The color of macroalgal proliferations changes over space and time, and may indicate changes in their ecological function. In the South Fork Eel River, California, USA, the extent and color of algal blooms (primarily Cladophora) vary spatially down drainage networks and seasonally with changes in their epiphyte assemblages. New growths of Cladophora are green, but turn yellow with early diatom colonization, and later rusty color with heavy loads of Epithemia spp. which contain N-fixing cyanobacteria.  We are developing a photogrammetric technique that documents spatial and temporal variation in macroalgal color over reach and basin scales, and investigating how well color transitions predict changes in ecological function (insect emergence or nitrogen loading). Color of algal assemblages was quantified using aerial photographs from a tree-mounted camera 41 m above the river.  Images were calibrated with streamside camera photographs, microscopic algal counts, and a color chart to assess the reliability of color associations with algal assemblage structure. Preliminary results show that rusty Cladophora that were densely epiphitized by Epithemia had significantly greater N-fixation rates than green or yellow Cladophora.  Rusty mats, as hot spots of nitrogen fixation, may supply significant nitrogen inputs to downstream riverine, and possibly nearshore marine ecosystems, as they senesce.