SCALING AQUATIC PRIMARY PRODUCTIVITY - EXPERIMENTS UNDER NUTRIENT-LIMITED AND LIGHT-LIMITED CONDITIONS

To explore the interactive effect of physical dimension and nutrient c onditions on primary productivity, experimental planktonic-benthic eco systems were initiated in different-sized cylindrical containers scale d 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 m (3). 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 illuminat ed with fluorescent and incandescent lights, and mixed by means of lar ge, slow-moving impellers. All experimental ecosystems received an exc hange of filtered estuarine water (10%/d). Nutrient concentrations, an d ecosystem primary productivity and respiration, were traced over tim e during spring, summer, and fall experiments. During the nutrient-ric h spring experiment, systems in the constant-shape series exhibited si milar gross primary productivity (GPP) when rates were expressed per u nit area or per unit Light energy received. When productivity was expr essed per unit volume, however, rates declined as the depth of the con tainers increased. We interpret this dimensional pattern of GPP in the spring experiment as a reflection of light limitation. During the sum mer experiment, when nutrient concentrations were low, GPP was constan t per unit volume, and it increased with increasing depth when express ed 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 exper iment when we added nutrients to the containers. During the fall exper iment, nutrient concentrations were intermediate between spring and su mmer, 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 arti facts of enclosure, such as differences in light attenuation and diffe rences in the ratio of wall area to the unit volume of the containers. Understanding both fundamental scaling effects and artifacts of enclo sure is key to the comparative analysis of processes among ecosystems, and to extrapolating results from experimental to natural ecosystems.