Rapid human population growth engenders landuse changes and alters nutrients, algal biomass (as chlorophyll a [chl a]), and dissolved O₂ concentrations (DO) in streams, but quantitative information on these relationships is limited in regions with a Mediterranean climate. We surveyed macroalgal % cover and chl a and physicochemical factors in spring and summer at 15 stream and estuarine sites in a catchment in southern California with a mosaic of undeveloped, agricultural, and urban areas to examine relationships among land use, nutrients, algae, and DO. We used nutrient diffusing substrata (NDS) at 12 sites to assess the nutrient(s) limiting algal growth. Algal chl a and pH often exceeded suggested or mandated water-quality impairment thresholds at sites affected by human activity, particularly those downstream from a wastewater treatment plant. Total N (TN) affected total and benthic algal chl a in spring and summer, total P (TP) affected benthic algal chl a in summer, and light influenced algae in summer. N was the sole or primary limiting nutrient at ½ of the sites and was colimiting with P at another ⅓ of the sites. Nutrient ratios (molar TN∶TP) were poor predictors of algal responses to nutrient enrichment. Diel changes in dissolved O₂ were related positively to discharge and negatively to algal chl a, particularly when floating macroalgae were present. In June, N, chl a, and macroalgal % cover were positively related to human landuse patterns at the subcatchment scale, whereas in September, P and chl a were related to landuse patterns at more local scales (500 m, 1000 m). Mediterranean streams present a variety of challenges for water-quality managers because of high seasonal and interannual variation in discharge and nutrient flux, high accumulations of algae during the dry season, low nutrient thresholds that generate nuisance algal blooms, and human population growth and associated changes in landuse patterns.