The filamentous green alga Cladophora hosts a complex epiphyte assemblage that varies in space and time, with important potential consequences for ecological function. In the South Fork Eel River watershed in northern California, the extent and color of macroalgal assemblages changes with watershed area and successional time from green to yellow-green to rusty-red in relation to increasing epiphyte load and changes in dominant epiphytes. Epithemia species, which are diatoms that contain nitrogen-fixing endosymbionts, can achieve high densities in rust colored macroalgal assemblages and thus may contribute to spatial and temporal variation in riverine N loading via N-fixation. In July 2008, N-fixation rates were measured in six replicate green, yellow-green, and rust-colored Cladophora mat fragments using acetylene reduction. In addition, differences in epiphyte density and structure on Cladophora mats of each color were determined by microscopic algal counts.

Results/Conclusions

The rates of N-fixation from rust-colored Cladophora assemblages were significantly higher than from green or yellow-green assemblages (p<0.001), achieving rates exceeding 200 μg/m²/h. Furthermore, rust-colored assemblages had significantly higher epiphyte loads than green or yellow-green assemblages. Green Cladophora assemblages were dominantly epiphytized by Cocconeis pediculus, while rust-colored Cladophora assemblages were overgrown by Epithemia turgida and E. sorex, both of which contain nitrogen-fixing cyanobacterial endosymbionts. High rates of N-fixation observed in rust-colored macroalgal assemblages suggest that epiphytization by Epithemia in late summer may represent an important source of nitrogen in this N-limited ecosystem. Furthermore, variation in N-fixation rates in space and time may be predicted by distinct shifts in Cladophora assemblage color, providing a potentially important new approach for estimating spatial and temporal variation in N-fixation at reach to watershed scales.