Preliminary Estimations of Insect Mediated Transfers of Mercury and Physiologically Important Fatty Acids from Water to Land

Abstract

Aquatic insects provide an energy subsidy to riparian food webs. However, most empirical studies have considered the role of subsidies only in terms of magnitude (using biomass measurements) and quality (using physiologically important fatty acids), negating an aspect of subsidies that may affect their impact on recipient food webs: the potential of insects to transport contaminants (e.g., mercury) to terrestrial ecosystems. To this end, I used empirical data to estimate the magnitude of nutrients (using physiologically important fatty acids as a proxy) and contaminants (total mercury (Hg) and methylmercury (MeHg)) exported by insects from rivers and lacustrine systems in each continent. The results reveal that North American rivers may export more physiologically important fatty acids per unit area (93.0 ± 32.6 Kg Km−2 year−1) than other continents. Owing to the amount of variation in Hg and MeHg, there were no significant differences in MeHg and Hg among continents in lakes (Hg: 1.5 × 10−4 to 1.0 × 10−3 Kg Km−2 year−1; MeHg: 7.7 × 10−5 to 1.0 × 10−4 Kg Km−2 year−1) and rivers (Hg: 3.2 × 10−4 to 1.1 × 10−3 Kg Km−2 year−1; MeHg: 3.3 × 10−4 to 8.9 × 10−4 Kg Km−2 year−1), with rivers exporting significantly larger quantities of mercury across all continents than lakes. Globally, insect export of physiologically important fatty acids by insect was estimated to be ~43.9 × 106 Kg year−1 while MeHg was ~649.6 Kg year−1. The calculated estimates add to the growing body of literature, which suggests that emerging aquatic insects are important in supplying essential nutrients to terrestrial consumers; however, with the increase of pollutants in freshwater systems, emergent aquatic insect may also be sentinels of organic contaminants to terrestrial consumers.

Feeding habits and multifunctional classification of soil‐associated consumers from protists to vertebrates

“…Diptera play a keystone role in cross‐habitat linkages through the transfer of matter and energy, which is very important at the boundaries of aquatic–terrestrial ecosystems (Hoekman et al ., 2011; Lafage et al ., 2019). Studies based on stable isotope (Collier, Bury & Gibbs, 2002; Sanzone et al ., 2003; Paetzold, Bernet & Tockner, 2006; Mellbrand et al ., 2011) and fatty acid analyses (Gladyshev, Arts & Sushchik, 2009; Gladyshev, Gladysheva & Sushchik, 2019; Moyo, 2020) showed that adult Diptera emergent from water can be an important or even the main source supporting the high density and diversity of invertebrates in adjacent terrestrial ecosystems (Gratton, Donaldson & Zanden, 2008; Korobushkin et al ., 2016; Radermacher et al ., 2020). Overall, Diptera are trophically diverse even though they may be morphologically similar.…”

Differential reliance on aquatic prey subsidies influences mercury exposure in riparian arachnids and songbirds

“…We found that aquatic energy subsidies comprised a large component of riparian predator diets, and species in riparian habitats foraging on aquatic-derived food items were more likely to have higher Hg exposure than those foraging on prey derived from terrestrial habitats. Contaminants like Hg are generally higher in aquatic than terrestrial environments; thus, these subsidies may represent substantial vectors of aquatic contaminants into terrestrial communities (Kraus et al, 2014;Moyo, 2020;Walters et al, 2008).…”

“…The flux of aquatic prey is an important nutrient source, as emergent insects have higher polyunsaturated fatty acids (PUFA) than terrestrial prey (Martin-Creuzburg et al, 2017;Moyo, 2020). Both terrestrial predatory invertebrates (e.g., arachnids) and songbirds often concentrate near aquatic habitats ostensibly to exploit the emergent insect subsidy during peak emergence times (Hagar et al, 2012;Uesugi & Murakami, 2007).…”

Trait–environment relationships could alter the spatial and temporal characteristics of aquatic insect subsidies at the macrospatial scale

“…Our results are indicative of potential characteristics of aquatic subsidies as they are transported into terrestrial ecosystems by adults. Although adults and their traits are sourced from aquatic communities, emergence rates are likely variable among taxa (Poepperl 2000, Moyo 2020) and location (Gratton and Vander Zanden 2009). The trait–environmental relationships we uncovered however, could be used in tandem with large‐scale analyses of aquatic secondary production (Patrick et al 2019) to refine estimates of aquatic insect deposition (Gratton and Vander Zanden 2009, Bartrons et al 2013).…”