Nutrient uptake in experimental estuarine ecosystems: Scaling and partitioning rates

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, PO₄^3-, 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 DINXPO₄^3- concentrations in the water column. Similarly, differences in net nutrient uptake rates among experimental ecosystems of different dimensions could be explained by the relative partitioning 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 general scaling relations for nutrient cycling in natural aquatic ecosystems.