Background/Question/Methods A major challenge in ecology is how to extrapolate the results of mesocosm experiments to larger scales such as entire ecosystems. Understanding how the physical dimensions of mesocosm tanks influence biological processes will aid the interpretation of experimental results. Toward this goal, we conducted an experiment to determine the singular and interactive effects of mesocosm volume and shape on the response of phytoplankton (as chlorophyll (chl a) concentrations) to a simple nutrient manipulation. Mesocosm volume varied over 5 levels and 4 orders of magnitude, ranging from 4 L buckets to 500,000 L experimental ponds. Within a volume level, mesocosm shape was manipulated by establishing containers with a high or low surface area to volume ratio (SA:V). However, mesocosms greater than 1000L were present at only one shape level due to constraints on the availability of large-sized vessels. Nutrient levels were elevated in half of the mesocosms through a single, large pulse addition of inorganic nitrogen and phosphorus. During the 7 day experiment most mesocosms were exposed to ambient weather conditions; the smallest volume mesocosms were maintained in an environmental chamber. Temperature and light intensity were similar among all mesocosm sizes. Mesocosm tank water was sampled for chl a and total phosphorus (total P) concentrations on days 0, 2, 3, 5, and 7.

Results/Conclusions Nutrient enrichment increased algal biomass and chl a : total P after day 0. Mesocosm volume influenced chl a concentrations, chl a : total P, and chl a effect size, however, there was no clear relationship between increasing volume and the response variables. Algal biomass, chl a : total P, and chl a effect size were higher in low SA:V mesocosms on days 2 and 3. Mesocosm shape did not influence algal response variables after day 3, indicating that the effects of SA:V on algae were transitory. Interactive effects between nutrient enrichment and mesocosm volume and shape suggest that mesocosm dimensions can influence algal response in complex ways. Overall, our results suggest that short term manipulations of water column algae may be influenced by mesocosm dimensions, but for experiments lasting a week, results appear to be independent of mesocosm size. This suggests that it is valid to compare results of experiments at various spatial scales.