Discussion

Recent attempts for the inclusion of parasites in food webs have shown parasites to be significant structuring forces that could potentially increase important properties of trophic networks. Understanding ecosystem processes and functioning inherently requires the assessment of the ecological role of parasites in food webs, particularly the quantification of parasite-predator-prey relationships. However, recent findings are based on topological food webs, and the energetic aspect in terms of strength of trophic links between parasites and free-living organisms remains to be tested. Digenean trematodes are essential components of aquatic food webs potentially affecting multiple trophic levels within an ecosystem. The aim of this study is to investigate the ecological role of trematodes in trophic interactions and their biomass contribution to energy flow in the food web of a subarctic freshwater system. From Lake Takvatn in northern Norway, a highly resolved parasite-inclusive topological network has been established, and trematodes, the most diverse parasite taxon in this ecosystem, represent an excellent model system to address the general questions on trematodes’ ecological relevance in food webs. During two extensive sampling periods in August and October 2017, we performed a series of field and laboratory experiments to quantify and evaluate the strength of trophic interactions between the trematode free-living stage, the cercariae, and several predatory organisms by obtaining data for estimation of three main ecological aspects: (i) cercarial emission, output rates and biomass, (ii) survival rates of cercariae, and (iii) predation rates on cercariae from direct and indirect (i.e. concomitant predation) consumption by different predators. Model organisms comprised trematodes of three genera, one invertebrate predator (amphipod) and three vertebrate species (fish). Our preliminary results on the mean daily output rates and chronobiology in cercariae emergence suggest an adaptive response to the unique subarctic light and climatic conditions (photo- and thermoperiod). High survival rates of cercariae at low temperatures indicate the existence of compensation mechanisms of trematodes for a narrow temporal transmission window in subarctic areas. Part of the cercarial biomass serves as food resource for non-host species in the Takvatn ecosystem, as evidenced by the frequent direct consumption of cercariae by amphipod and fish predators. An additional proportion of the trematode population was excluded via concomitant predation, when trematodes were consumed along with their snail hosts. This indicates that parasite-predator-prey interactions could have strong knock-on effects on trematode population dynamics, transmission success and infection levels in downstream hosts. Collectively, our preliminary estimates of trematode biomass and its loss within the lake ecosystem provide empirical evidence that parasites play a significant ecological role in ecosystem energetics and thereby ecosystem processes. Our data underline the importance of studying the role of both parasites and ambient environment on the energy flow and functioning of food webs in general.