Abstract
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.
Highlights
One of the main shortcomings in understanding the response of aquatic ecosystems to disturbances is the lack of a framework blending together physics and biology [1]
This holds in climate change studies in which most scenarios on the effects on aquatic ecosystems have focused on rising water temperature, variations in external nutrient loading, and resulting changes in nutrient concentrations [2,3,4], while one of the most important physical factors, water turbulence, and its effects on biological interactions have been ignored
Predation pressure by juvenile and adult stages of planktivorous fish usually results in a zooplankton community dominated by small species [12,13], whereas predation by invertebrates often leads to dominance by large-bodied zooplankton [14,15]
Summary
One of the main shortcomings in understanding the response of aquatic ecosystems to disturbances is the lack of a framework blending together physics and biology [1] This holds in climate change studies in which most scenarios on the effects on aquatic ecosystems have focused on rising water temperature, variations in external nutrient loading, and resulting changes in nutrient concentrations [2,3,4], while one of the most important physical factors, water turbulence, and its effects on biological interactions have been ignored. Turbulence affects aquatic ecosystems, e.g. via bottom-up regulation, because it influences nutrient cycling and light environment through sediment resuspension and resulting water turbidity Such effects have been widely studied [7,8] and have been included in climate change scenarios [3]. The importance of each factor is dependent on the characteristics of the predator and the prey
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