Habitat use of three sympatric whitefish Coregonus lavaretus forms was determined using hydroacoustics, pelagic trawling and epibenthic gillnetting in the subarctic Lake Muddusja¨rvi during the day and night in June, August and September. Whitefish constituted 97% of the numerical catches and whitefish with high number of gill rakers (DR) were the most abundant whitefish form. Forms with low numbers of gill rakers used only epibenthic habitats during both the day and night in all study periods: large whitefish with low numbers of gill rakers (LSR) dwelled mainly at depths 0-10 m, whereas small whitefish with low numbers of gill rakers (SSR) used deeper (>10 m) habitats. LSR and SSR whitefish consumed mainly benthic macroinvertebrates during all study occasions. The planktivorous DR whitefish used both epibenthic and pelagic habitats, but vertical habitat selection varied both over time of day and season.

A large‐scale biomanipulation trial was carried out on Lake Vesijärvi in Finland during 1989–1993. Following the mass removal of coarse fish the biomass of cyanobacteria collapsed from 1.4 g/m−3 to below 0.4 g/m−3, while total phosphorus concentration declined from 45 μg/L to 30 μg/L. No relevant changes in zooplankton communities were observed. The results suggest that the success of food web manipulation as a tool for lake restoration is not necessarily dependent on the grazing rate of zooplankton. The effects of reduced fish‐mediated internal loading and recycling of nutrients are in many cases stronger than those of reduced planktivory. Alternative stable states of water quality may also exist in lakes not covered by macrophytes, owing to the changes in the behavior of fish stocks. Year‐to‐year variation in the littoral zone may cause large oscillations in lake ecosystems—for example, through the recruitment of fish. In addition, the nutrients translocated by fish from the littoral zone may affect the nutrient dynamics of the pelagial plankton community. In terms of phytoplankton species composition and the ratio of phosphorus to chlorophyll a, the water quality in Lake Vesijärvi has improved in a stepwise fashion within the last 10 years. This is probably due to the fact that the five‐year mass removal of fish in Enonselkä fulfilled the requirement of sustained management of fish stocks in order to maintain nonequilibrial conditions between alternate stable states. The prediction of the water quality development is obscured, however, by spatial and temporal within‐lake variation, which sets high requirements for sampling programs.

The effects of clay turbidity and light on the predator–prey interaction between planktivorous smelts (Osmerus eperlanus) and phantom midge (Chaoborus flavicans) larvae were studied by means of laboratory experiments. Irrespective of light intensity, fish-mediated mortality of chaoborid larvae was highest at intermediate turbidity (20 nephelometric turbidity units (NTU)). Increases in light intensity enhanced the feeding rate of smelts at very low light intensities. A regression model describing the dependence of smelt-mediated mortality of chaoborids on light intensity and turbidity was fitted to the data. The model suggested that turbidity exceeding 30 NTU combined with light intensity below 0.1 µE·m–2·s–1 provides an efficient daytime refuge for chaoborids even in the presence of planktivorous smelts. In the field studies, the depth distribution of chaoborids followed the predictions of the model. The depth at which chaoborid density was highest depended on the existence and location of the thermocline, the densest swarms occurring beneath the turbidity maximum in the metalimnion. The smelts occupied water layers above the chaoborids, suggesting that the chaoborids used the steep turbidity gradient in the thermocline as a shelter against predation.

Predation has a fundamental role in aquatic ecosystems, but the relative importance of factors governing prey selection by predators remains controversial. In this study, we contrast five lakes of a subarctic watershed to explore how prey community characteristics affect prey selection and growth rate of the common top predator, brown trout ( Salmo trutta ). The lakes constitute a distinct gradient of different coregonid prey fish, ranging from monomorphic common whitefish ( Coregonus lavaretus ) to polymorphic whitefish co-occurring with vendace ( Coregonus albula ). The brown trout was a morph–species- and size-specific pelagic predator, selecting the small-sized, pelagic whitefish morph or vendace over the benthic whitefish morphs. In all lakes, the average prey size increased with predator size, but small-sized prey were also included in the diet of large predators. The selection of small-sized, pelagic prey fish appeared to be a favourable foraging strategy for the brown trout, yielding higher growth rates and an earlier ontogenetic shift to piscivory. The findings emphasize that piscivory appear to be shaped by the diversity, size-structure, and abundance of available prey in a given community.

Adaptive phenotypic divergence of sympatric morphs in a single species may have significant evolutionary consequences. In the present study, phenotypic impacts of predator on zooplankton prey populations were compared in two northern Finnish lakes; one with an allopatric whitefish, Coregonus lavaretus (L.), population and the other with three sympatric whitefish populations. First, we examined whether there were phenotypic associations with specific niches in allopatric and sympatric whitefish. Second, trait utility (i.e. number of gillrakers) of allopatric and sympatric whitefish in utilizing a pelagic resource was explored by comparing predator avoidance of prey, prey size in environment, and prey size in predator diet. The allopatric living large sparsely rakered (LSR) whitefish morph, was a generalist using both pelagic and benthic niches. In contrast, sympatric living whitefish morphs were specialized: LSR whitefish was a littoral benthivore, small sparsely rakered whitefish was a profundal benthivore and densely rakered (DR) whitefish was a pelagic planktivore. In the lake with allopatric whitefish, zooplankton prey did not migrate vertically to avoid predation whereas, in the lake with sympatric whitefish, all important prey taxa migrated significantly. Trait utility was observed as significantly smaller size of prey in environment and predator diet in the lake with DR whitefish than in the lake with only LSR whitefish.

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Summary The rate of non‐native fish introductions into freshwater ecosystems has more than doubled during the past three decades, posing a serious threat to native biodiversity. Despite potential benefits for fisheries, little is known about how introduced species interact with native communities at the food‐web level, or impact energy transfer dynamics and accumulation of contaminants in lake ecosystems. Here, we explored the trophic structure of a large, oligotrophic subarctic lake and assessed the trophic niche use and potential ecosystem‐wide consequences of two introduced salmonid species: piscivorous lake trout (Salvelinus namaycush) and zooplanktivorous vendace (Coregonus albula). We used a combination of diet, stable isotope and total mercury concentration data to test the hypotheses that the introduced fishes: (i) show partial niche overlap with the native fish community; (ii) increase total isotopic food‐web size and dietary linkages by increasing the diversity of niches present within the system, in comparison to analyses where only the native species were considered; and (iii) have differing mercury bioaccumulation rates from native species due to differences in trophic ecology and habitat preferences, being higher in pelagic than in littoral species. Trophic interactions between the introduced and native species were extensive, with evidence of reciprocal predation, resource competition and possible competitive exclusion apparent. Despite partial niche overlap with native species, the inclusion of introduced species in our analysis increased both total isotopic niche space and the number of dietary linkages present in the food web. On the basis of these findings, we suggest that introduced vendace may have led to a shift in system‐wide reliance on pelagically derived energy, whereas generalist foraging by piscivorous lake trout may have further integrated littoral and pelagic food‐web compartments. Mercury bioaccumulation rates were highly species‐specific and varied among habitats, but were generally higher in the pelagic food‐web compartment. However, contrary to expectations, vendace had lower mercury levels than native pelagic species, potentially reducing the extent of biomagnification within the lake. Our study demonstrates how introduced fishes may elicit complex and unpredictable responses in food‐web structure and ecosystem function, and thus complicate contaminant bioaccumulation and transfer processes within freshwater ecosystems.

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