Nitrogen cycling and dynamics in a macrophyte-rich stream as determined by a 15N-NH4+ release

1. A tracer release study was conducted in a macrophyte-rich stream, the River Lilleaa in Denmark. The objectives of the study were to compare uptake rates per unit area of by primary producers and consumers in macrophyte and non-macrophyte habitats, estimate whole-stream uptake rates of and compare this to other stream types, and identify the pathways and estimate the rate at which enters the food web in macrophyte and non-macrophyte habitats. 2. Macrophyte habitats had four times higher primary uptake rates and an equal uptake rate by primary consumers per unit habitat area as compared to non-macrophyte habitats. These rates represent the lower limit of potential macrophyte effects because the rates will be highly dependent on macrophyte bed height and mean bed height in the River Lilleaa was low compared to typical bed heights in many lowland streams. Epiphytes accounted for 30% of primary uptake in macrophyte habitats, illustrating a strong indirect effect of macrophytes as habitat for epiphytes. N flux per unit habitat area from primary uptake compartments to primary consumers was four times lower in macrophyte habitats compared to non-macrophyte habitats, reflecting much greater biomass accrual in macrophyte habitats. Thus, we did not find higher N flux from macrophyte habitats to primary consumers compared to non-macrophyte habitats. 3. Whole-stream uptake rate was 447 mgN m-2 day-1. On a habitat-weighted basis, fine benthic organic matter (FBOM) accounted for 72% of the whole-stream uptake rate, and macrophytes and epiphytes accounted for 19 and 8%, respectively. 4. We had expected a priori relatively high whole-stream N uptake in our study stream compared to other stream types mainly due to generally high biomass and the macrophytes role as habitat for autotrophic and heterotrophic organisms, but our results did not confirm this. In comparison with other release study streams, we conclude that nutrient concentration is the overall controlling factor for N uptake rates across streams, mostly as a result of high biomass of primary uptake compartments in streams with high nutrient concentrations in general and not in macrophyte streams in particular. 5. Our results indicate that macrophytes play an important role in the longer-term retention of N and thus a decrease in net downstream transport during the growing season compared to streams without macrophytes, through direct and indirect effects on the stream reach. Direct effects are high uptake efficiency, low turnover rate (partly due to no direct feeding on macrophytes) and high longevity. An indirect effect is increased sedimentation of FBOM in macrophytes compared to non-macrophyte habitats and streams which possibly also increase denitrification. Increased retention with macrophyte presence would decrease downstream transport during the growing season and thus the N loading on downstream ecosystems.