The influence of experimental scale on estimating the predation rate of Gammarus lacustris (Crustacea: Amphipoda) on Daphnia in an alpine lake

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Abstract
We examined the effect of experimental scale on the predation rate of Gammarus lacustris preying on Daphnia middendorffiana in 20 l cubitainers, 2800 l mesocosms and diurnal whole-lake surveys. The predation rate differed significantly among experimental scales. It was highest in the 20 l cubitainers, intermediate in the 2800 l mesocosms and lowest in the lake. At all experimental scales, the predation of G.lacustris on D.middendorffiana increased as a linear function of prey density (Type I functional response). Very different predation rates were calculated for the whole lake depending on whether D.middendorffiana densities were estimated from whole-water column hauls or stratified depth hauls. This highlights the importance of accurately determining the spatial distribution of prey for estimating reliable in situ predation rates. We also determined prey selection by G.lacustris. Daphnia middendorffiana was strongly selected over the calanoid copepod Hesperodiaptomus arcticus. When presented with different sized Daphnia, G.lacustris preyed size selectively on individuals ≥2 mm. Our results show that the predation rate of small invertebrate predators is sensitive to a wide range of experimental conditions and needs to be corroborated at a variety of scales. We conclude that the impact of G.lacustris on zooplankton communities would be strongest in small fishless lakes or ponds.

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