Emergent Freshwater Insects Serve as Subsidies of Methylmercury and Beneficial Fatty Acids for Riparian Predators Across an Agricultural Gradient

Abstract

Aquatic-to-terrestrial subsidies have the potential to provide riparian consumers with benefits in terms of physiologically important organic compounds like omega-3 long-chain polyunsaturated fatty acids (n-3 LCPUFAs). However, they also have a "dark side" in the form of exposure to toxicants such as mercury. Human land use intensity may also determine whether subsidies provide benefits or come at a cost for riparian predators. We sampled insects as well as Eastern Phoebe (Sayornis phoebe) chicks in 2015−2016 within the southern Finger Lakes region to understand how food quality, in terms of n-3 LCPUFAs and methylmercury (MeHg), of emergent freshwater insects compared with that of terrestrial insects and how land use affected the quality of prey, predator diet composition, and MeHg exposure. Across the landscape, freshwater insects had a significantly higher percentage of the n-3 LCPUFA eicosapentaenoic acid (EPA) compared to terrestrial insects and contained significantly more MeHg than terrestrial insects did. In spite of differences in MeHg concentrations between aquatic and terrestrial insects, chick MeHg concentrations were not related to diet composition. Instead, chick MeHg concentrations increased with several metrics of human land use intensity, including percent agriculture. Our findings suggest that freshwater subsidies provide predators with both risks and benefits, but that predator MeHg exposure can vary with human land use intensity.

Increased Mercury and Reduced Insect Diversity in Linked Stream–Riparian Food Webs Downstream of a Historical Mercury Mine

“…Notably, THg concentrations in bull trout observed in our study were also within the range of human consumption advisories (1 and 2 fish meals/week; 0.05–0.22 µg THg g −1 wet wt axial muscle, Lepak et al, 2016) and of having reproductive effects on piscivorous fish and birds (Ackerman et al, 2016; A. Jackson et al, 2016). Similarly, MeHg and THg concentrations in spiders collected downstream of mining (347–1140 ng MeHg g −1 dry wt; 453–2060 ng THg g −1 dry wt) were at the upper end of concentrations previously observed in riparian spiders and of potential risk to arachnivorous birds (Chaves‐Ulloa et al, 2016; Gann et al, 2015; Hannappel et al, 2021; Ortega‐Rodriguez et al, 2019; Otter et al, 2013; Pennuto & Smith, 2015; Twining et al, 2021). Similar concentrations in arachnids near Caddo Lake (a wetland–lake complex) in Texas and Louisiana (USA; 19.4–256 ng MeHg g −1 wet wt; 90–1182 ng THg g −1 dry wt) were reported to have posed a risk of physiological impairment for young black‐capped chickadees ( Poecile atricapillus ) ranging from 122% to 434% of wildlife values (Gann et al, 2015).…”

Combined influence of food availability and agricultural intensification on a declining aerial insectivore

“…Increased provisioning of these types of insects may lead to a more nutritive diet in less intensively cultivated landscapes as they typically contain high levels of highly unsaturated omega‐3 fatty acids (HUFA), which have been found to improve the mass gain, immunocompetence, and fledging success of Tree Swallow nestlings (Twining et al, 2018). That said, insects with an aquatic larval stage can also be loaded with various environmental contaminants, such as pesticides and heavy metals, that may impair the breeding behavior and success of swallows as well as their survival (Alberts et al, 2013; Custer, 2011; Twining et al, 2021). The costs and benefits of feeding on aquatic insects rich in HUFA within an agricultural context will thus require further attention before we can determine its real impact on Tree Swallow fitness components and demography.…”

Temperature and land use influence tree swallow individual health

“…In our study system, nestlings exhibited ~38% dietary reliance on emergent aquatic insects ( Sullivan et al , 2021 ). Despite the nutritional benefits conferred by emergent insects, this advantage may be compromised by emergent-insect facilitated biomagnification of contaminants such as mercury (Hg) ( Rowse et al , 2014 ; Twining et al , 2021 ). Thus, trade-offs between the benefits and risks associated with aquatic insects emerging from polluted aquatic ecosystems may contribute to explaining the negative relationship we observed between emergent insects and physiological stress.…”