Macrophyte-mediated fluctuations in stream ecosystem function

Macrophytes are important structural and biologically active components in many lowland streams. Although the direct and indirect roles of macrophytes on ecosystem structure and function have been assessed, their concurrent influence on whole-stream metabolism and nutrient cycling, particularly during periods of growth and senescence, is less clear. In our study, we sought to quantify the relationships between macrophyte phenology and seasonal patterns in stream structure and function. We measured variables related to biofilm abundance, whole-stream metabolism, and nutrient cycling in a macrophyte-dominated agricultural stream in Denmark on 9 sampling events throughout a year. Seasonal variation in macrophyte abundance and biomass led to concurrent variation in the biomass of epiphytic and benthic biofilms. Regarding ecosystem function, macrophyte biomass was the strongest predictor of NH₄⁺ uptake length (i.e., S_w) and was also a predictor of soluble reactive P (SRP) S_w and whole-stream metabolism. Furthermore, NH₄⁺ demand was predicted by gross primary production, and SRP uptake was predicted by ecosystem respiration, demonstrating tight coupling between autotrophic production and N cycling and between heterotrophic production and P. Our study adds to the few seasonal studies on the central role of macrophytes on ecosystem function in lowland streams, especially during the growing season via direct metabolic activity and nutrient assimilation as well as indirect effects, such as supporting epiphytic biofilms and enhancing organic matter retention.