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


Anika Bratt

Document Type

Senior Honors Project

Publication Date



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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