I used in situ, flow-through, hyporheic mesocosms to examine how streambed grain size distribution affects uptake and regeneration rates of dissolved inorganic nitrogen (DIN), phosphorus (SRP) along interstitial flow paths. A pilot study demonstrated that mixed coarse and fine sediments produced higher DIN concentrations than coarse sediments alone, but a similar mass of DIN due to faster exchange flow through the coarse sediments. I followed with more comprehensive investigations in a nitrogen-limited, forest stream and a phosphorus-limited, nitrogen-rich, agricultural stream, monitoring uptake and regeneration in fifteen distinct sediment treatments varying in median grain size and grain size distribution (2 replicates per treatment = 30 mesocosms per stream).
Regeneration was the dominant process for SRP and for nitrate, which composed 99-100% of detectable DIN, while uptake characterized ammonium and dissolved oxygen (O2). Sediment hydraulic conductivity (K) below 24 cm/s positively affected rate coefficients of nitrate regeneration and O2 uptake in both streams, and SRP regeneration in the nitrogen-limited forest stream (log-log regression: P < 0.0001, r2 > 0.77 for all). I attribute this effect to K control of interstitial water velocities, which determine boundary layer thickness and therefore diffusion rates at biofilm surfaces. In contrast, K negatively affected rates of O2 uptake above K = 24 cm/s (P = 0.0924, r2 = 0.548) and ammonium uptake above K = 5 cm/s (P = 0.0127, r2 = 0.425), suggesting substrate surface area limits biofilm metabolism more than K in very coarse sediments. Although SRP regeneration did not vary with K in the phosphorus-limited stream, the molar nitrogen:phosphorus ratio of regenerated nutrients in both streams varied significantly with K, with greater retention of the limiting nutrient at higher K indicating assimilation efficiencies of interstitial consumers increased with K. I compared modeled aerial SRP flux from streambeds driven by advective exchange flow to SRP flux estimated with a boundary layer diffusion model. Advective flux was more than two orders of magnitude higher than diffusive flux for high K sediments, and declined to diffusive flux levels only for K < 0.2 cm/s, suggesting diffusion models can greatly underestimate SRP exchange flux.
|Advisor:||Johnson, Michael L.|
|Commitee:||Ginn, Timothy R., Moyle, Peter B.|
|School:||University of California, Davis|
|School Location:||United States -- California|
|Source:||DAI-B 71/03, Dissertation Abstracts International|
|Subjects:||Ecology, Biogeochemistry, Limnology|
|Keywords:||Hyporheic, Nitrogen, Phosphorus, Regeneration, Sediments, Streams|
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