Moving on up: can results from simple aquatic mesocosm experiments be applied across broad spatial scales?

Aquatic ecologists use mesocosm experiments to understand mechanisms driving ecological processes. Comparisons across experiments, and extrapolations to larger scales, are complicated by the use of mesocosms with varying dimensions. We conducted a mesocosm experiment over a volumetric scale spanning five orders of magnitude (from 4 L to whole ponds) to determine the generality of algal responses to nutrient enrichment. Recognising that mesocosm dimensions may affect algal growth, we also manipulated the ratio of mesocosm surface area to volume (SA : V) over two levels (high versus low). We used mesocosm tanks of similar size and construction to those commonly used in aquatic experiments to increase the generality of our results. 2. Volume was generally a stronger determinant of algal responses than mesocosm shape (i.e. SA : V). However, the effects of both volume and shape on algae were weak and explained a small portion of the variance in response variables. In addition, there was no consistent, directional relationship (positive or neutral) between mesocosm volume and algal abundance (estimated by chlorophyll concentration). Combined, our findings suggest that results from small-scale experiments, examining the direct response of algae to nutrient enrichment, can probably be ‘moved on up’ and applied to larger, more natural aquatic systems. 3. Algal response to nutrient enrichment (e.g. nutrient use efficiency and effect size) varied strongly with time. This underscores the importance of choosing an experimental timescale appropriate to the biological and/or ecological process of interest. 4. We compared our results to those from a recent meta-analysis of nutrient-limitation studies that included 359 freshwater pelagic experiments, spanning a wide range of volumetric and temporal scales. Similar findings between this experiment and the meta-analysis indicate that algal response to nutrient enrichment varies little across spatial scales. Therefore, it is probable that results from small-scale pelagic algal nutrient-limitation experiments are relevant to large-scale processes, such as eutrophication.