Title

Nitrogen Fixation Dynamics and Biological Controls in the South Fork of the Eel River Watershed in Northern California

Authors

Anika Bratt

Document Type

Senior Honors Project

Publication Date

1-2011

Abstract

We explored the dynamics of nitrogen fixation (N-fixation) in Cladophora mat assemblages in the South Fork Eel River in northern California, an N-limited ecosystem. We estimated N-fixation rates and characterized epiphyte communities to evaluate: 1. The relationship between Cladophora mat color and N-fixation; 2. The effect of mat architecture and light availability on N-fixation; 3. Patterns in N-fixation with increasing drainage area and over successional time; and 4. The effect of nutrient addition (nitrogen (N) & phosphorous (P)) on N-fixation at 3 sites of increasing watershed area (125, 145 & 155 km2).

1. Rates of N-fixation from rust-colored Cladophora assemblages were significantly higher than from green or yellow-green assemblages, achieving rates exceeding 7,502 μg/m2/h. Rust-colored assemblages had significantly higher epiphyte loads than green or yellow-green assemblages. Rust-colored Cladophora assemblages were dominantly epipytized by diatoms of the order Rhopalodiaceae such as Epithemia, which are hosts to N-fixing endosymbionts. High rates of N-fixation in rust-colored macroalgal assemblages suggested that epiphytization by Epithemia in late summer may represent an important source of nitrogen in this ecosystem.

2. Rates of N-fixation and Rhopalodiaceae loads in both floating and benthic mat surfaces were significantly higher than in the shaded mat interior. Rates of N-fixation in Cladophora assemblages decreased significantly when shifted from full sunlight exposure to no light exposure. These data indicated that variation in N-fixation rates in space and time may be predicted by Cladophora mat architecture and light availability.

3. N-fixation rates and Rhopalodiaceae loads increased at the two downstream sites but N-fixation rates were initially higher upstream. This may be connected with the irregular precipitation pattern of the previous winter N-fixation rates at a downstream site increased substantially, yet Rhopalodiaceae loads increased minimally, indicating that another N-fixer, possibly heterotrophic cyanobacteria, may play an important role in the community composition at this site. Although our earlier work evaluating the relationship between Cladophora mat color and N-fixation rates suggested that rates may be predicted by mat color, these data indicated that this relationship is more complex.

4. Nutrient enrichments of N and P had no significant effect on rates of N-fixation in Cladophora assemblages, however at the most upstream site a trend of suppression of N-fixation rates in assemblages exposed to the N addition and an increase of rates in assemblages exposed to the P addition indicate that P limitation may occur in this ecosystem.

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