Estimating Nitrogen Fixation Rates, Importance, and Short-Term Efficiency in Small, Temperate Reservoirs Using Delta15N Techniques

<p>Nitrogen (N2) fixation can give certain species of cyanobacteria a competitive advantage in lake and reservoir phytoplankton. These species of cyanobacteria, along with others that cannot fix N2, can form toxic compounds that impair water quality when present in high concentrations. N2 fixation rates may be substantial in small (< 1.0 km2), temperate reservoirs since these systems experience thermal stratification and often nitrogen (N) limitation throughout a substantial proportion of the year. However, the effects of N2 fixation on N cycling, alleviation of short-term N limitation, and water quality are not well-understood. A mesocosm experiment and ecosystem-scale observational study were conducted to 1) determine the efficiency of N2 fixation under varying N relative to phosphorus (P) supply, 2) examine the effects of N2 fixation on autotrophic biomass accumulation and microcystin production, and 3) measure N2 fixation rates and importance to autotrophic N demand and zooplankton N assimilation. Results of the mesocosm experiment indicated that N2 fixation was increased at low N:P supply under high P. However, N2 fixation was inefficient at alleviating N limitation when fixed N was the primary source of N. Additionally, microcystin production occurred only at high N:P supply when N2 fixation was low, indicating that reducing external N inputs may have a positive effect on water quality. Results of whole-reservoir determination of N2 fixation using seston &#948;15N natural abundances indicated that N2 fixation rates throughout the warm season were substantial and influenced by water temperature. Annual N2 fixation rates ranged from 2.2 - 6.6 g N m-2 yr-1, and contributed up to 19% of the annual autotrophic N demand. Zooplankton were assimilating fixed N in most of the study reservoirs, representing a possible mechanism of ecosystem fixed N retention. Collectively, these results suggest that N2 fixation plays a substantial role in N cycling in small, temperate reservoirs, but likely cannot alleviate short-term N limitation. </p>

Name thesis:degree_name

Master of Science in Crop, Soil & Environmental Sciences (MS)

Level thesis:degree_level

Thesis

Year dc:date.available

2014

Author dc:creator

• Baker, Bryant Christopher

Advisor dc:contributor.advisor

• Scott, J. Thad

Contributors dc:contributor

• Evans-White, Michelle A.
• Haggard, Brian E.

• Nitrogen
• Nitrogen Fixation
• Phosphorus
• Reservoir
• Stable Isotope
• Biogeochemistry
• Environmental Monitoring
• Environmental Sciences

Repository record dc:identifier

https://scholarworks.uark.edu/etd/2272

OAI identifier oai:identifier

oai:scholarworks.uark.edu:etd-3811