This study examined the relative influence of nutrients (nitrogen and phosphorus) and habitat on algal biomass in five agricultural regions of the United States. Sites were selected to capture a range of nutrient conditions, with 136 sites distributed over five study areas. Samples were collected in either 2003 or 2004, and analyzed for nutrients (nitrogen and phosphorous) and algal biomass (chlorophyll a). Chlorophyll a was measured in three types of samples, fine-grained benthic material (CHL sub(FG)), coarse-grained stable substrate as in rock or wood (CHL sub(CG)), and water column (CHL sub(S)). Stream and riparian habitat were characterized at each site. TP ranged from 0.004-2.69mg/l and TN from 0.15-21.5mg/l, with TN concentrations [...]