Parmar, Tarn Preet

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Aquatic-terrestrial linkages : export of polyunsaturated fatty acids from aquatic ecosystems via emerging insects and potential consequences for terrestrial invertebrate consumers

2023, Parmar, Tarn Preet

Freshwater and surrounding terrestrial ecosystems are tightly connected via bidirectional fluxes of organisms and materials that can support and maintain adjacent food webs. Emergent aquatic insects represent an important pathway through which freshwater-derived energy and nutrients are transferred across the aquatic-terrestrial ecosystem boundary. During their larval stage, aquatic insects incorporate essential biochemical nutrients that become available to riparian predators as they emerge into the terrestrial ecosystem as adults. Algae, unlike terrestrial plants, contain high proportions of long-chain polyunsaturated fatty acids (LC-PUFA) that are transferred to higher trophic levels (i.e., emergent insect larvae). Emergent insects are an important dietary energy and LC-PUFA source for riparian terrestrial predators such as birds, bats, and spiders. LC-PUFA, such as eicosapentaenoic acid (EPA), play an important role in animal physiology as they influence growth and reproduction in both aquatic and terrestrial consumers. In addition, they have many important cellular functions, such as maintaining membrane fluidity and serving as eicosanoid precursors. Consumers that cannot biosynthesize LC-PUFA from C18 PUFA precursors rely on a dietary supply of LC-PUFA. The general aim of this thesis was to better understand emergent aquatic insects from lentic ecosystems and their potential importance for riparian terrestrial predators. To achieve this, I together with colleagues, collected aquatic emergent insects, terrestrial insects, birds, and web-building spiders from and around several lakes. We used a combination of fatty acid analysis, stable isotopes (δ13C, δ15N), and compound-specific stable isotope analysis (CSIA) to quantify LC-PUFA export and the incorporation of those LC-PUFA into riparian predators. With this research, I demonstrated the importance of aquatic insects, as opposed to terrestrial insects, to potential riparian predators due to higher LC-PUFA content (e.g., EPA). My colleagues and I were also able to show higher emergent aquatic insect biomass from shallower lakes, in general, and in shallower areas of lakes compared to their pelagic areas. We also demonstrated the importance of fishponds in providing LC-PUFA to riparian predators in surrounding terrestrial ecosystems via emergent aquatic insects. We were also able to demonstrate the importance of lakes and riparian zones as critical habitats where riparian predators, such as web-building spiders, can have greater access to LC-PUFA as opposed to further inland. Finally, we demonstrated that future global warming may disrupt the timing of bird-breading season and peak aquatic insect emergence, resulting in lower availability of physiologically important LC-PUFA in a changing world. My research showed that emergent aquatic insects are an important vector of how LC-PUFA can be transferred from aquatic ecosystems to riparian terrestrial food webs. Future work can build on these findings and further quantify the LC-PUFA biosynthesis ability of riparian birds and spiders as a way to predict the physiological effect of a changing world with lower LC-PUFA availability on riparian terrestrial predators.