To identify trematode diversity and life-cycles in the sub-Arctic Lake Takvatn, Norway, we characterised 120 trematode isolates from mollusc first intermediate hosts, metacercariae from second intermediate host fishes and invertebrates, and adults from fish and invertebrate definitive hosts, using molecular techniques. Phylogenies based on nuclear and/or mtDNA revealed high species richness (24 species or species-level genetic lineages) and uncovered trematode diversity (16 putative new species) from five families typical in lake ecosystems (Allocreadiidae, Diplostomidae, Plagiorchiidae, Schistosomatidae and Strigeidae). Sampling potential invertebrate hosts allowed matching of sequence data for different stages, thus achieving molecular elucidation of trematode life-cycles and exploration of host-parasite interactions. Phylogenetic analyses also helped identify three major mollusc intermediate hosts (Radix balthica, Pisidium casertanum and Sphaerium sp.) in the lake. Our findings increase the known trematode diversity at the sub-Arctic Lake Takvatn, showing that digenean diversity is high in this otherwise depauperate sub-Arctic freshwater ecosystem and indicating that sub-Arctic and Arctic ecosystems may be characterised by unique trematode assemblages.

We review three long-term research programs performed over the last four decades on the ecology and management of oligotrophic lake systems with different fish communities at 69 N in Norway. Through whole-lake perturbation experiments, intensive culling of stunted fish removed 35 tons (1984)(1985)(1986)(1987)(1988)(1989)(1990)(1991) of Arctic charr Salvelinus alpinus in Takvatn (15 km 2 ) and 153 tons (1981)(1982)(1983)(2002)(2003)(2004) of European whitefish Coregonus lavaretus in Stuorajavri (25 km 2 ). In Takvatn, the overcrowded charr population decreased to 20% of the initial abundance, whereas brown trout Salmo trutta abundance increased. Somatic growth improved strongly in both species. In charr, ontogenetic habitat shifts broke down, the diet changed to more benthos, and plankton-borne parasites decreased. High abundance of juvenile, littoral charr provided new prey for trout, creating an alternative, predator-regulated stable state. Similar density reductions, positive effects on growth and reduced parasite loads occurred in whitefish in Stuorajavri. Despite the heavy culling, however, a new stable state did not occur and the fish community returned to the pre-culling situation. In the Pasvik watercourse, vendace Coregonus albula invaded around 1990 after an upstream introduction. The population of this non-native, highly specialized planktivore increased rapidly, resulting in steep density declines in zooplankton and the native planktivorous whitefish morph, and large changes in energy flow and structure and dynamics of the lacustrine food web. These programs show that long-term research is essential for understanding the ecology of manmade disturbances and providing a scientific basis for management efforts.

Competition is assumed to shape niche widths, affecting species survival and coexistence. Expectedly, high interspecific competition will reduce population niche widths, whereas high intraspecific competition will do the opposite. Here we test in situ how intra-and interspecific competition affects trophic resource use and the individual and population niche widths of two lacustrine fish species, Arctic charr and brown trout, covering a 40 year study period with highly contrasting competitive impacts prior to and following a large-scale fish culling experiment. Initially, an overcrowded Arctic charr population dominated the study system, with brown trout being nearly absent. The culling experiment reduced the littoral Arctic charr density by 80%, whereupon brown trout gradually increased its density in the system. Thus, over the study period, the Arctic charr population went from high to low intraspecific competition, followed by increasing interspecific competition with brown trout. As hypothesized, the relaxed intraspecific competition following the experimental culling reduced individual diet specialization and compressed population niche width of Arctic charr. During the initial increase of the brown trout population, there was a large dietary overlap between the two species. Over the subsequent intensified interspecific competition from the population build-up of brown trout, their trophic niche overlap chiefly declined due to a dietary shift of Arctic charr towards enhanced zooplankton consumption. Contrary to theoretical expectations, the individual and population niche widths of Arctic charr increased with intensified interspecific competition. In contrast, the diet and niche width of brown trout remained stable over time, confirming its competitive superiority. The large-scale culling experiment and associated long-term research revealed pronounced temporal dynamics in trophic niche and resource use of the inferior competitor, substantiating that intra-and interspecific competition have large and contrasting impacts on individual and population niches.

The trophic niches of Arctic charr and brown trout differ when the species occur in sympatry. Their trophically transmitted parasites are expected to reflect these differences. Here, we investigate how the infections of Diphyllobothrium dendriticum and D. ditremum differ between charr and trout. These tapeworms use copepods as their first intermediate hosts and fish can become infected as second intermediate hosts by consuming either infected copepods or infected fish. We examined 767 charr and 368 trout for Diphyllobothrium plerocercoids in a subarctic lake. The prevalence of D. ditremum was higher in charr (61.5%) than in trout, (39.5%), but the prevalence of D.dendriticum was higher in trout (31.2%) than in charr (19.3%). Diphyllobothrium spp. intensities were elevated in trout compared to charr, particularly for D. dendriticum. Large fish with massive parasite burdens were responsible for the high Diphyllobothrium spp. loads in trout. We hypothesize that fish prey may be the most important source for the Diphyllobothrium spp. infections in trout, whereas charr predominantly acquire Diphyllobothrium spp. by feeding on copepods. Our findings support previous suggestions that the ability to establish in a second piscine host is greater for D. dendriticum than for D. ditremum.

Free‐living parasite life stages may contribute substantially to ecosystem biomass and thus represent a significant source of energy flow when consumed by non‐host organisms. However, ambient temperature and the predator's own infection status may modulate consumption rates towards parasite prey. We investigated the combined effects of temperature and predator infection status on the consumer functional response of three‐spined sticklebacks towards the free‐living cercariae stages of two common freshwater trematode parasites (Plagiorchis spp., Trichobilharzia franki). Our results revealed genera‐specific functional responses and consumption rates towards each parasite prey: Type II for Plagiorchis spp. and Type III for T. franki, with an overall higher consumption rate on T. franki. Elevated temperature (13°C) increased the consumption rate on Plagiorchis spp. prey for sticklebacks with mild cestode infections (<5% fish body weight) only. High consumption of cercarial prey by sticklebacks may impact parasite population dynamics by severely reducing or even functionally eliminating free‐living parasite life stages from the environment. This supports the potential role of fish as biocontrol agents for cercariae with similar dispersion strategies, in instances where functional response relationships have been established. Our study demonstrates how parasite consumption by non‐host organisms may be shaped by traits inherent to parasite transmission and dispersal, and emphasises the need to consider free‐living parasite life stages as integral energy resources in aquatic food webs.