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While nutrient availability is an important controlling factor of primary production in aquatic systems, evidence has shown that nutrient concentration alone is an ineffective predictor of autotrophic production in flowing waters. Effects of nutrient enrichment differ in lotic systems as upstream fluxes resupply nutrients to primary producers downstream. Understanding these effects and describing the nutrient kinetics of a system are important objectives when setting ecological nutrient thresholds or predicting possible shifts in aquatic communities. To better understand the impact of enrichment on stream metabolism (i.e. gross primary production and ecosystem respiration) and the response of assimilation (i.e. nutrient uptake) to varying nutrient supply, we designed a factorial nutrient addition experiment at Silver River. The benthos box, a Plexiglas chamber, inserts into stream sediments and allows characterization of the isolated benthos (a 0.6m x 0.6m area), as nutrient supply is blocked and thus depleted over time. The chamber was coupled with a clustered nutrient bioassay to include one control chamber paired with three treatment chambers, which were deployed adjacent to each other for one week at a unique location along the river. Each treatment chamber was enriched with one of seven nutrient combinations of nitrate, phosphate, and iron. Within each box, a HOBO dissolved oxygen (DO) probe and a light logger captured DO and light intensity dynamics at 15-minute intervals. From these data, stream metabolism and light use efficiency measurements are estimated, and ancillary factors (e.g. canopy and algal cover, porewater chemistry, etc.) are tested as other potential explanatory factors of metabolic variation. From this study, we will determine 1) the effect of nutrient enrichment on stream metabolism and 2) how primary production and nutrient assimilation change under nutrient supply that varies from above saturation to below ambient. |