Hamilton, S.K.
Presented at the All Scientist Meeting (1999-07-20 to 1999-07-21 )
The Lotic Intersite Nitrogen Experiment (LINX) is an NSF-funded cross-site comparison study that includes streams at several LTER sites across North America. The KBS site is Eagle Creek, located near Augusta in Ross Township. At each site we performed whole-stream 15N enrichment experiments for six weeks, following standardized protocols. The 15N was added as ammonium at levels that did not produce a fertilizing effect. The 15N enrichment revealed nitrogen flow through aquatic food webs as well as molecular transformations mediated by microbial activity (e.g., nitrification). All ten experiments were conducted in 1997 and 1998, and the LINX group is presently working on data analysis and publication of the results.Eagle Creek is one of the largest and most nutrient-rich streams among the LINX sites. In short-term nutrient additions (N or P), uptake from the stream water could not be detected over a 500-m reach. Nutrient-diffusing substrates did not stimulate algal growth, suggesting that low light availability due to canopy shading limits primary production within the stream more than nutrients do. Thus the stream would appear to act as a conduit for nutrients. However, the d 15N of dissolved ammonium and nitrate changed markedly over the reach, revealing that these labile nutrients exist in a dynamic steady-state in which ammonium is being produced and nitrified, and nitrate is being assimilated and/or denitrified, thus maintaining stable concentrations.Aquatic invertebrates in the creek, which are diverse and abundant, became labeled with 15N to varying degrees depending on their food sources and growth and turnover rates. Insect larvae that scrape algae from hard substrates became most labeled, followed by invertebrates known to be benthic particle feeders. Filter feeders became only slightly labeled. The 15N enrichment was harder to detect in animals with the slowest turnover (fishes and clams), but measurements of the digestive gland of the clams did reveal isotopic enrichment.The results of this experiment show the relative importance of algal assimilation vs. microbial transformations in determining the fate of nitrogen in the stream, and provide a comprehensive picture of food-web linkages and the relative roles of autochthonous primary production and allochthonous litter inputs in supporting stream food webs leading to fishes.
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