Scaling Aquatic Primary Productivity: Experiments Under Nutrient- and Light-Limited Conditions

Abstract

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 m 3 . The other series was scaled for a constant shape (radius/depth ϭ 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.

Lake metabolism and the diel oxygen technique: State of the science

“…Although the method has several advantages and avoids the caveats of bottle and chamber incubations (Gerhart and Likens 1975;Bender et al 1987;Petersen et al 1997Petersen et al , 1999Chen et al 2000), a number of uncertainties and assumptions are, however, also associated with the current application of the technique that need consideration (Table 7). Regardless of these issues, buoy-based data are being collected from an increasing number of lakes around the world, making it possible to investigate changes in metabolism over large gradients in time and space.…”

“…In the past, metabolism has been measured in bottles and chambers incubated at varying irradiance levels, using either dissolved oxygen or carbon (as 14 C) as an elemental tracer. By extrapolating bottle rates to represent 24-h periods, it is possible to calculate depth-and time-integrated pelagic rates of GPP, NEP, and R; and when combined with sediment chamber incubations, it is possible to estimate GPP, NEP, and R of entire ecosystems (e.g., Kemp et al 1997;Gazeau et al 2005). While enclosing part of an aquatic system within bottles is attractive because it allows precise measurements with replication, incubations are significantly affected by "container effects," which hamper estimates of ecosystem level metabolism (Bender et al 1987).…”

“…The main uncertainty with the container approaches concerns problems of scale (Gerhart and Likens 1975;Chen et al 2000) and is related to natural heterogeneity of the benthos and variability in accounting for the natural light conditions. Also, comparisons among container studies are difficult because many studies fail to report key details of the containers used (Petersen et al 1997(Petersen et al , 1999. In comparison, it was hoped that free-water techniques integrate a signal over the entire benthic-littoral and pelagic region in much the same way that a terrestrial gas-flux tower integrates gas-fluxes over a region (Valentini et al 1996).…”

Assessing pelagic and benthic metabolism using free water measurements

“…Free-water methods have considerable advantages over incubation approaches in either light and dark bottles or benthic chambers. Container effects including enclosure in bottles, sediment chambers, and various other devices can be significant, and comparison among container studies is difficult because many studies fail to report key details of the containers used (Petersen et al 1997;Petersen et al 1999). Additionally, container approaches suffer from problems of scale (Gerhart and Likens 1975;Chen et al 2000).…”

Biomass and production of plankton in shallow and deep lakes : are there general patterns ?

“…Mazumder (1994) demonstrated that the phosphorus-chlorophyll relationship is influenced by thermal mixing regime (with or without stratification) and foodweb structure (presence or absence of large-bodied Daphnid). Lake depth is suspected to have a great influence on the plankton communities and ecosystem functions through light limitation (Petersen et al 1997 …”