Department Surface Waters - Research and Management

In a warming world, the input of glacier meltwater to inland water ecosystems is predicted to change, potentially affecting their productivity. Meta-ecosystem theory, which posits that the nutrient availability in the recipient ecosystem can determine the extent of cross-ecosystem boundary utilization, can be useful for studying landscape-scale influences of glacier meltwater on inland waters. Here, we investigate how the input of glacier meltwater in a river system in Southern Greenland influences the utilization of marine subsidies in freshwater fish. Our study system comprised four sites, with controls for glacial meltwater and marine subsidies, harboring a partially migrating population of arctic char, meaning that some individuals migrate to the ocean and others remain in freshwaters, and two fully resident populations as a freshwater reference. We assessed the incorporation of marine carbon in freshwater resident char using both bulk and amino acid stable isotope analysis of muscle tissue. In the population with partial migration, marine subsidies were a significant resource for resident char individuals, and estimates of trophic position suggest that egg cannibalism is an important mechanism underlying the assimilation of these marine subsidies. In proglacial streams, namely those with high glacial meltwater, the total dependence on marine subsidies increased and reached 83% because char become cannibals at smaller sizes. In the configuration of our focal meta-ecosystem, our results suggest that the importance of marine subsidies to freshwater fish strengthens within increasing meltwater flux from upstream glaciers.

1. Global change drives multiple facets of biodiversity including interaction diversity, which is fundamental for ecosystem functioning. However, studying trophic interactions is challenging in meta-ecosystems, that is ecosystems connected by spatial flows of energy, materials and organisms across ecosystem boundaries. While analytical methods based on abundances of polyunsaturated fatty acids (PUFAs) and stable isotopes of amino acids (AAs) are being increasingly used, it has never been explored if both approaches could be: (i) combined in mixing models to enhance precision in dietary inference (ii) compared to disentangle transfers of various PUFAs and proteins in food webs in the wild.
2. We explore the utility of analytical approaches based on PUFA abundances and AA isotopes to resolve resource transfers in a natural riparian food web. We focus on spiders and their potential prey from blue, green and brown sources to address three important and persisting methodological issues in food-web ecology, namely whether (i) essential AA carbon isotopes can resolve protein origin from blue, green and brown resources, (ii) PUFA relative abundance and AA isotopes can be combined in a mixing model to provide higher precision estimates (i.e. narrower intervals) and (iii) combining the two approaches can unveil the coupling of protein and PUFA transfers in food webs. 3. Our research demonstrates the power of AA isotopes and PUFAs to distinguish blue, green, and brown sources and their transfer up to consumers. We show that combining PUFA relative abundance and AA isotopes in a mixing model provides overall estimates similar to the individual estimates but significantly increases precision. In addition, we showcase how combining approaches unveil the coupling of protein and PUFA transfers. For instance, we show that most PUFAs are less concentrated from prey to predators, relative to proteins, highlighting uncoupling of PUFAs and protein transfer along food chains.
4. We show for the first time the effectiveness of combining AA isotopes and PUFA abundances, particularly relevant for complex trophic interactions in a meta-ecosystem context. Our study illustrates the trophic uncoupling of proteins and PUFAs, highlighting the necessity in combining both approaches.

1. Lakes are recipients of allochthonous organic matter and nutrients. However, the importance of these subsidies for food webs and how they vary with lake trophic status remains unclear, especially for large lakes.
2. We assessed the source and fate of organic matter and nutrients in seven perialpine lakes across a gradient of trophic status. We measured carbon and nitrogen stable isotopes of amino acids of lake-residing Atlantic trout, Salmo trutta, to determine the source of primary production (i.e., how carbon is fixed in the ecosystem) and how it is transferred through food webs, respectively. Based on essential amino acid carbon fingerprinting, we estimated the probability of organic carbon originating from autochthonous (algal), allochthonous (terrestrial plant), and recycled (bacterial) sources. In addition, we used amino acid δ¹⁵N to track how this primary production is transferred to consumers in general, and by using different trophic amino acids (glutamic acid and alanine), identify the trophic pathways involving either metazoan or protozoans.
3. We found a high likelihood of autochthonous origin of organic carbon (86 ± 9%) in trout that contrasted with allochthonous origins of particulate organic matter and some sediments. We showed that those estimates are good proxies of source reliance. Our results also highlighted the importance of bacterial origin of organic carbon in fish (12%). The likely autochthonous origin of this carbon was supported by trophic markers (Ala δ¹⁵N) that suggest the role of protists in transferring recycled organic carbon up the food web. While the sources of nitrogen sustaining food webs varied among lakes, we found a conserved carbon fingerprinting of fish. Overall, this suggests an uncoupling between the source of nutrients and organic carbon in large perialpine lakes.
4. Across a wide range of trophic status (c. 2 orders of magnitude range of phosphorus concentration), several lines of evidence suggested that perialpine lake food webs shared a common reliance on autochthonous and bacterial production.
5. Our study is the first to quantify the dependence on allochthonous organic carbon in lake food webs based on new amino acid stable isotope markers (carbon fingerprinting and Ala δ¹⁵N) and shows promise for estimating the source of carbon fixation in ecosystems. Our results support previous suggestions that terrestrial organic carbon is a relatively minor source for aquatic consumers despite contributing to the pool of organic matter, and more importantly, its contribution does not vary substantially with trophic status in perialpine lakes.