Quantitative Assessment of Drivers of Ecosystem Functions in Headwater Stream Networks
Author: Kaitlin Jean Farrell
Publisher:
Published: 2017
Total Pages: 348
ISBN-13:
DOWNLOAD EBOOKEffectively valuing the contribution of small headwater streams to the functioning of river networks requires an understanding of how physical and biological drivers are linked to ecosystem functions, and of how drivers and functions vary, both within and among stream networks. Despite a large body of work on individual 1st and 2nd order streams, fewer studies have examined patterns in structure or function across different sized streams within a network. Here, I combined field experiments and cross-biome syntheses to link stream structure and function through two focal lenses; the quantity and quality of basal resources, and the role of stream consumers. When developing quantitative relationships between drivers and rates of ecosystem respiration (ER), gross primary production (GPP), and ammonium (NH4) uptake, I found that within a headwater stream network, coarse and fine benthic organic matter standing stocks were positively correlated with ER, while light availability was a strong driver of GPP. Based on extrapolation to the stream network, small 1st and 2nd order streams are expected to play a substantial role in ER and NH4 uptake compared to larger streams. Across four distinct stream networks, benthic organic matter carbon to nitrogen and phosphorus ratios were lower in fine than course fractions. This higher relative nutrient content in fine benthic organic matter suggested that macroinvertebrates feeding on coarse and fine organic matter could be broadly macronutrient or carbon limited, respectively, with implications for in-stream resource cycling. Stream consumers had limited detectable effects on measured metrics of structure and function, both within and among headwater networks. Larval salamanders did not initiate top-down trophic cascades, but reduced stream NH4 uptake rates, potentially through nitrogen excretion. We also did not detect significant consumer effects across stream networks from five biomes, despite differences in consumer biomass and trophic position. However, across biomes, basal resources were strong drivers of both ER and GPP, which indicated that study networks were bottom-up, rather than top-down controlled. Collectively, these four studies highlight the importance of linking measurements of structure and function in headwater streams, to quantify their role in larger stream networks, and as such, advocate for their protection.