Scaling aquatic primary productivity: Experiments under nutrient- and light-limited conditions

To explore the interactive effect of physical dimension and nutrient conditions on primary productivity, experimental planktonic-benthic ecosystems were initiated in different-sized cylindrical containers scaled in two ways. One series of experimental ecosystems was scaled for a constant depth (1.0 m) as volume was increased from 0.1 to 1.0 to 10 m³. The other series was scaled for a constant shape (radius/depth = 0.56) across an identical range of volumes. Triplicate systems of each size and shape were housed in a temperature-controlled room illuminated with fluorescent and incandescent lights, and mixed by means of large, slow-moving impellers. All experimental ecosystems received an exchange of filtered estuarine water (10%/d). Nutrient concentrations, and ecosystem primary productivity and respiration, were traced over time during spring, summer, and fall experiments. During the nutrient-rich spring experiment, systems in the constant-shape series exhibited similar gross primary productivity (GPP) when rates were expressed per unit area or per unit light energy received. When productivity was expressed per unit volume, however, rates declined as the depth of the containers increased. We interpret this dimensional pattern of GPP in the spring experiment as a reflection of light limitation. During the summer experiment, when nutrient concentrations were low, GPP was constant per unit volume, and it increased with increasing depth when expressed per unit area. This reversed dimensional pattern is consistent with expectations under nutrient-limited conditions. Indeed, GPP increased and the scaling pattern returned to that observed in the spring experiment when we added nutrients to the containers. During the fall experiment, nutrient concentrations were intermediate between spring and summer, and the dimensional pattern of GPP exhibited characteristics of both light and nutrient limitation. Differences in productivity in the constant-depth series were less extreme and can be attributed to artifacts of enclosure, such as differences in light attenuation and differences in the ratio of wall area to the unit volume of the containers. Understanding both fundamental scaling effects and artifacts of enclosure is key to the comparative analysis of processes among ecosystems, and to extrapolating results from experimental to natural ecosystems.