In aquatic ecosystems, predation is affected both by turbulence and visibility, but the combined effects are poorly known. Both factors are changing in lakes in the Northern Hemisphere; the average levels of turbulence are predicted to increase due to increasing wind activities, while water transparency is decreasing, e.g., due to variations in precipitation, and sediment resuspension. We explored experimentally how turbulence influenced the effects of planktivorous fish and invertebrate predators on zooplankton when it was combined with low visibility caused by high levels of water color. The study was conducted as a factorial design in 24 outdoor ponds, using the natural zooplankton community as a prey population. Perch and roach were used as vertebrate predators and Chaoborus flavicans larvae as invertebrate predators. In addition to calm conditions, the turbulent dissipation rate used in the experiments was 10−6 m2 s−3, and the water color was 140 mg Pt L−1. The results demonstrated that in a system dominated by invertebrates, predation pressure on cladocerans increased considerably under intermediate turbulence. Under calm conditions, chaoborids caused only a minor reduction in the crustacean biomass. The effect of fish predation on cladocerans was slightly reduced by turbulence, while predation on cyclopoids was strongly enhanced. Surprisingly, under turbulent conditions fish reduced cyclopoid biomass, whereas in calm water it increased in the presence of fish. We thus concluded that turbulence affects fish selectivity. The results suggested that in dystrophic invertebrate-dominated lakes, turbulence may severely affect the abundance of cladocerans. In fish-dominated dystrophic lakes, on the other hand, turbulence-induced changes in planktivory may considerably affect copepods instead of cladocerans. In lakes inhabited by both invertebrates and fish, the response of top-down regulation to turbulence resembles that in fish-dominated systems, due to intraguild predation. The changes in planktivorous predation induced by abiotic factors may possibly cascade to primary producers.

Summary Small‐scale turbulence is a key factor in increasing encounter rates but also dispersing prey patches of planktonic organisms and can thus contribute to their distribution. Our hypotheses were that turbulence can (i) disturb the vertical refuge use of fourth instar Chaoborus flavicans larvae and (ii) also disperse them horizontally. Both were tested experimentally, across a gradient of turbulence. The vertical experiments were conducted in 46‐L cylinders, providing the larvae a vertical refuge by darkening the bottom layer. The horizontal experiments were conducted in a 200‐L aquarium, providing them a horizontal refuge by darkening one end of the aquarium. With both set‐ups, two light treatments were tested: uniform darkness as a control and the bottom layer of the cylinder or either end of the aquarium darkened to provide a refuge. In addition to non‐turbulent treatment, five different root‐mean‐square (RMS) velocities were generated, ranging from 0.6 to 10.2 cm s−1 in the vertical and to 10.9 cm s−1 in the horizontal experiment. Increasing turbulence had a significant effect on the distribution of C. flavicans larvae in both vertical and horizontal directions and increased the proportion of larvae in the non‐preferred illuminated environment. In the vertical direction, the proportion of larvae in the refuge decreased linearly with increasing turbulence, showing a decline from 84% under calm conditions to 61% at the highest RMS velocity (10.2 cm s−1). The effect of turbulence was more prominent in the horizontal direction; without turbulence, 86% of the larvae occupied the refuge, but the proportion declined to 32% at 5.4 cm s−1 RMS velocity. In experiments with uniform darkness, the distribution of chaoborids was not affected by turbulence either in the vertical or in the horizontal direction. Our results indicate that turbulence has substantial effects on refuge use and distribution of C. flavicans larvae. Increasing turbulence can greatly diminish the ability of prey species to use their vertical refuges. Once away from their vertical refuge, chaoborids are highly vulnerable to horizontal dispersion, thus upsetting their horizontal escape behaviour. Small‐scale turbulence is therefore likely to have considerable effects on food‐web dynamics in lakes.

The effects of water turbulence on rotifer communities were experimentally studied under different predation pressures. When the larvae of the phantom midge (Chaoborus flavicans) were present in turbulent water, the abundance of most rotifer taxa was enhanced. Especially the genera Chromogaster, Keratella, Polyarthra, and Trichocerca, increased in abundance. In calm water, chaoborids did not affect the rotifer community. In turbulent water predation by chaoborids was targeted more towards cladocerans (Bosmina sp.) and predation pressure on rotifers was relaxed. Additionally, reduced competition with cladocerans probably contributed to the increase of rotifer abundance. Turbulence alone had no significant effect on rotifer abundance because their individual size was small compared with the diameter of the turbulent eddies. The study suggested that the effects of turbulence on rotifers is not direct but takes place through changed predator-prey relations, i.e., the effect depends on the abundance of invertebrate predators. In aquatic ecosystems with a high density of chaoborids, increasing turbulence can considerably increase the abundance of rotifers.

The coexistence of competing species relies on niche partitioning. Competitive exclusion is likely inevitable at high niche overlap, but such divide between competitors may be bridged if environmental circumstances displace competitor niches to enhance partitioning. Foraging‐niche dimension can be influenced by environmental characteristics, and if competitors react differently to such conditions, coexistence can be facilitated. We here experimentally approach the partitioning effects of environmental conditions by evaluating the influence of water turbulence on foraging‐niche responses in two competing fish species, Eurasian perch Perca fluviatilis and roach Rutilus rutilus, selecting from planktonic and benthic prey. In the absence of turbulence, both fish species showed high selectivity for benthic chironomid larvae. R. rutilus fed almost exclusively on zoobenthos, whereas P. fluviatilis complemented the benthic diet with zooplankton (mainly copepods). In turbulent water, on the other hand, the foraging‐niche widths of both R. rutilus and P. fluviatilis increased, while their diet overlap simultaneously decreased, caused by 20% of the R. rutilus individuals turning to planktonic (mainly bosminids) prey, and by P. fluviatilis increasing foraging on littoral/benthic food sources. We show that moderate physical disturbance of environments, such as turbulence, can enhance niche partitioning and thereby coexistence of competing foragers. Turbulence affects prey but not fish swimming capacities, with consequences for prey‐specific distributions and encounter rates with fish of different foraging strategies (pause‐travel P. fluviatilis and cruise R. rutilus). Water turbulence and prey community structure should hereby affect competitive interaction strengths among fish species, with consequences for coexistence probability as well as community and system compositions.

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