into the land [18], and this nutrient subsidy can significantly affect primary production in nutrientlimited ecosystems adjacent to waterbodies [9]. In one study, midge deposition peaked at 12 kg N·ha -1 ·yr -1 near shore during a high midge-emergence year [9], and this N contribution from midges can be three to five times the level of background atmospheric deposition in the subarctic [112].…”

“…As most adults of aquatic insects deposited within 100 m into the land [18], and this nutrient subsidy can significantly affect primary production in nutrientlimited ecosystems adjacent to waterbodies [9]. In one study, midge deposition peaked at 12 kg N·ha -1 ·yr -1 near shore during a high midge-emergence year [9], and this N contribution from midges can be three to five times the level of background atmospheric deposition in the subarctic [112]. Consequently, % N dry weight of willow leaves in high midge sites was 8-11% higher than low-midge sites, and this was further beneficial to the herbivorous insects (Hydriomena furcata Thunberg) with 4-6 times higher density and 72% heavier individual biomass [111].…”

Variation in reciprocal subsidies between lakes and land: perspectives from the mountains of California

“…However, the resources flowing out of lakes (mostly in the form of animal biomass) tend to be high-quality food items, and aquatic resources can be just as important to terrestrial animals as terrestrial resources are to aquatic animals (Bartels et al 2012). The emergence of aquatic insects (Gratton and Vander Zanden 2009;Bartrons et al 2013;Dreyer et al 2015) and amphibians (Regester et al 2006;Gibbons et al 2006;Schriever et al 2013) are the best studied lake-to-land resource flows, and these subsidies can have important effects on terrestrial ecosystems (Richardson and Sato 2015). For example, changes in consumer density and behavior associated with emerging aquatic insects can have cascading top-down effects on lower trophic levels in riparian areas surrounding streams (Henschel et al 2001;Murakami and Nakano 2002;Sabo and Power 2002), and the deposition of aquatic insect carcasses can represent an important source of nutrients to terrestrial plants near lakes (Hoekman et al 2012), with consequent bottom-up effects on higher trophic levels (Bultman et al 2014).…”