Nutrient uptake in experimental estuarine ecosystems:scaling and partitioning rates

Abstract

Studies of nutrient cycling and enrichment in aquatic ecosystems are commonly conducted in enclosed experimental ecosystems. Although there is considerable information about how the dimensions of natural aquatic ecosystems influence nutrient cycling processes, little is known on how nutrient cycling studies might be affected by the physical scales of experimental enclosures. In the present study, replicate (n = 3) cylindrical containers of 5 dimensions with 3 volumes (0.1, 1.0, 10 m3), 3 depths (0.46, 1.0, 2.15 m), and 5 diameters (0.35, 0.52, 1.13, 2.44, 3.57 m) were established and subjected to pulsed additions of dissolved inorganic nutrients (DIN, Pod3-, Si) in summer and autumn experiments. Consistent with common experimental protocols, walls of these containers were not cleaned of periphytic growth during the 8 wk studies. Nutrient concentrations in experimental ecosystems were low prior to nutrient-pulse additions and exhibited exponential depletion following treatments. Overall, larger containers had lower net uptake rates and higher nutrient concentrations than did smaller tanks. Relative contributions of planktonic, benthic and wall periphytic communities to total nutrient uptake varied in relation to dimensions of experimental systems. In general, net uptake rates by planktonic communities were inversely related to water depth, with higher rates associated with increased mean Light-energy in shallower systems. Indirect estimates of benthic uptake rates, which were relatively low in all but the shallowest systems, tended also to be inversely related to depth and directly proportional to light levels at the sediment surface. In contrast, nutrient uptake by wall communities (per water volume) was inversely related to the radius of experimental containers. Differences in the 2 container dimensions, depth and radius, accounted for more than 90 % of the variance in both net nutrient uptake by the whole ecosystem and the molar ratio of DIN/POd3-concentrations in the water column. Similarly, differences in net nutrient uptake rates among experimental ecosystems of different dimensions could be explained by the relative partition-, ing of rates among planktonic, periphytic, and benthic habitats. These results demonstrated that the physical dimensions of experimental ecosystems can have profound effects on measured nutrient dynamics. We also suggest that many of these experimental observations may be relevant also to more genera1 scaling relations for nutrient cycling in natural aquatic ecosystems.

Multiscale Experiments in Coastal Ecology: Improving Realism and Advancing Theory

“…Results from this series of experiments were also generally consistent with our expectation that the relative contribution of wall periphyton to total system productivity and associated nutrient uptake would decrease with increasing radius (Chen et al , 2000. The results do not, however, fall cleanly along the line predicted by the radius-scale hypothesis (figure 7a).…”

Effect of oysters Crassostrea virginica and bottom shear velocity on benthic-pelagic coupling and estuarine water quality

“…Within the first 3 d of each experiment we added a nutrient spike of ammonium and soluble reactive phosphorus (SRP) at the Redfield ratio, increasing initial ammonium and SRP levels in the tanks by about 25 and 1.6 µM, respectively, to stimulate a phytoplankton bloom. We cleaned mesocosm walls of wall periphyton biweekly or more often to minimize wall growth (Chen et al 1997(Chen et al , 2000.…”

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.…”

“…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.…”