Implicit Scaling in the Design of Experimental Aquatic Ecosystems

Abstract

We undertook a quantitative literature review to assess implicit scaling choices made in the design of enclosed experimental aquatic ecosystems (mesocosms). A database was constructed with information on temporal scale, spatial scale, and other design characteristics for 360 experiments reported in the literature. We found that key scaling variables such as complete physical dimensions were often not reported. Overall mesocosm experiments had a median volume of 1.7 m 3 and median duration of 49 d. Volume and duration varied by habitat type, experimental treatment, number of trophic levels included, and the response variable under investigation. A number of experimental design characteristics varied with mesocosm size. For instance, characteristics that decreased with increasing mesocosm size included the number of replicates and treatments, and the degree of experimental control over species composition and the physical environment. We also found a bias towards scaling cylindrical containers for a constant ratio of radius to depth as size was increased. This design choice inevitably alters surface-area-to-volume relationships as well as associated ecological variables such as gas and light energy flux, benthic-pelagic coupling, and the relative dominance of periphyton growth on mesocosm walls. Our results indicate the need for both 'scale sensitive' experiments, that explicitly consider scale in design and interpretation of results, and multi-scale' experiments that manipulate temporal and spatial attributes in order to test specific hypotheses regarding the ecological impact of modification in scale. Both types of experiments are prerequisites for improved mesocosm design and for the systematic extrapolation of information from experimental ecosystems to nature.