Implicit scaling in the design of experimental aquatic ecosystems

We undertook a quantitative literature review to assess implicit scaling ch oices made in the design of enclosed experimental aquatic ecosystems (mesoc osms). A database was constructed with information on temporal scale, spati al scale, and other design characteristics for 360 experiments reported in the literature. We found that key scaling variables such as complete physic al dimensions were often not reported. Overall mesocosm experiments had a m edian volume of 1.7 m(3) and median duration of 49 d. Volume and duration v aried by habitat type, experimental treatment, number of trophic levels inc luded, and the response variable under investigation. A number of experimen tal design characteristics varied with mesocosm size. For instance, charact eristics that decreased with increasing mesocosm size included the number o f replicates and treatments: and the degree of experimental control over sp ecies composition and the physical environment. We also found a bias toward s scaling cylindrical containers for a constant ratio of radius to depth as size was increased. This design choice inevitably alters surface-area-to-v olume relationships as well as associated ecological variables such as gas and light energy flux, benthicpelagic coupling, and the relative dominance of periphyton growth on mesocosm walls. Our results indicate the need for b oth 'scale sensitive' experiments, that explicitly consider scale in design and interpretation of results, and;'multi-scale' experiments that manipula te temporal and spatial attributes in order to test specific hypotheses reg arding the ecological impact of modification in scale. Both types of experi ments are prerequisites for improved mesocosm design and for the systematic extrapolation of information from experimental ecosystems to nature.